Amperometric determination of oxidizable solutes in water with a solution exchange technique

ANAmIcA CHIMICA

AC-IA ELSEVIER

Analytica Chimica Aeta 300 ( 1995) 53-57

Amperometric determination of oxidizable solutes in water with a solution exchange technique Charles Gartske ‘, Calvin 0. Huber * Department of Chemistry, University of Wisconsin-Milwaukee,

P.O. Box 413, Milwaukee, WI 53201, USA

Received 9 May 1994; revised manuscript received 22 July 1994

Abstract Determination of oxidizable solutes in water is accomplished using amperometric detection in a solution exchange apparatus. The applied potential (E,J used is + 0.5 V vs. SCE at a nickel working electrode. Electrode pretreatment is performed at - 1.0 V vs. SCE. The background electrolyte is 0.1 M NaOH. Analyte solution drawn into the cell replaces background solution. No further convection occurs. Peak anodic current relative to the baseline is used as the analytical signal. The linear range extends down to 1 PM for sucrose, diethylenetriamine,phenol, and ethanolamine.The determinationtime is 2 min per sample. Keywords: Arnperometry; Nickel oxide

1. Introduction

Amperometry commonly employs controlled convection. Often the reproducibility of the convection is one of the most demanding aspects of the method. The investigation described here included efforts to develop a measurement method which eliminates the need for stirring or continuous flow. Further advantages sought were small sample volume, simple operation, rapid response, and applicability to a wide array of oxidizable solutes. The amperometric system selected for this purpose included an anodic nickel working electrode in basic solution. The analytical electrode process can be described as oxidative attack on analyte by higher * Corresponding

author. ’ Present address: SC. Johnson Wax, 1525 Howe Street, Racine, WI 53403, USA. 0003-2670/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDZOOO3-2670(94)00356-4

potential species on the electrode surface lattice [ l-31. Aliphatic as well as aromatic hydroxy and amine functionalities are oxidized. The rate determining step includes an alpha-hydrogen abstraction leaving a radical species. Typically, cyano and carboxylate products are produced from amine and hydroxy functionalities, respectively. The goal for the project reported here was to design and examine the performance of a simple device for nickel oxide anodic amperomehic measurements of typical organic solutes in water. Analytes were selected partly to reflect the oxidizable organic pollutants expected in waste water. The analytes chosen included sucrose (carbohydrates, cellulosics, etc.), phenol (phenols), glycine, ethanolamine and diethylenetriamine (amino acids, peptides, proteins). Under the experimental conditions used, hydrocarbons can not be oxidized and so were not tested.

54

C. Gut&-,

C.O. Huber / Analytica Chimica Acta 3t.W (199s) 53-57

2. Experimental

ment step, 25 ml of analyte solution was drawn into the cell and the resulting current was measured. For storage between uses the cell was left dry and at open circuit.

2.1. Apparatus

The diagram for the device is shown in Fig. 1. The main channel of the cell consists of 6 mm i.d. poly (vinyl chloride), Tygon( R) @tubing. The amperometric detector consisted of a three electrode system. The reference electrode was a saturated calomel electrode (Coming, Cat. No. 476350) which contacted the solution via a l/2 inch poly(viny1 chloride) tee. The working and counter electrodes were 0.6 mm diameter nickel wires inserted through the flexible poly(viny1 chloride) Tygon( R) @tubing with a nominal working area of 0.01 cm2. The potentiostat and current follower circuits were based on operational amplifiers (device type 324) powered by two 6-V dry cell batteries. Analytical measurement readings were taken using an auto ranging ( f 0.1 mV) digital multimeter (Micronta Model 22-193) or a potentiometric strip chart recorder (Linear Model 1200). Applied potential was typically + 0.5 V vs. SCE. 2.2. Procedure Using the syringe (Fig. 1), background electrolyte was drawn into the cell to just above the reference electrode. Pretreatment consisted of applying - 1.0 V for 15 s followed by the applied analytical potential, typically + 0.50 V, for 30 s. To perform the measure-

25 MLSYRINGE

Fig. 1. Solution exchange apparatus.

2.3. Reagents All chemicals were reagent grade. Calibration solutions were prepared using a background 0.1 M NaOH solution. Samples were adjusted to be 0.1 M in NaOH.

