- Email: [email protected]
A fast assay of water hardness ions based on alkaline earth metal induced coacervation (HALC)
Talanta 80 (2010) 2049–2056
Contents lists available at ScienceDirect
Talanta journal homepage: www.elsevier.com/locate/talanta
A fast assay of water hardness ions based on alkaline earth metal induced coacervation (HALC) George Z. Tsogas, Dimosthenis L. Giokas, Athanasios G. Vlessidis ∗ Laboratory of Analytical Chemistry, Department of Chemistry, University of Ioannina, 45110, Ioannina, Greece
a r t i c l e
i n f o
Article history: Received 12 August 2009 Received in revised form 23 October 2009 Accepted 3 November 2009 Available online 10 November 2009 Keywords: Alkaline earth metals Coacervation Spectrophotometry Water hardness
a b s t r a c t In this work, a rapid assay of water hardness is presented based on the formation of a coacervate phase made up of multilamellar vesicles and close bilayers produced upon the reaction of alkaline earth metals with a carboxylate anionic surfactant in the presence of a co-surfactant (methanol). The procedure exploits the light scattering abilities of the coacervate phase which can be logarithmically linked to total hardness as CaCO3 equivalents via spectrophotometric detection at 350 nm. The method, abbreviated as HALC, stemming from hardness by alkaline earth metal coacervation, is straightforwardly applicable overcoming the requirement for regulation of the experimental parameters involved in the determination procedure. In total, 28 water samples with various matrix compositions and hardness contents were analyzed with satisfactory accuracy as evidenced by comparison of the results with EDTA complexometric titration. The method is free from interferences from environmentally significant metal cations and inorganic anions affording detection limits as low as 13.5 mg L−1 CaCO3 with the aid of a correction factor, which is determined by the non-linear absorbance properties of the solution mixture, and satisfactory reproducibility (RSD = 4.21–8.08%). © 2009 Elsevier B.V. All rights reserved.
1. Introduction Real time monitoring of water quality parameters is of critical importance in environmental and industrial fields, including the water quality control in drinking water treatment plants. Among these parameters, total hardness, defined as the total content of calcium (Ca2+ ) and magnesium (Mg2+ ) in water, is one of the most popular indicators of water quality since it is not only related to the deposition of mineral deposits in water pipes but it is also reported to affect the toxicity of certain metal ions in aquatic ecosystems [1]. Furthermore, water hardness contributes to the daily uptake of Ca2+ and Mg2+ which are physiologically important elements for the human body with significant negative effects encountered in cases of deficiency [2]. The most popular methods for the determination of water harness are those based on complexometric titration [3], flame atomic absorption spectrometry [4] and ion-selective electrodes [5,6]. Complexometric methods are probably the most popular methods due to their simplicity but they are prone to the analysts’ efficiency unless an automated system is used that can accurately determine the end point of the titration. On the other hand, flame AAS methods are highly accurate for benchtop analysis but they
∗ Corresponding author. Tel.: +30 26510 08401; fax: +30 26510 08781. E-mail address: [email protected] (A.G. Vlessidis). 0039-9140/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.talanta.2009.11.003
cannot be used for outdoor studies. Potentiometric ion-selective electrodes (ISEs) are the technique of choice when continuous in situ measurements in an automatic manner are required combining simplicity of operation and low analysis cost. However, the selectivity of measurements is not often sufficient to enable the primary ion to be directly determined; interferences arising from other species with similar size and charge, matrix effects and mediocre day-to-day reproducibility may have serious repercussions in the accuracy of the analysis [5]. To overcome these problems, several techniques have been employed based on physical separation of sample components (i.e., chromatography), modeling the potentiometric response of the analytes in the presence of interferences (Nikolskii–Eisenman equation) or chemometric procedures [5,7]. In the pursue of combining the advantages of the above methods in the determination of water hardness, the phenomenon of coacervation may offer a convenient alternative. By definition, coacervation is the separation of colloidal systems into two liquid phases, a colloid-rich and a colloid-poor phase [8]. In that sense, coacervates can be defined as colloid-rich liquids immiscible with the colloid-poor aqueous equilibrium solution from which it is separated. In this context, if appropriate interactions between the coacervate phase and the analytes of interest can be established, coacervation can be efficiently employed as an alternative to the solvent-free extractions. Up to date, several such systems have appeared in the literature enabling the determination of organic
2050
G.Z. Tsogas et al. / Talanta 80 (2010) 2049–2056
and inorganic analytes via hydrophobic, electrostatic and hybrid interactions [9–13]. The coacervation of organic macromolecules induced by metallic ions is a well-known phenomenon and several studies have been devoted to the characterization of the morphological changes induced upon their interaction [14]. In this direction, alkaline earth metals, and especially Ca2+ , have received considerable attention due to their high reactivity with anionic surfactants leading either to the formation of well-known lime soaps or to complex vesicle and lamellar phases with interesting properties [14,15]. Nevertheless, the analytical utility of these interactions towards the determination of alkaline earth metals has not been reported despite the fact that vesicle-based extractions of metal ions exploiting their attraction with the anionic head group of anionic surfactants have been described [9,16,17]. Considering the high reactivity of alkaline earth metals with anionic surfactants, this work describes the first analytical application of the phenomenon of coacervation for the determination of water hardness. The proposed method is based on the formation of a lamellar colloid-rich phase induced by alkaline earth metals with sodium dodecanoate in the presence of a co-surfactant (water miscible organic solvent). This reaction results in the formation of a dispersed vesicular phase which scatters visible light in manner related to metal ion concentration. Based on this principle, the absorbance of the coacervate phase formed by Ca2+ and Mg2+ with sodium dodecanoate was monitored spectrophotometrically yielding a concentration depended response to metal ion concentration. To the best of our knowledge, this is the first time that metal ion induced coacervation is being exploited directly towards the determination of water hardness whish is one of the most popular water quality parameters. 2. Experimental section 2.1. Reagents All reagents were of analytical-grade and they were used without further purification. Standard metal solutions used for the experiments were prepared by sequential dilution of 1000 mg L−1 stock solutions prepared by dissolving appropriate amounts of Ca(NO3 )2 (Merck, Darmstadt, Germany) and Mg(NO3 )2 ·6H2 O (Fluka, Buchs, Switzerland). Metal standard solutions used for the interference study were prepared by sequential dilution of 1000 mg L−1 of Titrisol stock solutions (Merck). Lauric acid in the form of Sodium dodecanoate (Sigma grade, 99–100%, Sigma–Aldrich, Greece) was dissolved in HPLC grade methanol (Labscan, Dublin, Ireland.) to prepare a 5% (w/v) stock solution. 2.2. Instrumentation Absorbance measurements were performed with matched quartz cells of 1-cm path length in a Jenway 6405 UV–vis spectrophotometer. A pH meter, WTW 552 model glass electrode was employed for the pH measurements for the solutions. 2.3. HALC analytical procedure In 10 mL of water sample previously equilibrated at room temperature, 1 mL of Lauric acid solution (5% (w/v) in methanol) is added. The mixture is agitated and its absorbance is directly recorded at 350 nm against reagents blank.
