Simultaneous spectrophotometric determination of calcium and magnesium in water

513

Analyrrca Chrmrca Acta, 249 (1991) 513-518 Elsevter Sctence Pubhshers B V.. Amsterdam

Simultaneous spectrophotometric determination and magnesium in water E. Gomez, Department

of Chemutry

J.M. Estela and V. Cerda

of calcium

*

Faculty of Scrences, Umverslty of the Baleanc Islands, 07071 Palma de Maliorca (Spain) (Recetved

12th December

1990)

Abstract

A method for the stmultaneous spectrophotometnc determmatton of calcntm and magnesmm based on the formation of thetr complexes wtth 4-(2-pyndylazo)resorcmol ts proposed The absorbance band yielded by the mixture was resolved by applying a computer multtlinear regresston program to the corrected, standardized spectrum of each metal ton as standard. The proposed method ts straightforward and rapid and provtdes a linear determmation range of 0.10-4.0 pg ml-’ for calcmm and 0 15-2 5 pg ml-’ for magnestum It was sattsfactorily applied to the analysis of various types of water Keyworcis

Spectrophotometry;

Calcium,

Magnesium;

Waters

The determination of the concentrations of calcium and magnesium ions, whether individually or as overall hardness, is one of the usual steps involved in water analysis. Routine laboratories commonly use volumetric methods [l] involving EDTA as complexant and murexide or Eriochrome Black T as indicator or atomic absorption spectrometric (AAS) methods for this purpose. The latter are less laborious as they permit direct determinations and are more sensitive, but they also more expensive. These two elements can also be determined by various photometric methods. Thus, Sweetser and Bricker [2] were the first to use spectrophotometric measurements to determine the end-points of EDTA titrations, which they applied to calcium and magnesium. Whereas all these methods determine the two elements in two stages, the flow-injection spectrophotometric method developed by Blanc0 et al. [3] allows their simultaneous determination in a single step over the ranges 0.2-1.5 and 0.1-1.0 pg ml-‘, respec0003-2670/91/$03

50

0 1991 - Elsevter

Science

Publishers

tively, by using Arsenazo III and a diode-array detector. 4-(2-Pyridylazo)resorcinol (PAR) is a chromogenic reagent widely used in photometric determinations for a number of metals, either directly [4] or indirectly in post-column reactions by various chromatographic techniques [5-91 because it forms water-soluble complexes with high molar absorptivities (ca. 104) compared with those of other reagents such as Arsenazo I or III. Argue110 and Fritz [lo] reported a method for the separation of Ca(I1) and Mg(I1) in hard water samples based on ion-exchange chromatography followed by spectrophotometric detection with a chromogenic reagent (Arsenazo I), which allowed concentrations as low as a few pg ml-’ to be determined. This paper reports a method for the simultaneous spectrophotometric determination of calcium and magnesium also based on the use of a diode-array detector. The concentrations of the B.V. All nghts

reserved

514

E Gi)MEZ

two metals were obtained from their respective PAR complexes by using the program MULTIC [ll], which resolved the peak absorbance of the unknown sample using the standardized spectrum of each element as standard. The proposed method has a detection limit of ca. 0.1 I-18ml-’ and a broad determination range, the upper limit of which is determined by the [PAR]/[ion] ratio, which should always be above 5. Practical application of the method requires only a diode-array spectrophotometer and the corresponding software, thereby dispensing with the complexity of the flow-injection design and the use of a dual injection valve.

EXPERIMENTAL

Reagents

All reagents were of analytical-reagent grade and solutions were prepared in distilled water purified using a Millipore Milli-Q system. A stock PAR solution (3 X lop3 M) was prepared by dissolving 0.0766 g of the monosodium salt in 100 ml of water. The solution was stored in a dark polyethylene bottle and was spectrophotometrically stable for at least 1 week. Tris(hydroxymethyl)aminomethane (Tris) buffer (0.5 M, pH 9.6, adjusted with hydrochloric acid) was prepared. A calcium standard solution (1000 pg ml-‘) was prepared by dissolving calcium carbonate in 1% nitric acid and a magnesium standard solution (1000 pg ml-‘) by dissolving MgCl, .6H,O water and standardizing with EDTA and Eriochrome Black T. Apparatus

and software

Spectra were recorded on a Hewlett-Packard Model 8452A diode-array spectrophotometer connected to an HP Vectra ES/12 personal computer and printer. The programs used to obtain and process the spectra were supplied by the spectrophotometer manufacturer as bundled software. Mixtures were resolved from the spectra of each component by using a multiple linear regression mathematical algorithm included in the program MULTIC [ll].