3. Results and discussion Initial tubing diameters were chosen to be small, typically l-2 mm i.d., in order to minimize sample volume. Very poor precision was encountered. Careful visual examination showed gas build-up at the working and counter electrode surfaces. At the working electrode (anode) the background current includes oxidation of water to produce oxygen gas. Correspondingly, hydrogen gas is produced at the counter electrode. In the case of smaller tubing diameters, the overall ratio of low polarity tubing surface area to solution volume is large. This encourages the build-up of oxygen and hydrogen bubbles at the electrode surfaces where bubble formation, with occasional dislodging, prevents good precision. With larger tubing diameters the ratio of tubing surface area to solution volume decreases, resulting in decreased bubble formation. 6 mm i.d. poly( vinyl chloride), Tygon( R) @tubing was found to be adequate to prevent bubble formation, while not requiring prohibitively large sample volumes. 3.1. Transient response signal shape The analytical response current/time profiles are shown in Fig. 2. At the higher concentrations, a sharp maximum decays to a plateau value. At medium concentrations the peaks become less sharp, and at low concentration the analytical signal is a step response. These anodic currents have been shown by Fleishmann et al. [ 11 to be principally controlled by surface reaction kinetics rather than by mass transport. Accordingly, in non-stirred solution horizontal current vs. time response would be expected and were observed for concentrations below a few tenths millimolar. At higher concentrations initial peaks were seen. These peaked responses were attributed either to mass transport influ-

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C. Gartske, C.O. Huber / Analytica Chimica Acta 300 (1995) 53-57

or to product desorption kinetics. The former were considered unlikely, because estimation of the apparent diffusivity using the profile shown in Fig. 2 for 2 mM concentration yielded a value of 5 X 10p6, indicating a rate about an order of magnitude lower than diffusion control in the solution phase, yet well above the value for solid or semisolid diffusion in the electrode material. Thus, it was concluded that the peaked-shaped response was attributable to adsorptionldesorption phenomena. Peak currents were used as analytical response rather than plateau currents because peak values provided a larger signal at higher concentrations. The background current due to the oxidation of the 0.1 M NaOH solution was concluded to be kinetically controlled, because the magnitude of the current was small ( 1.5 fl) for such high concentration species, and because when the solution was stirred, the background current was independent of stirring rate. Currents for analytes, in contrast, appear to be mass transport influenced based on the fact that flow rate changes affect current. ewes

\

1

2.0 mM

1 0

I

30

60

TIME, S Fig. 2. Time-dependent

response for sucrose solutions.

3530 -

3.2. Current versus concentration profile 6

Fig. 3, depicting amperometric current versus sucrose concentration shows that up to about 0.5 mM sucrose the plot appears linear, whereas at the higher

CONCENTRATION, mM Fig. 3. Amperometric Table 1 Amperometric

response for sucrose.

response for sucrose with and without pretreatment

of 0.005 to 0.5 mM Intercept (@)

Slope (&mM) Sucrose without pretreatment Sucrose with pre-

for concentrations

LOD ’ (mM)

39.6

0.13

0.06

225.6

- 1.63

0.04

treatment

a LOD = Lower limit of detection = 3 X standard error of the estimate divided by slope Table 2 Amperometric

responses for five analytes Slope (film

Sucrose Glycine Diethylenetriamine Phenol Ethanolamine

226 165 340 98 114

M)

Intercept ( jA)

LOD a (mM)

- 1.63 1.43 2.64 - 0.42 4.34

0.04 0.02 0.10 0.03 0.10

a Lower limit of detection, three times the standard error of the estimate divided by the slope.

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C. Gartske, CO. Huber /Analytica

Table 3 Anodic currents

Chimica Acta 300 (1995) 53-57

3.4. Calcium effects on amperometry

Sample (in 0.1 M NaOH)

Convective amperometry

Solution exchange method

0.1 mM glycine 0.1 mM glycine + 0.1 mM calcium 0.1 mM glycine + 0.1 mM calcium + 0.1 mM oxalate

12 CLA

12 fi

6.1

21

6-8

18

concentrations there is a distinct levelling effect. The levelling of the response curve may also be attributed to adsorption/desorption effects at the electrode surface. Reciprocal current versus reciprocal concentration plots were not linear, indicating non-adherence to a simple Langmuir adsorption model. 3.3. Electrode pretreatment Analytical signals showed variations of as much as 10% due to the apparent oxidation state of the nickel oxide surface lattice. To obtain a more reproducible nickel oxide lattice a negative or reducing potential was applied to the electrode as a pretreatment before measurements. A pretreatment voltage range of -0.84 to - 1.09 V vs. SCE was investigated. This range of pretreatment potentials was selected to be sufficiently cathodic to reduce all nickel oxides to nickel( 0) while avoiding hydrogen evolution [ 41. The pretreatment time was 30 s. Optimum precision occurred at - 1.0 V vs. SCE. Pretreatment times ranging from 5 to 60 s were examined. Precision was not improved for pretreatment times of 15 s or greater. A pretreatment time of 15 s was adopted for subsequent work. An added advantage from the pretreatment procedure was an approximately five-fold enhancement of sensitivity. The enhanced slope with pretreatment can be attributed to a higher activity surface generated by the freshly formed nickel oxide lattice following the pretreatment. Table 2 summarizes measurements over the 0.005 to 0.50 mM linear concentration range for five analytes. Comparison of signal size for the five analytes agreed with earlier reports that amines are more reactive than hydroxyls and that oxidation current is enhanced by multiple reactive functional groups [ 21. For these typical compounds the sensitivity per oxidative site seldom differs by more than a factor of three.