chased from local stores, surface waters (6 samples) collected from Acherontas river (N.W. Greece) in various locations from springs to estuary and water samples from six (6) springs used for water supply in Thesprotia region were collected in HNO3 -cleaned glass containers, filtered through a Whatman No. 40 filter (0.45 m) and stored at room temperature. Tap water was collected directly form the drinking water network supply, filtered and analyzed directly without storage. A series of samples from underground water aquifers were collected along the coastal line (≤2 km from sea) of north Corfu Island (N.W. Greece) from four (4) different drillings. Infiltration of seawater was qualitatively determined by the addition of AgNO3 leading to the formation of white AgCl precipitate. Two (2) seawater samples were collected from the coastal line of Thesprotia (N.W. Greece). The first sample was collected in the estuaries of Kalamas River and the other by direct sampling of seawater 10 m from the coast. 3. Results and discussion 3.1. Optimization of experimental conditions The optimization of the experimental conditions that enable the determination of water hardness with the proposed coacervation scheme is not a straightforward objective. That is because Ca2+ and Mg2+ upon reaction with sodium dodecanoate, in the presence of methanol as a co-surfactant (which affects aggregates size) [18], yield different lamellar structures [17]. According to the SEM micrographs of Fig. 1, Ca2+ favours the formation of spongelike phases (Fig. 1b) and Mg2+ the formation of bilayer structures (Fig. 1c) while mixtures of alkaline earth metals lead to the formation of a complex network of organized surfactant assemblies containing both multilamellar and bilayer vesicular aggregates (Fig. 1d). From a technical standpoint, these phenomena can be explained taking into consideration the organization phenomena occurring in the oriented molecular layers and the different radii of the hydrated cation condensation [9,17]. However, in the context of the analytical application intended in this study, these structural diversifications of the coacervate phase bring about significant changes in the absorption of each solution. According to the results of the calibration curves of Fig. 2, Ca2+ yields a linear response up to a concentration of 100 mg L−1 while Mg2+ response is rectilinear only up to 15 mg L−1 , above which absorbance values gradually reaches a plateau. In the linear range of the calibration curves, the ratio of slopes (Mg2+ /Ca2+ ) is approx. 8 which means that Mg2+ induces, on average, 8 times more intense absorption than Ca2+ at the same concentration level at 350 nm. Beyond ion type and concentration, the measured signal strongly depends on other parameters related to the formation and stability of the vesicular (coacervate) phase, like sodium dodecanoate concentration, ionic strength, hydroxyl or hydrogen concentration and temperature [9,10,17]. To this end, we decided to prepare a set of model solutions containing a high variety of Ca2+ and Mg2+ concentrations and use them as workings solutions for the optimization of the working conditions. The composition of these samples, gathered in Table 1, covers a wide range of Ca2+ and Mg2+ concentrations (as well as their ratio) so as to provide a representative evaluation of most natural waters. All samples had a total volume of 10 mL and measurements were performed at 350 nm. 3.2. Effect of sodium dodecanoate concentration
2.4. Real samples In total, 28 water samples of various matrix composition and hardness content were assessed. Bottled waters (9 samples) pur-
The first parameter assessed was sodium dodecanoate concentration in the range of 0.1–2% (w/v). Aqueous solutions (10 mL) were spiked with 0.1–2 mL of 5% (w/v) sodium dodecanoate and the
G.Z. Tsogas et al. / Talanta 80 (2010) 2049–2056
2051
Fig. 1. SEM micrographs of the coacervate phase formed by sodium dodecanoate, methanol and various cations. (a) Na+ (50 m at 10 kV), (b) Ca2+ (5 m at 20 kV), (c) Mg2+ (5 m at 20 kV) and (d) Ca2+ and Mg2+ (1 m at 20 kV).
absorbance was immediately recorded at 350 nm. According to the data presented in Fig. 3, 0.5% (w/v) sodium dodecanoate provides the optimum absorption for a wide variety of water hardness ranging from soft to very hard water. For lower sodium dodecanoate concentrations, the vesicle’s charge was probably counterbalanced due to inadequacy of reactive sites and in combination with the respective low amounts of co-surfactant content did not favour the formation of a lamellar phase. On the other hand, with increasing sodium dodecanoate and co-surfactant concentrations, the precipitate swelled, increasing the absorbance of the solution, either due to transition from vesicles to stacked membranes [17], or due to the increase of surfactant concentration above cmc which complicates solubilization and dispersion of the alkaline earth metal coacer-
Fig. 2. Absorbance vs. concentration plots for Ca2+ and Mg2+ .
vates due to the presence of micellar aggregates. In either case, the analytical signal is deteriorated either due to low UV absorption or due to intense turbidity that reduces the working and measuring range of the method. Therefore, the concentration of 0.5% (w/v) was selected for the remaining work. It is worth mentioning that at the optimum sodium dodecanoate concentration, sample volume did not have any significant effect on the absorbance of the solutions (±6%). However, to minimize uncertainty it is advisable to maintain a constant sample volume throughout method application.
Table 1 Composition of model solutions used for the optimization study. Sample
Ca2+
Mg2+
Total hardness (mg L−1 CaCO3 )
Absorbance (A.U.) at = 350 nm
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
5 10 5 10 20 40 60 5 10 20 40 60 5 10 20 40 60 5 10 20 40 60
0 0 5 5 5 5 5 10 10 10 10 10 15 15 15 15 15 20 20 20 20 20
12.5 25.0 33.1 45.6 70.5 120.5 170.4 53.7 66.1 91.1 141.1 191.0 74.2 86.7 111.7 161.6 211.6 94.8 107.3 132.3 182.2 232.2
0.075 0.215 0.619 0.762 1.141 1.402 1.729 1.043 1.284 1.511 1.660 2.168 1.649 1.740 1.820 2.085 2.340 1.909 2.035 2.136 2.347 2.503
2052
G.Z. Tsogas et al. / Talanta 80 (2010) 2049–2056
Fig. 3. Effect of sodium dodecanoate concentration in the absorbance of aqueous solutions containing various levels of water hardness.
3.3. Reaction kinetics The incubation time necessary for sodium dodecanoate to react with alkaline earth metals towards the formation of a lamellar structure was the next parameter assessed for its influence on the analytical signal in terms of intensity, measuring range and reproducibility. Absorbance values recorded over the range of 0–240 min showed an increasing absorbance pattern with reaction time. From the data of Fig. 4 it can be inferred that absorbance increases with increasing water hardness gradually limiting the applicability of the method in hard waters since the signal eventually soared up to “out-of-range” absorbance values (not shown). Furthermore, the reproducibility of the measurements was deteriorated probably because vesicle membranes were not homogeneously dispersed in the vial which increases uncertainty. On the other hand, samples analyzed immediately after sodium dodecanoate addition gave the best reproducibility. Based on this observation, samples were measured after addition of sodium dodecanoate followed by vortex agitation to facilitate the reaction of sodium dodecanoate
Fig. 5. Absorbance signal as a function of temperature in soft water.
with alkaline earth metals and improve dispersion of the lamellar phase. 3.4. Effect of temperature Temperature was the next parameter assessed for its effect on the analytical response (Fig. 5). Cold conditions (<10 ◦ C) induced significant signal amplification due to the formation of a viscoelastic phase while warm conditions reduced the absorbance due to increased solubility of the lamellar phase and dehydration of the amphiphile head group inducing thermo-sensitive transformations [16,17]. A stable and reproducible response was obtained between 20 and 30 ◦ C, therefore room temperature (approx. 25 ◦ C) was selected for the analysis of real samples. 3.5. Effect of solution pH The influence of hydrogen or hydroxyl concentration present in the sample was assayed by addition of increasing amounts of dilute HCl or NaOH. At acidic pH values sodium dodecanoate is protonated which increases competition with alkaline earth metals while the opposite is observed in alkaline media where the anionic group of sodium dodecanoate is deprotonated which aids complexation with metal species. In either case, the concentration of Ca2+ and Mg2+ in the sample was found to be an important factor in the absorbance of the solution as a function of pH. At acidic media higher Ca2+ and Mg2+ concentrations were necessary to compete for the reactive sites of sodium dodecanoate while in alkaline conditions complexation was favoured with increasing pH and water hardness. In this context, a priori knowledge of total hardness is necessary in order to decide on the appropriate regulation of the solution pH which has no practical meaning since the determination of hardness is of concern. Furthermore, the pH of the solution upon addition of sodium dodecanoate was close to 6.5 which is not far from the usual pH of most natural waters and far less than the solubility limit of both Ca2+ and Mg2+ in aqueous solutions. Therefore it was decided to apply the method in an unregulated pH environment. 3.6. Calibration procedure
Fig. 4. Effect of incubation time on the absorbance of sodium dodecanoate-alkaline earth cation coacervates.