ETAL

Procedure

The experimental determination of Ca(I1) and Mg(I1) concentrations in the water samples involved the following sequence. Recording of spectra. In a 25-ml volumetric flask were mixed 5 ml of 3 x lop3 PAR solution, 5 ml of 0.5 M Tris buffer (pH 9.6) and a volume of sample solution to obtain Ca and Mg concentrations over their respective linear determination ranges (0.10-4.0 pg ml-’ for Ca and 0.15-2.5 pg ml-’ for Mg). The solution was then diluted to the mark with water and swirled to homogenize the mixture. An aliquot of this solution was placed in a quartz cuvette of l-cm path-length and its absorbance spectrum was recorded with the diode-array spectrophotometer over the wavelength range 470-650 nm at an integration time of 1.0 s. Processing of spectra. First, the above-described procedure was used to obtain the spectra of PAR and its Ca and Mg complexes. The PAR concentration used in each case was 6 X lop4 M. The solution containing PAR alone was prepared in triplicate and its spectrum was recorded three times on each solution near being considered as the PAR standard. The spectra of the two metal ions were recorded on four solutions containing different concentrations within their linear determination range. Each of the spectra of the PAR-Ca and PARMg complexes were then corrected by subtracting the standard spectrum of the ligand using a subroutine of the spectrophotometer’s bundled software. Once corrected, the spectra were standardized by dividing them into the Ca and Mg concentrations at which they were obtained. This resulted in four corrected, standardized spectra for each element, so each corresponded to a 1 pg ml-’ concentration. Finally, the average spectrum of each group was obtained and used as a standard for each ion. These two standard spectra, together with the working wavelength range (470650 nm) and wavelength scan interval (2 nm), were stored in the corresponding computer subdirectory as a working model for subsequent determinations. Figure la shows the absorption bands yielded by PAR and its two complexes, and Fig. lb shows the bands of the two standards,

SIMULTANEOUS

SPECTROPHOTOMETRIC

DETERMINATION

OF Ca AND

squares fitting. This treatment tail in various sources [12,13].

with maxima at 490 nm (Mg) and 502 nm (Ca), and that of a mixture. For resolution of the mixed spectra, once the spectrum of the unknown solution had been obtained, it was also corrected by subtracting that of PAR and resolved by means of the program MULTIC over the wavelength range 470-650 nm on the basis of the standard spectra of Ca and Mg. The program, under the assumption that Beer’s law is obeyed and the absorbances are additive, solves the matrix corresponding to a mixture of N absorbing components at J wavelengths from

RESULTS

q,bc, + k,

r=l

where k, 1s the independent term corresponding to the experimental error (residuals), A, and E,, are the absorbances of the mixture and the molar absorptivity of each species i, respectively, at the different wavelengths of the working range, j and c, is the concentration of each species i. The erJ values are obtained from the standard spectra of the I components and are reflected as a (J X i) matrix. The above matrix was solved by using a multilinear regression treatment based on determining the concentrations of the I components of the unknown sample with a minimum experimental error by matrix calculus. The result should comtide with that which one would obtain by least-

2 99

05gj

qa

-0

01

AND

is described

m de-

DISCUSSION

First, the working medium for the determination was investigated. The PAR complexes of the two elements were found to be unstable in ammonia, which resulted in a gradual decrease m their absorbances, probably as a result of hydroxide precipitation by the two ions. Likewise, an ammonia-ammonium chloride medium provided unsatisfactory results because of the added instability of PAR, which yielded an absorption band at 340 nm at the expense of a gradual decrease in its characteristic band at 412 nm was also assayed. A 0.1 M Tris buffer of pH 9.6 following literature recommendations and the complexes were found to be stable in it for several hours and the pH remained constant throughout the analyses, so this was adopted as the determination medium. The effect of the molar concentration ratio of PAR and each metal was also investigated; ratios above 5 resulted in virtually constant absorbances. However, halving this ratio resulted in an absorbance decrease of 12% for Ca and 37% for Mg. For the above reasons a PAR concentration of 6 x lop4 M was adopted as optimum; this provided a wide enough working range while causing no spectrophotometric interference. If half this

c=N

A, = c

515

MS IN WATER

b

i,r,,,,,& 470

520

570

Wavelength

670

520

570

620

670

Wavelength

Fig. 1 Absorbance bands yielded by (a) the PAR complexes formed from 6 x 10m4 M PAR and 1 /.tg ml-’ Ca(I1) and M&II) standards and a rmxture of Ca and Mg contammg 2 ~1 ml-’ of each. (a) -, PAR; _-__-Ca-Mg, - - -, Ca(I1) standard; - - - - - -, Mg(I1) standard. , PAR-Mg(I1). (b) -,