The calibration procedure was performed with a series of model solutions (10 mL sample volume) containing various amounts of water hardness and Ca2+ /Mg2+ ratios, by extending the
G.Z. Tsogas et al. / Talanta 80 (2010) 2049–2056
2053
The following classification was used according to the level of total hardness: very soft (0–70 mg L−1 CaCO3 ), soft (70–140 mg L−1 CaCO3 ), slightly hard (140–210 mg L−1 CaCO3 ), moderately hard (210–320 mg L−1 CaCO3 ), hard (320–530 mg L−1 CaCO3 ) and very hard (>530 mg L−1 CaCO3 ). 3.7. Analytical characteristics of the method
Fig. 6. Logarithmic calibration curve at 350 nm.
experimental data of Table 1, in an unregulated ionic strength environment to simulate the composition of natural waters. The concentration vs. absorbance line plotted in Fig. 6 shows that the absorbance is linearly depended on water hardness up to 60 mg L−1 (as CaCO3 ) but above this value a logarithmic relationship is blatantly observed. This is reasonable since Mg2+ -sodium dodecanoate coacervates scavenge the light more intensely than the respective Ca2+ -sodium dodecanoate coacervates. As a result, the same absorbance value can be produced by at two combinations, the first involving high Ca2+ levels and the second low Mg2+ levels. From the concentration curves of Fig. 2 it is inferred that an absorbance value of 1 (a.u.) can be obtained by either 56.7 mg L−1 Ca2+ or 5 mg L−1 Mg2+ . In order to account for all occasions (samples with different water hardness giving similar absorbance values analyzed in the same batch) the overall fit to the experimental data was logarithmically expressed as a function of total water hardness. The calibration data were fitted to the expression A = 0.9347 × Ln(total hardness) − 2.6626 r = 0.9765
(1)
which was considered satisfactory considering the large number of data points and the high variability in the concentration of the individual data points included in the calibration curve. This equation was therefore applied to the analysis of real samples and the results were evaluated from a qualitative, semi-quantitative and quantitative perspective. It is conceivable that appropriate dilution can bring the sample within the linear range of the calibration curve, provided that an a priori assessment of total hardness has been undertaken. Starting already from the first attempts to apply HALC method to the analysis of real samples it was revealed that a correction factor (Cf ) to the calculated concentration was necessary. This came as no surprise considering the aforementioned discussion regarding the influence of Mg2+ in comparison to Ca2+ . Despite the fact that in most real samples situations were Ca2+ is totally absent or Mg2+ significantly surpasses Ca2+ concentration are rather scarce, we persisted on accounting for all cases, irrespectively of their scarcity and appearance rate in the natural environment. The calculated values of total hardness were multiplied by a factor of 1.80 ± 0.41 when the absorbance lied within the range 0.31 < Abs350 < 2 and by a factor of 0.70 ± 0.13 when 2 < Abs350 < 0.31. By using this approach the total hardness of real samples was calculated and the results are presented below in more detail. In is conceivable that if another calibration curve equation is used then a re-examination of the correction factors should be applied although no significant differences were observed in model solutions examined for that purpose.
Due to the lack of linearity of the calibration curve the determination of method detection limit and the quantitation limit as multiples of the signal-to-noise ratio cannot be calculated because of the non-linear properties of the calibration curve which results in variable slope and, as previously discussed, similar responses to more than one combination of Ca2+ and Mg2+ levels. Therefore, the lower limit of detection (LLD), which is the lowest amount or concentration of analyte which can be distinguished from the blank with a reasonable confidence, was determined by the mean absorbance of the blank sample plus k times the standard deviation of the absorbance obtained on the blank sample, i.e., ALLD = Amean blk + k × ASDblk , where k is a constant related to the statistical level of confidence [19]. This approach is similar to that proposed in DG SANCO decision were the detection capability (CCa ) is defined “the lowest concentration level at which a method can discriminate with a statistical certainty of 1 – ˛ whether the identified analyte is present” [20]. For k = 3, the LLD is 19.4 mg L−1 CaCO3 and corresponds to the uncorrected concentration determined directly by equation (1). Since the absorbance at this concentration is below 0.31, multiplication by the respective Cf = 0.70, results in a final detection limit of 13.5 mg L−1 CaCO3 . The reproducibility of the method, expressed by the relative standard deviation (RSD) of repeated assays was 4.21–8.08% (n = 7) from very soft to very hard waters, which satisfies the requirements for the application intended in this study. 3.8. Interferences In view of the low selectivity of the spectrophotometric detector and the competitive reaction ambience where other metallic ions may react with sodium dodecanoate, various metallic cations and inorganic anions were investigated for their effect on the absorbance signal. The tolerance limit of a foreign species was defined as the concentration that causes a relative error greater than ±5%. Trace elements like Pb+2 , Cd+2 , Sb+3 , As+3 , Co2+ , Ni+2 and Cr3+ had no effect on the analytical response at concentrations up to 50 g L−1 , which is well above the usual concentration levels. Important elements like Fe3+ , Al3+ and Zn2+ had no effect up to the maximum concentration levels examined (500 g L−1 ), while Cu2+ could be tolerated up to 200 g L−1 . Regarding the interference by anions, phosphates (25 mg L−1 ), chloride (250 mg L−1 ), nitrates (50 mg L−1 ), nitrites (0.5 mg L−1 ), fluoride (1.5 mg L−1 ) and sulphate (250 mg L−1 ), which are the indicative values proposed by the 98/83/EC directive, did not interfere with the determination of water hardness. Furthermore, organic matter in the form of humic acids (10 mg L−1 ) did not exhibit any adverse effects on the analytical response of HALC. 4. Method application 4.1. Qualitative analysis As previously discussed the reaction of sodium dodecanoate with alkaline earth metals yields a coacervate phase depending on the total water hardness which is further determined by the relative abundance of Ca2+ and Mg2+ . In most natural waters where Mg2+ is less or equal to Ca2+ the coacervate phase yields a response similar to that of Fig. 7. As we can observe, although the visual
2054
G.Z. Tsogas et al. / Talanta 80 (2010) 2049–2056
Fig. 7. A photograph of real samples taken after the addition of sodium dodecanoate in the presence of various water hardness content. (A) Very soft water at 35 mg L−1 CaCO3 , (B) soft water at 105 mg L−1 CaCO3 , (C) moderately hard water at 265 mg L−1 CaCO3 , (D) hard water at 425 mg L−1 CaCO3 and (E) very hard water at 600 mg L−1 CaCO3 . Experimental conditions as defined in the text.
differentiation between soft to moderately hard and hard to very hard water requires some degree of familiarization, the classification to soft, moderately hard and hard water can be accomplished by direct visual inspection even without any previous experience in the method. Therefore, the method is directly applicable to the qualitative assay of water hardness by visual inspection which is not offered by any of the usual methods for the determination of water hardness.
4.2. Semi-quantitative analysis The method was then applied to the classification of water hardness based on the concentration ranges assigned to each type of water adopted in this work (very soft to very hard). The data gathered in Table 2 show that method application successfully assigned almost all samples to the correct concentration range thus providing a good classification of water samples based on the level
Table 2 Determination of water hardness in real samples and comparison with standard methods. Sample
Total hardness HALCa
Total hardnessa , b
Classificationc
Relative error (%)
Bottled waters 1 2 3 4 5 6 7 8 9
218 158 285 15 161 222 105 232 218
183 168 253 17 157 241 149 246 233
Failed Success Success Success Success Success Success Success Success
19.0 −6.0 12.5 −9.5 2.4 −7.8 −29.8 −5.6 −6.3
Average relative deviation (ARD)d
0.11% Tap water
122
134
Success
−8.6 –
Acherontas river Site 1 Site 2 Site 3 Site 4 Site 5 Site 6
103 105 109 255 245 247
81 96 93 237 230 233
Success Success Success Success Success Success
26.3 8.9 17.9 7.7 6.3 6.3 0.12%
Natural water springs Aulotopos Ligoneri Sikia Milos Mboustere Mpousmpou Kedros Frosini Goura
80 88 201 197 76 159 148 247 114
103 105 182 179 96 184 150 222 127
Success Success Success Success Success Success Success Success Success
−22.3 −16.6 10.4 9.9 −20.5 −13.4 −1.1 11.5 −10.3 0.13%
Expressed in mg L−1 CaCO3 . b Based on reported values or measured from the EDTA titration method. c Very soft (0–70 mg L−1 CaCO3 ), soft (70–140 mg L−1 CaCO3 ), slightly hard (140–210 mg L−1 CaCO3 ), moderately hard (210–320 mg L−1 CaCO3 ), hard (320–530 mg L−1 CaCO3 ) and very hard (>530 mg L−1 CaCO3 ). a
d
ARD = (1/n)
n
((XiSTD − XiHALC )/XiSTD ) where n is the number of experimental data points, XiSTD are the measured values of total hardness using a standard method and
i=1
XiHALC are the measured values from the proposed (HALC) method.
G.Z. Tsogas et al. / Talanta 80 (2010) 2049–2056
2055
of total hardness. This is advantageous when a fast estimation of water hardness is of concern, thus minimizing experimental effort. 4.3. Quantitative analysis The ability of the method to provide an accurate measure of total water hardness is also shown in Table 2. The average relative error ranges, in terms of absolute values, from 0.87 to 29.7% with an average value of 10.7 ± 7.9 mg L−1 CaCO3 and a standard error of 1.58 mg L−1 CaCO3 . The distribution of relative experimental error shows that the method affords a satisfactory assessment of water hardness. The relative average deviation (ARD) of all samples, which is a measure of precision, was 0.11 ± 0.08% to 0.13 ± 0.1% which is very satisfactory. It is also worth mentioning that the lowest error was observed in bottled waters where the concentrations of both Ca2+ and Mg2+ were known thus enabling an accurate calculation of total hardness as CaCO3 equivalents, using the equation: [CaCO3 ] = 2.497 × [Ca2+ ] + 4.117 × [Mg2+ ]
(2)
which derives from the ratio of the molar masses of CaCO3 , Ca2+ and Mg2+ . On the other hand, all other samples were analyzed with the EDTA titration method with visual inspection of the end point of the EDTA titration procedure. 4.4. Analysis of salt containing samples The analysis of real samples containing inorganic salts was further considered. It is well known that inorganic electrolytes decrease the electrostatic repulsion among adjacent sodium dodecanoate molecules leading to the formation of planar or bent structures (Fig. 1a) while they compete Ca2+ and Mg2+ for the available reactive sites [9,17]. Therefore the applicability of the method in the presence of NaCl, which is the most common inorganic salt in the environment, was investigated. Considering the competition of Na+ counter-ions for Ca2+ and Mg2+ the influence of increasing NaCl was investigated at concentrations from 0 to 0.75 M which is higher than the usual value of seawater (35‰) by direct spiking in salt-free water samples (bottled waters) with variable total hardness content. However, values above 0.5 M could not be employed because the coacervate phase became insoluble forming a thick membrane that remained on the top of the vial especially with increasing total hardness. The analysis of salt containing samples revealed that the absorbance signal soars up to 0.1 M NaCl irrespectively of water hardness, but above this value it descents, increases or reaches a plateau depending on total hardness (Fig. 8), which is attributed to the relative competition of Na+ with Ca2+ and Mg2+ for the reactive sites of sodium dodecanoate. From a practical standpoint, the presence of salt deteriorated the relative experimental error between EDTA titration and the proposed method providing no improvement in the measured concentrations. Furthermore, no specific relationship between salt concentration and water hardness could be established. Empirically, it was observed that the influence of
Fig. 8. Influence of ionic strength on the absorbance yield of HALC in variable water hardness.
ionic strength is related to the relative abundance of individual alkaline earth metals to that of salt counter-ion but the wide distribution of the experimental measurements did not enable the provision of a mathematical expression that could account for these phenomena. Since no clear relationship between ionic strength and water hardness was found, it was decided to investigate the influence of NaCl on the analytical response of HALC method empirically by employing real rather than artificial samples. A series of four NaCl-containing samples from different drillings located along the coastal line of north Corfu (<2 km from sea) and two (2) seawater samples from the Thesprotia region were analyzed for that purpose applying various dilution factors (Table 3). The optimum results were obtained at 2-fold dilution of underground water samples and 50-fold for seawater while for higher dilution the relative error deteriorated significantly and was attributed to the high Mg2+ content of seawater, compared to Ca2+ [21]. Therefore, although the proposed method may provide a fair approximation of water hardness to salt containing samples after appropriate dilution, its application is not a straightforward task and requires the experimental validation of the results before a secure conclusion can be reached. In this sense, samples that contain appreciable amounts of salts should be analyzed in parallel with another method in order to decide on the best dilution factor which enables HALC method to provide an accurate estimate of total hardness. In the same direction, Mg2+ -rich waters could be treated in a similar manner in order to reduce the influence of Mg2+ in the final measured signal. However, once this is done, the HALC method can be routinely applied to monitor water hardness in salt containing and/or Mg2+ -rich samples.
Table 3 Analysis of high ionic strength water samples (calculations and units as in Table 2). Sample
Total hardness HALC
Total hardness
Classification
Relative error (%)
Underground water Drilling 1 Drilling 2 Drilling 3 Drilling 4
417 438 984 1007
570 607 1024 1059
Failed Failed Success Success
26.9 27.9 3.9 4.9
Average relative deviation (ARD)
0.16 ± 0.1 Seawater Seawater Estuary water
6520 1255
6856 1311
Success Success
4.9 4.3 0.02 ± 0.01
2056
G.Z. Tsogas et al. / Talanta 80 (2010) 2049–2056
4.5. Speciation of water hardness Despite the non-selective determination of Ca2+ and Mg2+ ions afforded by HALC, further development towards the individual determination of water hardness components, which may is of interest especially in biological samples, can be pursued. As previously discussed, both Ca2+ and Mg2+ yield a linear response with increasing concentration suggesting that the method can be conveniently applied to their separate determination. Since most samples of environmental, biological and food origin contain both species, the selective removal or determination of one species must precede the speciation protocol, the other being determined as the difference between total hardness and known ion concentration according to Eq. (2). Ion-selective electrodes or flame atomic absorption spectrometry could be employed towards this direction [4,5,7,22]. 5. Conclusions A rapid and facile method abbreviated as HALC (hardness by alkaline earth metal coacervation) is described in this study for the determination of water hardness exploiting the phenomenon of coacervation of alkaline earth metals in aqueous solution upon their reaction with an anionic surfactant (sodium dodecanoate). Investigation of the experimental conditions that afford the optimum analytical performance was elucidated suggesting that the method is straightforwardly applicable by measuring the absorbance of the solutions at 350 nm, without the need for any adjustment of the usual analytical parameters like pH, ionic strength, temperature and incubation time. Due to the different response of Mg2+ and Ca2+ in the final absorbance signal, a non-linear calibration curve based on the logarithmic expression of absorbance vs. concentration was constructed and a concentration correction factor was established. The method was successfully applied to various natural water samples yielding a fair estimation of water hardness in terms of qualitative, semi-quantitative and quantitative analysis. The interference from salts (above 0.1 M) or too high
Mg2+ concentrations can easily be overcome by simple dilution, enabling method application in a wide variety of samples, including seawater. References [1] J.H. Yim, K.W. Kim, S.D. Kim, J. Hazard. Mater. 138 (2006) 16. [2] J. Cotruvo, J. Bartram (Eds.), Calcium and Magnesium in Drinking-water: Public Health significance, World Health Organization, Geneva, 2009. [3] A.D. Eaton, L.A. Clesceri, A.E. Greenberg (Eds.), APHA-AWWA-WEF, Standard Methods for the Examination of Water and Wastewater, 19th ed., APHA, Washington, 1995. [4] Official Methods of Analysis, vol. 2, 16th ed., Association of Official Analytical Chemists, Arlington, 1995. [5] J. Saurina, E. López-Aviles, A. Le Moal, S. Hernández-Cassou, Anal. Chim. Acta 464 (2002) 89. [6] V.K. Gupta, S. Chandra, R. Mangla, Sens. Actuators B 86 (2002) 235. [7] P. Kane, D. Diamond, Talanta 44 (1997) 1847. [8] IUPAC Compendium of Chemical Terminology 31 (1972) 611. [9] G.Z. Tsogas, D.L. Giokas, E.K. Paleologos, A.G. Vlessidis, N.P. Evmiridis, Anal. Chim. Acta 537 (2004) 239. [10] G.Z. Tsogas, D.L. Giokas, A.G. Vlessidis, N.P. Evmiridis, Spectrochim. Acta, Part B 59 (2004) 957. [11] F.J. Ruiz, S. Rubio, D. Perez-Bendito, Anal. Chem. 78 (2006) 7229. [12] F.J. Ruiz, S. Rubio, D. Perez-Bendito, Anal. Chem. 79 (2007) 7473. [13] N.I. Kapakoglou, D.L. Giokas, G.Z. Tsogas, A.G. Vlessidis, Anal. Chem. 80 (2008) 9787. [14] A. Zapf, R. Beck, G. Platz, H. Hoffmann, Adv. Colloid Interface Sci. 100–102 (2003) 349. [15] D. Grabner, T. Matsuo, E. Hoinkis, C. Thunig, H.J. Hoffmann, Colloid Interface Sci. 236 (2001) 1. [16] E.K. Paleologos, A.G. Vlessidis, M.I. Karayannis, N.P. Evmiridis, Anal. Chim. Acta 477 (2003) 223. [17] D.L. Giokas, G.Z. Tsogas, A.G. Vlessidis, M.I. Karayannis, Anal. Chem. 76 (2004) 1302. [18] M. Mao, J. Huang, B. Zhu, H. Yin, H. Fu, Langmuir 18 (2002) 3380. [19] J.O. Westgard, Basic Method Validation, third ed., WESTGARD QC Publ, USA, 2008, p. 320. [20] Commission decision laying down performance criteria for the analytical methods to be used for certain substances and residues there of in live animals and animal products according to Council Directive 96/23/EC, Draft SANCO/1805/2000, 2000, Rev. 1. [21] M. Sadiq, Toxic Metal Chemistry in Marine Environments, vol. 1, CRC Press, 1992. [22] D.L. Giokas, E.K. Paleologos, P.G. Veltsistas, M.I. Karayannis, Talanta 56 (2002) 415.