Ca(II) and Mg(I1); (b) - - -, PAR-Ca(II),

E GbMEZ

516 TABLE

1

Cahbration

graphs

obtamed

by the proposed

method

Element

No. of pomts

Intercept (a + 95% confidence

Ca Mg

6 6

0.0223 f 0.0086 0.0036 k 0.0122

r = Correlation

coefficient;

s~,~ = standard

deviation

hrmt)

of the ions that occur most frequently in waters were investigated. The amounts of the different interferents tolerated by the proposed simultaneous spectrophotometric

sY/X

hrmt) 0.9996 0.9996

0 0043 0.0057

determination of 0.5 pg ml-’ Ca(I1) and Mg(I1) are listed in Table 2. All the metal ions except Na(I), K(I), Al(II1) and Cr(II1) increased the absorbance at ratios above those given by formation of coloured complexes with PAR. While quantitatively significant, the interferences from these ions posed no insurmountable problems for the determination of calcium and magnesium in typical water samples as a result of the dilution required (usually greater than 25 : 1) to bring their concentrations within the linear determination range, which also decreased the concentrations of potential interferents to below their tolerated levels unless metal-polluted waters were being dealt with. If the concentration of any of these metal ions was still above that tolerated by the proposed method, the interference can be overcome by determining the interfering metal together with the two alkaline earths, which can be most readily accomplished by using the program MULTIC. TABLE

3

Results and 95% confidence lmrits obtained m the stmultaneous determmation of Ca and Mg m synthetic nuxtures

2

Effect of foreign tons on the simultaneous pg ml-’ calcmm and magnesmm ion

determmatton

of 0.5

Tolerated

Ilonl/l~~ytel ratio Chlonde, carbonate, nitrate, Na(I), K(1) Sulphate, phosphate Ba(I1) Al(III), Cr(II1) Pb(I1) Cd(II), Cu(II), Hg(II), Fe(III) Co(II), Nt(I1) Mn(II), Zn(I1) a Maximum

a

ra

of the ftttmg

Effect of foreign ions The potential interferences

Foreign

Slope (b f 95% confidence 0.0927 i- 0.0038 0.1868 f 0 0080

concentration had been used, the sensitivity of the method would have been significantly decreased. The PAR and buffer solutions were mixed prior to the experiment because, according to Jezorek and Freiser [6], PAR solutions buffered with Tris are absorbed by the walls of plastic bottles after 1 day’s storage. Once the optimum working conditions had been established, calibration graphs were obtained that showed the proposed method to be applicable over the ranges 0.10-4.0 pg ml-’ for Ca and 0.15-2.5 pg ml-’ for Mg; the detection limit was calculated as three times the standard deviation of the blank. The results obtained are listed in Table 1. As can be seen, the points on each graph were well correlated and deviated only slightly from the fitted lines.

TABLE

ET AL

concentratton

assayed

100 a 40 10 1 02 0.05 0.02 0.01

Sam-

Added

pie

(pg ml-‘)

Found

(pg ml-‘)

Ca

Mg

Ca

Mg

1 2 3 4 5 6 7 8 9 10 11 12

0.50 2.00 2.00 2 50 2.00 2.00 0.50 1.00 0.50 0.25 0.00 0.00

000 000 0.25 0.50 100 2.00 0.50 2.00 2 50 2.00 2.00 0.50

0.53 f 0.01 1.90*0.04 2.04 f 0 03 2.35*004 201kOO6 1 88 f 0.06 0 51 f 0.02 1 03 f 0.05 0.51 f 0.04 0.22 + 0 02 0.00 f 0.02 0.00*001

0.04*0.01 000*0.02 023+002 0.57 f 0 02 1.03 f 0 03 2.08 k 0.03 0.57 * 0.01 2.18*0.03 251kOO2 206kO.04 1 98 f 0.01 0.51 f 0.01

SIMULTANEOUS

SPECTROPHOTOMETRIC

DETERMINATION

OF Ca AND

517

MS IN WATER

TABLE 4 Results and 95% confidence1mCtsobtamed m the simultaneousdetermmatlonof Ca and Mg m reai water sampies AAS reference method (ug ml-‘)

Proposed method (/.tg ml-‘)

Sample

Ca

Drlutton factor

Ca

Mmeral waters: Zone 1 (Gerona) Zone 2 (Mallorca)