Contents lists available at ScienceDirect
Talanta journal homepage: www.elsevier.com/locate/talanta
A fast assay of water hardness ions based on alkaline earth metal induced coacervation (HALC) George Z. Tsogas, Dimosthenis L. Giokas, Athanasios G. Vlessidis ∗ Laboratory of Analytical Chemistry, Department of Chemistry, University of Ioannina, 45110, Ioannina, Greece
a r t i c l e
i n f o
Article history: Received 12 August 2009 Received in revised form 23 October 2009 Accepted 3 November 2009 Available online 10 November 2009 Keywords: Alkaline earth metals Coacervation Spectrophotometry Water hardness
a b s t r a c t In this work, a rapid assay of water hardness is presented based on the formation of a coacervate phase made up of multilamellar vesicles and close bilayers produced upon the reaction of alkaline earth metals with a carboxylate anionic surfactant in the presence of a co-surfactant (methanol). The procedure exploits the light scattering abilities of the coacervate phase which can be logarithmically linked to total hardness as CaCO3 equivalents via spectrophotometric detection at 350 nm. The method, abbreviated as HALC, stemming from hardness by alkaline earth metal coacervation, is straightforwardly applicable overcoming the requirement for regulation of the experimental parameters involved in the determination procedure. In total, 28 water samples with various matrix compositions and hardness contents were analyzed with satisfactory accuracy as evidenced by comparison of the results with EDTA complexometric titration. The method is free from interferences from environmentally significant metal cations and inorganic anions affording detection limits as low as 13.5 mg L−1 CaCO3 with the aid of a correction factor, which is determined by the non-linear absorbance properties of the solution mixture, and satisfactory reproducibility (RSD = 4.21–8.08%). © 2009 Elsevier B.V. All rights reserved.
1. Introduction Real time monitoring of water quality parameters is of critical importance in environmental and industrial fields, including the water quality control in drinking water treatment plants. Among these parameters, total hardness, defined as the total content of calcium (Ca2+ ) and magnesium (Mg2+ ) in water, is one of the most popular indicators of water quality since it is not only related to the deposition of mineral deposits in water pipes but it is also reported to affect the toxicity of certain metal ions in aquatic ecosystems [1]. Furthermore, water hardness contributes to the daily uptake of Ca2+ and Mg2+ which are physiologically important elements for the human body with significant negative effects encountered in cases of deficiency [2]. The most popular methods for the determination of water harness are those based on complexometric titration [3], flame atomic absorption spectrometry [4] and ion-selective electrodes [5,6]. Complexometric methods are probably the most popular methods due to their simplicity but they are prone to the analysts’ efficiency unless an automated system is used that can accurately determine the end point of the titration. On the other hand, flame AAS methods are highly accurate for benchtop analysis but they
∗ Corresponding author. Tel.: +30 26510 08401; fax: +30 26510 08781. E-mail address: [email protected] (A.G. Vlessidis). 0039-9140/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.talanta.2009.11.003
cannot be used for outdoor studies. Potentiometric ion-selective electrodes (ISEs) are the technique of choice when continuous in situ measurements in an automatic manner are required combining simplicity of operation and low analysis cost. However, the selectivity of measurements is not often sufficient to enable the primary ion to be directly determined; interferences arising from other species with similar size and charge, matrix effects and mediocre day-to-day reproducibility may have serious repercussions in the accuracy of the analysis [5]. To overcome these problems, several techniques have been employed based on physical separation of sample components (i.e., chromatography), modeling the potentiometric response of the analytes in the presence of interferences (Nikolskii–Eisenman equation) or chemometric procedures [5,7]. In the pursue of combining the advantages of the above methods in the determination of water hardness, the phenomenon of coacervation may offer a convenient alternative. By definition, coacervation is the separation of colloidal systems into two liquid phases, a colloid-rich and a colloid-poor phase [8]. In that sense, coacervates can be defined as colloid-rich liquids immiscible with the colloid-poor aqueous equilibrium solution from which it is separated. In this context, if appropriate interactions between the coacervate phase and the analytes of interest can be established, coacervation can be efficiently employed as an alternative to the solvent-free extractions. Up to date, several such systems have appeared in the literature enabling the determination of organic
2050
G.Z. Tsogas et al. / Talanta 80 (2010) 2049–2056
and inorganic analytes via hydrophobic, electrostatic and hybrid interactions [9–13]. The coacervation of organic macromolecules induced by metallic ions is a well-known phenomenon and several studies have been devoted to the characterization of the morphological changes induced upon their interaction [14]. In this direction, alkaline earth metals, and especially Ca2+ , have received considerable attention due to their high reactivity with anionic surfactants leading either to the formation of well-known lime soaps or to complex vesicle and lamellar phases with interesting properties [14,15]. Nevertheless, the analytical utility of these interactions towards the determination of alkaline earth metals has not been reported despite the fact that vesicle-based extractions of metal ions exploiting their attraction with the anionic head group of anionic surfactants have been described [9,16,17]. Considering the high reactivity of alkaline earth metals with anionic surfactants, this work describes the first analytical application of the phenomenon of coacervation for the determination of water hardness. The proposed method is based on the formation of a lamellar colloid-rich phase induced by alkaline earth metals with sodium dodecanoate in the presence of a co-surfactant (water miscible organic solvent). This reaction results in the formation of a dispersed vesicular phase which scatters visible light in manner related to metal ion concentration. Based on this principle, the absorbance of the coacervate phase formed by Ca2+ and Mg2+ with sodium dodecanoate was monitored spectrophotometrically yielding a concentration depended response to metal ion concentration. To the best of our knowledge, this is the first time that metal ion induced coacervation is being exploited directly towards the determination of water hardness whish is one of the most popular water quality parameters. 2. Experimental section 2.1. Reagents All reagents were of analytical-grade and they were used without further purification. Standard metal solutions used for the experiments were prepared by sequential dilution of 1000 mg L−1 stock solutions prepared by dissolving appropriate amounts of Ca(NO3 )2 (Merck, Darmstadt, Germany) and Mg(NO3 )2 ·6H2 O (Fluka, Buchs, Switzerland). Metal standard solutions used for the interference study were prepared by sequential dilution of 1000 mg L−1 of Titrisol stock solutions (Merck). Lauric acid in the form of Sodium dodecanoate (Sigma grade, 99–100%, Sigma–Aldrich, Greece) was dissolved in HPLC grade methanol (Labscan, Dublin, Ireland.) to prepare a 5% (w/v) stock solution. 2.2. Instrumentation Absorbance measurements were performed with matched quartz cells of 1-cm path length in a Jenway 6405 UV–vis spectrophotometer. A pH meter, WTW 552 model glass electrode was employed for the pH measurements for the solutions. 2.3. HALC analytical procedure In 10 mL of water sample previously equilibrated at room temperature, 1 mL of Lauric acid solution (5% (w/v) in methanol) is added. The mixture is agitated and its absorbance is directly recorded at 350 nm against reagents blank.
chased from local stores, surface waters (6 samples) collected from Acherontas river (N.W. Greece) in various locations from springs to estuary and water samples from six (6) springs used for water supply in Thesprotia region were collected in HNO3 -cleaned glass containers, filtered through a Whatman No. 40 filter (0.45 m) and stored at room temperature. Tap water was collected directly form the drinking water network supply, filtered and analyzed directly without storage. A series of samples from underground water aquifers were collected along the coastal line (≤2 km from sea) of north Corfu Island (N.W. Greece) from four (4) different drillings. Infiltration of seawater was qualitatively determined by the addition of AgNO3 leading to the formation of white AgCl precipitate. Two (2) seawater samples were collected from the coastal line of Thesprotia (N.W. Greece). The first sample was collected in the estuaries of Kalamas River and the other by direct sampling of seawater 10 m from the coast. 3. Results and discussion 3.1. Optimization of experimental conditions The optimization of the experimental conditions that enable the determination of water hardness with the proposed coacervation scheme is not a straightforward objective. That is because Ca2+ and Mg2+ upon reaction with sodium dodecanoate, in the presence of methanol as a co-surfactant (which affects aggregates size) [18], yield different lamellar structures [17]. According to the SEM micrographs of Fig. 1, Ca2+ favours the formation of spongelike phases (Fig. 1b) and Mg2+ the formation of bilayer structures (Fig. 1c) while mixtures of alkaline earth metals lead to the formation of a complex network of organized surfactant assemblies containing both multilamellar and bilayer vesicular aggregates (Fig. 1d). From a technical standpoint, these phenomena can be explained taking into consideration the organization phenomena occurring in the oriented molecular layers and the different radii of the hydrated cation condensation [9,17]. However, in the context of the analytical application intended in this study, these structural diversifications of the coacervate phase bring about significant changes in the absorption of each solution. According to the results of the calibration curves of Fig. 2, Ca2+ yields a linear response up to a concentration of 100 mg L−1 while Mg2+ response is rectilinear only up to 15 mg L−1 , above which absorbance values gradually reaches a plateau. In the linear range of the calibration curves, the ratio of slopes (Mg2+ /Ca2+ ) is approx. 8 which means that Mg2+ induces, on average, 8 times more intense absorption than Ca2+ at the same concentration level at 350 nm. Beyond ion type and concentration, the measured signal strongly depends on other parameters related to the formation and stability of the vesicular (coacervate) phase, like sodium dodecanoate concentration, ionic strength, hydroxyl or hydrogen concentration and temperature [9,10,17]. To this end, we decided to prepare a set of model solutions containing a high variety of Ca2+ and Mg2+ concentrations and use them as workings solutions for the optimization of the working conditions. The composition of these samples, gathered in Table 1, covers a wide range of Ca2+ and Mg2+ concentrations (as well as their ratio) so as to provide a representative evaluation of most natural waters. All samples had a total volume of 10 mL and measurements were performed at 350 nm. 3.2. Effect of sodium dodecanoate concentration
2.4. Real samples In total, 28 water samples of various matrix composition and hardness content were assessed. Bottled waters (9 samples) pur-
The first parameter assessed was sodium dodecanoate concentration in the range of 0.1–2% (w/v). Aqueous solutions (10 mL) were spiked with 0.1–2 mL of 5% (w/v) sodium dodecanoate and the
G.Z. Tsogas et al. / Talanta 80 (2010) 2049–2056
2051
Fig. 1. SEM micrographs of the coacervate phase formed by sodium dodecanoate, methanol and various cations. (a) Na+ (50 m at 10 kV), (b) Ca2+ (5 m at 20 kV), (c) Mg2+ (5 m at 20 kV) and (d) Ca2+ and Mg2+ (1 m at 20 kV).
absorbance was immediately recorded at 350 nm. According to the data presented in Fig. 3, 0.5% (w/v) sodium dodecanoate provides the optimum absorption for a wide variety of water hardness ranging from soft to very hard water. For lower sodium dodecanoate concentrations, the vesicle’s charge was probably counterbalanced due to inadequacy of reactive sites and in combination with the respective low amounts of co-surfactant content did not favour the formation of a lamellar phase. On the other hand, with increasing sodium dodecanoate and co-surfactant concentrations, the precipitate swelled, increasing the absorbance of the solution, either due to transition from vesicles to stacked membranes [17], or due to the increase of surfactant concentration above cmc which complicates solubilization and dispersion of the alkaline earth metal coacer-
Fig. 2. Absorbance vs. concentration plots for Ca2+ and Mg2+ .
vates due to the presence of micellar aggregates. In either case, the analytical signal is deteriorated either due to low UV absorption or due to intense turbidity that reduces the working and measuring range of the method. Therefore, the concentration of 0.5% (w/v) was selected for the remaining work. It is worth mentioning that at the optimum sodium dodecanoate concentration, sample volume did not have any significant effect on the absorbance of the solutions (±6%). However, to minimize uncertainty it is advisable to maintain a constant sample volume throughout method application.
Table 1 Composition of model solutions used for the optimization study. Sample
Ca2+
Mg2+
Total hardness (mg L−1 CaCO3 )
Absorbance (A.U.) at = 350 nm
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
5 10 5 10 20 40 60 5 10 20 40 60 5 10 20 40 60 5 10 20 40 60
0 0 5 5 5 5 5 10 10 10 10 10 15 15 15 15 15 20 20 20 20 20
12.5 25.0 33.1 45.6 70.5 120.5 170.4 53.7 66.1 91.1 141.1 191.0 74.2 86.7 111.7 161.6 211.6 94.8 107.3 132.3 182.2 232.2
0.075 0.215 0.619 0.762 1.141 1.402 1.729 1.043 1.284 1.511 1.660 2.168 1.649 1.740 1.820 2.085 2.340 1.909 2.035 2.136 2.347 2.503
2052
G.Z. Tsogas et al. / Talanta 80 (2010) 2049–2056
Fig. 3. Effect of sodium dodecanoate concentration in the absorbance of aqueous solutions containing various levels of water hardness.
3.3. Reaction kinetics The incubation time necessary for sodium dodecanoate to react with alkaline earth metals towards the formation of a lamellar structure was the next parameter assessed for its influence on the analytical signal in terms of intensity, measuring range and reproducibility. Absorbance values recorded over the range of 0–240 min showed an increasing absorbance pattern with reaction time. From the data of Fig. 4 it can be inferred that absorbance increases with increasing water hardness gradually limiting the applicability of the method in hard waters since the signal eventually soared up to “out-of-range” absorbance values (not shown). Furthermore, the reproducibility of the measurements was deteriorated probably because vesicle membranes were not homogeneously dispersed in the vial which increases uncertainty. On the other hand, samples analyzed immediately after sodium dodecanoate addition gave the best reproducibility. Based on this observation, samples were measured after addition of sodium dodecanoate followed by vortex agitation to facilitate the reaction of sodium dodecanoate
Fig. 5. Absorbance signal as a function of temperature in soft water.
with alkaline earth metals and improve dispersion of the lamellar phase. 3.4. Effect of temperature Temperature was the next parameter assessed for its effect on the analytical response (Fig. 5). Cold conditions (<10 ◦ C) induced significant signal amplification due to the formation of a viscoelastic phase while warm conditions reduced the absorbance due to increased solubility of the lamellar phase and dehydration of the amphiphile head group inducing thermo-sensitive transformations [16,17]. A stable and reproducible response was obtained between 20 and 30 ◦ C, therefore room temperature (approx. 25 ◦ C) was selected for the analysis of real samples. 3.5. Effect of solution pH The influence of hydrogen or hydroxyl concentration present in the sample was assayed by addition of increasing amounts of dilute HCl or NaOH. At acidic pH values sodium dodecanoate is protonated which increases competition with alkaline earth metals while the opposite is observed in alkaline media where the anionic group of sodium dodecanoate is deprotonated which aids complexation with metal species. In either case, the concentration of Ca2+ and Mg2+ in the sample was found to be an important factor in the absorbance of the solution as a function of pH. At acidic media higher Ca2+ and Mg2+ concentrations were necessary to compete for the reactive sites of sodium dodecanoate while in alkaline conditions complexation was favoured with increasing pH and water hardness. In this context, a priori knowledge of total hardness is necessary in order to decide on the appropriate regulation of the solution pH which has no practical meaning since the determination of hardness is of concern. Furthermore, the pH of the solution upon addition of sodium dodecanoate was close to 6.5 which is not far from the usual pH of most natural waters and far less than the solubility limit of both Ca2+ and Mg2+ in aqueous solutions. Therefore it was decided to apply the method in an unregulated pH environment. 3.6. Calibration procedure
Fig. 4. Effect of incubation time on the absorbance of sodium dodecanoate-alkaline earth cation coacervates.
The calibration procedure was performed with a series of model solutions (10 mL sample volume) containing various amounts of water hardness and Ca2+ /Mg2+ ratios, by extending the
G.Z. Tsogas et al. / Talanta 80 (2010) 2049–2056
2053
The following classification was used according to the level of total hardness: very soft (0–70 mg L−1 CaCO3 ), soft (70–140 mg L−1 CaCO3 ), slightly hard (140–210 mg L−1 CaCO3 ), moderately hard (210–320 mg L−1 CaCO3 ), hard (320–530 mg L−1 CaCO3 ) and very hard (>530 mg L−1 CaCO3 ). 3.7. Analytical characteristics of the method
Fig. 6. Logarithmic calibration curve at 350 nm.