1: 10 1.50

26.8 rt 0.4 100 f 2

4.4 f 26 f

0.2 1

Well waters (Mallorca). Zone 1 Zone 2

1 25 1 50

45 110

f f

1 1

27.2 f 22.8 f

04 09

55 104

25 7 26 7

Tap waters (Palma de Mallorca): Zone 1 Zone 2 Zone 3 Zone 4

1:50 1: 50 1:50 1.100

112 122 138 250

* f f f

2 2 2 4

21 3 * 21.0 f 23 f 120 f

0.9 0.7 1 2

92 110 135 250

194 23 0 22 110

Sea waters (Mallorca). Zone 1 (Palma Bay) Zone 2 (Alcudra Bay)

1.500 1 1000

450 374

*22 *30

Mg

Appbcations By using the proposed method Ca and Mg were determined simultaneously in a set of synthetic samples containing different Ca and Mg proportions. The results obtained are listed in Table 3; the concentrations found were consistent with those added. The method was then applied to the simultaneous determination of Ca and Mg in waters from various sources. Table 4 gives the dilutions used TABLE 5 Results and 95X confidence hrmts obtamed in the simultaneous determmatron of Ca and Mg using first-derivative absorbance spectra Sam-

Added (pg ml-‘)

Found (ug ml-‘)

pie

Ca

Mg

Ca

Mg

1 2 3 4 5 6 7 8 9 10 11 12

0.50 2.00 2.00 2.50 2.00 2.00 0 50 100 0.50 0.25 0.00 0.00

000 0.00 0.25 0.50 1.00 2.00 0.50 2.00 2.50 200 200 0.50

0.55 f 0.02

0.00*0.01 0.00*0.02 0 34 * 0.05 0.56 f 0.06 0.99 f 0.10 209+009 0 54*0.01 2.14k0.19 2.64kO.14 2.26 f 0.06 2.05 f 0.04 0.48 f 0 02

1.87 f 0.03 2.02 + 0.06 2.30k0.11 1 98 f 0.20 1.85 kO.16 0.56kO.02 122*0 37 048kO25 023&008 0.00*0.05 0.02 f 0.02

1450 1470

*20 &27

and the results obtained

23 1 88

440 400

Mg

4.6 31

1400 1410

using both the proposed

AAS procedure. The results obtained by the two methods were in satisfactory agreement. Attempts to resolve the mixtures from first-derivative spectra (Table 5) met with more deviant results those provided by the normal absorbance mode.

method

and a standard

Conclwons The proposed spectrophotometric method for the simultaneous determination of Ca and Mg in water based on the use of the program MULTIC and the absorbance spectra of the PAR complexes of the ions provides good results, despite the overlap of the bands of the two complexes. The procedure is a straightforward, inexpensive means of determining the concentrations of the two metals as it requires no special instrumental set-up. Finally, judging from these results, a method for the simultaneous determination of toxic metals that form complexes of high absorptivity could be developed on the same basis. The authors express their gratitude to the DGICyT (Spanish Council for Research in Science and Technology) for financial support granted for the realization of Project PA 86-0033.

518 REFERENCES 1 R. Cervenka and M. Korvova, Chem. Lusty, 50 (1956) 306 2 P.B Sweetser and C.E Bncker, Anal Chem., 26 (1954) 195. 3 M. Blanco, J Coe.110, J Gene, H. Iturnaga and S. Maspoch, Anal. Chrm. Acta, 224 (1989) 23. 4 S Shrbata, Chelates m Analyttcal Chemtstry, Vol. 4, Dekker, New York, 1972 5 J S. Fntz and J N. Story, Anal. Chem., 46 (1974) 825. 6 J.R Jezorek and H. Fretser, Anal. Chem., 51 (1979) 373. 7 R.M. Casstdy and S Elchuk, J. Chromatogr. Set., 19 (1981) 503

E &MEZ

ET AL

8 R.M. Casstdy, S Elchuck and J. McHugh, Anal. Chem., 54 (1982) 727. 9 D Yan and G. Schwedt, Fresemus’ Z Anal. Chem., 320 (1985) 325. 10 M.D. Argiiello and J S. Fntz, Anal. Chem., 49 (1977) 1595 11 G. Sala, S. Maspoch, H. Iturriaga, M. Blanco and V Cerda, J. Pharm. Boomed. Anal., 6 (1988) 765. 12 N. Draper and H. Srmth, Apphed Regressron Analysrs, Wtley, New York, 1980. 13 P. Segura, Tratamrento de Datos y An&hsis de Error, Promoctbn Pubhcactones Umversttanas, Barcelona, 1985