experimental data of Table 1, in an unregulated ionic strength environment to simulate the composition of natural waters. The concentration vs. absorbance line plotted in Fig. 6 shows that the absorbance is linearly depended on water hardness up to 60 mg L−1 (as CaCO3 ) but above this value a logarithmic relationship is blatantly observed. This is reasonable since Mg2+ -sodium dodecanoate coacervates scavenge the light more intensely than the respective Ca2+ -sodium dodecanoate coacervates. As a result, the same absorbance value can be produced by at two combinations, the first involving high Ca2+ levels and the second low Mg2+ levels. From the concentration curves of Fig. 2 it is inferred that an absorbance value of 1 (a.u.) can be obtained by either 56.7 mg L−1 Ca2+ or 5 mg L−1 Mg2+ . In order to account for all occasions (samples with different water hardness giving similar absorbance values analyzed in the same batch) the overall fit to the experimental data was logarithmically expressed as a function of total water hardness. The calibration data were fitted to the expression A = 0.9347 × Ln(total hardness) − 2.6626 r = 0.9765
(1)
which was considered satisfactory considering the large number of data points and the high variability in the concentration of the individual data points included in the calibration curve. This equation was therefore applied to the analysis of real samples and the results were evaluated from a qualitative, semi-quantitative and quantitative perspective. It is conceivable that appropriate dilution can bring the sample within the linear range of the calibration curve, provided that an a priori assessment of total hardness has been undertaken. Starting already from the first attempts to apply HALC method to the analysis of real samples it was revealed that a correction factor (Cf ) to the calculated concentration was necessary. This came as no surprise considering the aforementioned discussion regarding the influence of Mg2+ in comparison to Ca2+ . Despite the fact that in most real samples situations were Ca2+ is totally absent or Mg2+ significantly surpasses Ca2+ concentration are rather scarce, we persisted on accounting for all cases, irrespectively of their scarcity and appearance rate in the natural environment. The calculated values of total hardness were multiplied by a factor of 1.80 ± 0.41 when the absorbance lied within the range 0.31 < Abs350 < 2 and by a factor of 0.70 ± 0.13 when 2 < Abs350 < 0.31. By using this approach the total hardness of real samples was calculated and the results are presented below in more detail. In is conceivable that if another calibration curve equation is used then a re-examination of the correction factors should be applied although no significant differences were observed in model solutions examined for that purpose.
Due to the lack of linearity of the calibration curve the determination of method detection limit and the quantitation limit as multiples of the signal-to-noise ratio cannot be calculated because of the non-linear properties of the calibration curve which results in variable slope and, as previously discussed, similar responses to more than one combination of Ca2+ and Mg2+ levels. Therefore, the lower limit of detection (LLD), which is the lowest amount or concentration of analyte which can be distinguished from the blank with a reasonable confidence, was determined by the mean absorbance of the blank sample plus k times the standard deviation of the absorbance obtained on the blank sample, i.e., ALLD = Amean blk + k × ASDblk , where k is a constant related to the statistical level of confidence [19]. This approach is similar to that proposed in DG SANCO decision were the detection capability (CCa ) is defined “the lowest concentration level at which a method can discriminate with a statistical certainty of 1 – ˛ whether the identified analyte is present” [20]. For k = 3, the LLD is 19.4 mg L−1 CaCO3 and corresponds to the uncorrected concentration determined directly by equation (1). Since the absorbance at this concentration is below 0.31, multiplication by the respective Cf = 0.70, results in a final detection limit of 13.5 mg L−1 CaCO3 . The reproducibility of the method, expressed by the relative standard deviation (RSD) of repeated assays was 4.21–8.08% (n = 7) from very soft to very hard waters, which satisfies the requirements for the application intended in this study. 3.8. Interferences In view of the low selectivity of the spectrophotometric detector and the competitive reaction ambience where other metallic ions may react with sodium dodecanoate, various metallic cations and inorganic anions were investigated for their effect on the absorbance signal. The tolerance limit of a foreign species was defined as the concentration that causes a relative error greater than ±5%. Trace elements like Pb+2 , Cd+2 , Sb+3 , As+3 , Co2+ , Ni+2 and Cr3+ had no effect on the analytical response at concentrations up to 50 g L−1 , which is well above the usual concentration levels. Important elements like Fe3+ , Al3+ and Zn2+ had no effect up to the maximum concentration levels examined (500 g L−1 ), while Cu2+ could be tolerated up to 200 g L−1 . Regarding the interference by anions, phosphates (25 mg L−1 ), chloride (250 mg L−1 ), nitrates (50 mg L−1 ), nitrites (0.5 mg L−1 ), fluoride (1.5 mg L−1 ) and sulphate (250 mg L−1 ), which are the indicative values proposed by the 98/83/EC directive, did not interfere with the determination of water hardness. Furthermore, organic matter in the form of humic acids (10 mg L−1 ) did not exhibit any adverse effects on the analytical response of HALC. 4. Method application 4.1. Qualitative analysis As previously discussed the reaction of sodium dodecanoate with alkaline earth metals yields a coacervate phase depending on the total water hardness which is further determined by the relative abundance of Ca2+ and Mg2+ . In most natural waters where Mg2+ is less or equal to Ca2+ the coacervate phase yields a response similar to that of Fig. 7. As we can observe, although the visual
2054
G.Z. Tsogas et al. / Talanta 80 (2010) 2049–2056
Fig. 7. A photograph of real samples taken after the addition of sodium dodecanoate in the presence of various water hardness content. (A) Very soft water at 35 mg L−1 CaCO3 , (B) soft water at 105 mg L−1 CaCO3 , (C) moderately hard water at 265 mg L−1 CaCO3 , (D) hard water at 425 mg L−1 CaCO3 and (E) very hard water at 600 mg L−1 CaCO3 . Experimental conditions as defined in the text.
differentiation between soft to moderately hard and hard to very hard water requires some degree of familiarization, the classification to soft, moderately hard and hard water can be accomplished by direct visual inspection even without any previous experience in the method. Therefore, the method is directly applicable to the qualitative assay of water hardness by visual inspection which is not offered by any of the usual methods for the determination of water hardness.
4.2. Semi-quantitative analysis The method was then applied to the classification of water hardness based on the concentration ranges assigned to each type of water adopted in this work (very soft to very hard). The data gathered in Table 2 show that method application successfully assigned almost all samples to the correct concentration range thus providing a good classification of water samples based on the level
Table 2 Determination of water hardness in real samples and comparison with standard methods. Sample
Total hardness HALCa
Total hardnessa , b
Classificationc
Relative error (%)
Bottled waters 1 2 3 4 5 6 7 8 9
218 158 285 15 161 222 105 232 218
183 168 253 17 157 241 149 246 233
Failed Success Success Success Success Success Success Success Success
19.0 −6.0 12.5 −9.5 2.4 −7.8 −29.8 −5.6 −6.3
Average relative deviation (ARD)d
0.11% Tap water
122
134
Success
−8.6 –
Acherontas river Site 1 Site 2 Site 3 Site 4 Site 5 Site 6
103 105 109 255 245 247
81 96 93 237 230 233
Success Success Success Success Success Success
26.3 8.9 17.9 7.7 6.3 6.3 0.12%
Natural water springs Aulotopos Ligoneri Sikia Milos Mboustere Mpousmpou Kedros Frosini Goura
80 88 201 197 76 159 148 247 114
103 105 182 179 96 184 150 222 127
Success Success Success Success Success Success Success Success Success
−22.3 −16.6 10.4 9.9 −20.5 −13.4 −1.1 11.5 −10.3 0.13%
Expressed in mg L−1 CaCO3 . b Based on reported values or measured from the EDTA titration method. c Very soft (0–70 mg L−1 CaCO3 ), soft (70–140 mg L−1 CaCO3 ), slightly hard (140–210 mg L−1 CaCO3 ), moderately hard (210–320 mg L−1 CaCO3 ), hard (320–530 mg L−1 CaCO3 ) and very hard (>530 mg L−1 CaCO3 ). a
d
ARD = (1/n)
n
((XiSTD − XiHALC )/XiSTD ) where n is the number of experimental data points, XiSTD are the measured values of total hardness using a standard method and
i=1
XiHALC are the measured values from the proposed (HALC) method.
G.Z. Tsogas et al. / Talanta 80 (2010) 2049–2056
2055
of total hardness. This is advantageous when a fast estimation of water hardness is of concern, thus minimizing experimental effort. 4.3. Quantitative analysis The ability of the method to provide an accurate measure of total water hardness is also shown in Table 2. The average relative error ranges, in terms of absolute values, from 0.87 to 29.7% with an average value of 10.7 ± 7.9 mg L−1 CaCO3 and a standard error of 1.58 mg L−1 CaCO3 . The distribution of relative experimental error shows that the method affords a satisfactory assessment of water hardness. The relative average deviation (ARD) of all samples, which is a measure of precision, was 0.11 ± 0.08% to 0.13 ± 0.1% which is very satisfactory. It is also worth mentioning that the lowest error was observed in bottled waters where the concentrations of both Ca2+ and Mg2+ were known thus enabling an accurate calculation of total hardness as CaCO3 equivalents, using the equation: [CaCO3 ] = 2.497 × [Ca2+ ] + 4.117 × [Mg2+ ]
(2)
which derives from the ratio of the molar masses of CaCO3 , Ca2+ and Mg2+ . On the other hand, all other samples were analyzed with the EDTA titration method with visual inspection of the end point of the EDTA titration procedure. 4.4. Analysis of salt containing samples The analysis of real samples containing inorganic salts was further considered. It is well known that inorganic electrolytes decrease the electrostatic repulsion among adjacent sodium dodecanoate molecules leading to the formation of planar or bent structures (Fig. 1a) while they compete Ca2+ and Mg2+ for the available reactive sites [9,17]. Therefore the applicability of the method in the presence of NaCl, which is the most common inorganic salt in the environment, was investigated. Considering the competition of Na+ counter-ions for Ca2+ and Mg2+ the influence of increasing NaCl was investigated at concentrations from 0 to 0.75 M which is higher than the usual value of seawater (35‰) by direct spiking in salt-free water samples (bottled waters) with variable total hardness content. However, values above 0.5 M could not be employed because the coacervate phase became insoluble forming a thick membrane that remained on the top of the vial especially with increasing total hardness. The analysis of salt containing samples revealed that the absorbance signal soars up to 0.1 M NaCl irrespectively of water hardness, but above this value it descents, increases or reaches a plateau depending on total hardness (Fig. 8), which is attributed to the relative competition of Na+ with Ca2+ and Mg2+ for the reactive sites of sodium dodecanoate. From a practical standpoint, the presence of salt deteriorated the relative experimental error between EDTA titration and the proposed method providing no improvement in the measured concentrations. Furthermore, no specific relationship between salt concentration and water hardness could be established. Empirically, it was observed that the influence of
Fig. 8. Influence of ionic strength on the absorbance yield of HALC in variable water hardness.
ionic strength is related to the relative abundance of individual alkaline earth metals to that of salt counter-ion but the wide distribution of the experimental measurements did not enable the provision of a mathematical expression that could account for these phenomena. Since no clear relationship between ionic strength and water hardness was found, it was decided to investigate the influence of NaCl on the analytical response of HALC method empirically by employing real rather than artificial samples. A series of four NaCl-containing samples from different drillings located along the coastal line of north Corfu (<2 km from sea) and two (2) seawater samples from the Thesprotia region were analyzed for that purpose applying various dilution factors (Table 3). The optimum results were obtained at 2-fold dilution of underground water samples and 50-fold for seawater while for higher dilution the relative error deteriorated significantly and was attributed to the high Mg2+ content of seawater, compared to Ca2+ [21]. Therefore, although the proposed method may provide a fair approximation of water hardness to salt containing samples after appropriate dilution, its application is not a straightforward task and requires the experimental validation of the results before a secure conclusion can be reached. In this sense, samples that contain appreciable amounts of salts should be analyzed in parallel with another method in order to decide on the best dilution factor which enables HALC method to provide an accurate estimate of total hardness. In the same direction, Mg2+ -rich waters could be treated in a similar manner in order to reduce the influence of Mg2+ in the final measured signal. However, once this is done, the HALC method can be routinely applied to monitor water hardness in salt containing and/or Mg2+ -rich samples.
Table 3 Analysis of high ionic strength water samples (calculations and units as in Table 2). Sample
Total hardness HALC
Total hardness
Classification
Relative error (%)
Underground water Drilling 1 Drilling 2 Drilling 3 Drilling 4
417 438 984 1007
570 607 1024 1059
Failed Failed Success Success
26.9 27.9 3.9 4.9
Average relative deviation (ARD)
0.16 ± 0.1 Seawater Seawater Estuary water
6520 1255
6856 1311
Success Success
4.9 4.3 0.02 ± 0.01
2056
G.Z. Tsogas et al. / Talanta 80 (2010) 2049–2056
4.5. Speciation of water hardness Despite the non-selective determination of Ca2+ and Mg2+ ions afforded by HALC, further development towards the individual determination of water hardness components, which may is of interest especially in biological samples, can be pursued. As previously discussed, both Ca2+ and Mg2+ yield a linear response with increasing concentration suggesting that the method can be conveniently applied to their separate determination. Since most samples of environmental, biological and food origin contain both species, the selective removal or determination of one species must precede the speciation protocol, the other being determined as the difference between total hardness and known ion concentration according to Eq. (2). Ion-selective electrodes or flame atomic absorption spectrometry could be employed towards this direction [4,5,7,22]. 5. Conclusions A rapid and facile method abbreviated as HALC (hardness by alkaline earth metal coacervation) is described in this study for the determination of water hardness exploiting the phenomenon of coacervation of alkaline earth metals in aqueous solution upon their reaction with an anionic surfactant (sodium dodecanoate). Investigation of the experimental conditions that afford the optimum analytical performance was elucidated suggesting that the method is straightforwardly applicable by measuring the absorbance of the solutions at 350 nm, without the need for any adjustment of the usual analytical parameters like pH, ionic strength, temperature and incubation time. Due to the different response of Mg2+ and Ca2+ in the final absorbance signal, a non-linear calibration curve based on the logarithmic expression of absorbance vs. concentration was constructed and a concentration correction factor was established. The method was successfully applied to various natural water samples yielding a fair estimation of water hardness in terms of qualitative, semi-quantitative and quantitative analysis. The interference from salts (above 0.1 M) or too high
Mg2+ concentrations can easily be overcome by simple dilution, enabling method application in a wide variety of samples, including seawater. References [1] J.H. Yim, K.W. Kim, S.D. Kim, J. Hazard. Mater. 138 (2006) 16. [2] J. Cotruvo, J. Bartram (Eds.), Calcium and Magnesium in Drinking-water: Public Health significance, World Health Organization, Geneva, 2009. [3] A.D. Eaton, L.A. Clesceri, A.E. Greenberg (Eds.), APHA-AWWA-WEF, Standard Methods for the Examination of Water and Wastewater, 19th ed., APHA, Washington, 1995. [4] Official Methods of Analysis, vol. 2, 16th ed., Association of Official Analytical Chemists, Arlington, 1995. [5] J. Saurina, E. López-Aviles, A. Le Moal, S. Hernández-Cassou, Anal. Chim. Acta 464 (2002) 89. [6] V.K. Gupta, S. Chandra, R. Mangla, Sens. Actuators B 86 (2002) 235. [7] P. Kane, D. Diamond, Talanta 44 (1997) 1847. [8] IUPAC Compendium of Chemical Terminology 31 (1972) 611. [9] G.Z. Tsogas, D.L. Giokas, E.K. Paleologos, A.G. Vlessidis, N.P. Evmiridis, Anal. Chim. Acta 537 (2004) 239. [10] G.Z. Tsogas, D.L. Giokas, A.G. Vlessidis, N.P. Evmiridis, Spectrochim. Acta, Part B 59 (2004) 957. [11] F.J. Ruiz, S. Rubio, D. Perez-Bendito, Anal. Chem. 78 (2006) 7229. [12] F.J. Ruiz, S. Rubio, D. Perez-Bendito, Anal. Chem. 79 (2007) 7473. [13] N.I. Kapakoglou, D.L. Giokas, G.Z. Tsogas, A.G. Vlessidis, Anal. Chem. 80 (2008) 9787. [14] A. Zapf, R. Beck, G. Platz, H. Hoffmann, Adv. Colloid Interface Sci. 100–102 (2003) 349. [15] D. Grabner, T. Matsuo, E. Hoinkis, C. Thunig, H.J. Hoffmann, Colloid Interface Sci. 236 (2001) 1. [16] E.K. Paleologos, A.G. Vlessidis, M.I. Karayannis, N.P. Evmiridis, Anal. Chim. Acta 477 (2003) 223. [17] D.L. Giokas, G.Z. Tsogas, A.G. Vlessidis, M.I. Karayannis, Anal. Chem. 76 (2004) 1302. [18] M. Mao, J. Huang, B. Zhu, H. Yin, H. Fu, Langmuir 18 (2002) 3380. [19] J.O. Westgard, Basic Method Validation, third ed., WESTGARD QC Publ, USA, 2008, p. 320. [20] Commission decision laying down performance criteria for the analytical methods to be used for certain substances and residues there of in live animals and animal products according to Council Directive 96/23/EC, Draft SANCO/1805/2000, 2000, Rev. 1. [21] M. Sadiq, Toxic Metal Chemistry in Marine Environments, vol. 1, CRC Press, 1992. [22] D.L. Giokas, E.K. Paleologos, P.G. Veltsistas, M.I. Karayannis, Talanta 56 (2002) 415.