Previous studies with the nickel oxide electrode and anodic convective amperometry [ 51 showed that calcium strongly inhibited the amperometric currents. Typical data are shown in Table 3. The inhibition effect for the convective amperometry method was virtually independent of flow rate. The results shown in column three of Table 3 show an increase in anodic current due to calcium, i.e., opposite to results obtained using the convective amperometry method. Oxalate only partially masked the enhancement caused by calcium. Similar results were obtained with glucose, glutamate and ethanol as analytes. To further examine the effects of solution movement, subsequent exchanges of sample solution were made. Samples without calcium give equal analytical peak currents, but when the calcium containing solution was injected a second time, an inhibited signal was obtained. To interpret the remarkable difference in the calcium effect, positive interference for the solution exchange method presented here and negative interference for conventional flow stream amperometry, it must be noted that the only significant distinction is that of convection of the solution during the measurement step. In the method presented here, i.e., solution exchange, there is convection only during the exchange process with no imposed convection before or after. By contrast, in the convective amperometric technique there is constant convection. This observation suggests that mass transport phenomena, i.e., concentration gradient thickness, at the electrode/solution interface dictates the nature of the calcium effect. Apparently, calcium species interact with the electrode surface oxide lattice. At the solution pH the predominant calcium species are Ca2+ and CaOH+. The Table 4 Effect of hydroxide: interference on signal for 0.05 mM glycine by 0.1 mM calcium Hydroxide concentration

Percent interference

0.03 0.1 0.3 1.0

+8 +9 +3 -15

C. Garde,

C.O. Huber IAnalytica Chimica Acta 300 (1995) 53-57

relative amounts of Ca2’ and CaOH+ are roughly the same [6]. At the levels of calcium added (i.e., 0.1 mM), no calcium will exist as Ca( 0H)2, because its solubility limit is 1.0 mM [ 61. The equilibrium expression is: [Ca2+]

=

1023

[W~W21,[H’12 For an electrode reaction controlled primarily by kinetics in a stirred solution, the surface pH can be expected to be near that of the bulk (i.e., pH 13). At this pH calcium oxide bridges with the metal oxide lattice can be speculated, e.g., calcium oxide bridging at the electrode surface would then compete with analyte for adjacent active nickel sites, resulting in a net decrease in analyte reaction. The solution exchange technique, in contrast, has minimal stirring so that a greater decrease in pH at the electrode surface occurs due to background and analytical anodic processes. At the lower interfacial pH the divalent calcium cation species would be the predominant form of calcium and it would be the only divalent cation. The enhancement of analytical current under these conditions may then be attributable to calcium cation occupation of exchangeable lattice cation sites, which would otherwise be occupied by protons that lower surface pH. Additionally, for anionic analytes, calcium ionic bridges might stabilize surface intermediates during alpha-carbon electron abstraction. The rationale for the calcium effects outlined above infers that at a sufficiently high solution pH, the solution exchange method should show an inhibition, rather than an enhancement, due to the presence of calcium. Correspondingly, data were taken over a range of hydroxide concentrations. Applied potentials were selected at each hydroxide concentration in order to yield constant background currents. The data in Table 4 shows that as hydroxide concentration is increased from 0.03 to 1.0 M, the observed enhancement effect of calcium is replaced by an inhibition effect. These observations correspond with the proposed calcium effect surface chemistry.

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3.5. Comparison with BUD testing An application for an amperometric method for oxidizable solutes is the determination of oxidizable (i.e., oxygen demand) impurities in municipal wastewater. Historically, the biological oxygen demand (BOD) test has been used as the acceptable method [7]. Amperometric measurements by the present method were compared with BOD measurements. For amperometric measurements, samples were adjusted to 0.1 M NaOH with NaOH crystals. The results showed that while the present method showed a 25% decrease in oxidizable species during the wastewater treatment, the BOD method registered more than 90% decrease. This can be attributed to the fact that the amperometric method responds to true solutes, while in the BOD method reaction with suspended matter is included in the results. Thus, the present method offers a rapid assay of soluble oxidizable species.

Acknowledgements We thank Racine Sewage Treatment Plant (Racine, WI, USA) for their assistance.

References [l] M. Fleischmann,

K. Korinek and D. Pletcher, J. Chem. Sot., Perkin Trans., 2 ( 1972).

[ 21 N. Botros and CO. Huber, Anal. Chim. Acta, 208 (1988) 247. [3] B.S. Hui and C.O. Huber, Anal. Chim. Acta, 134 (1982) 211. [4] Atlas of Electrochemical Equilibria and Aqueous Solutions, National Association of Corrosion Engineers, Houston, TX, 1974, p. 333. [ 51 B.S. Hui, Examinations of Nickel Oxide Electrodes for Electroanalysis, Ph.D. Thesis, University of WisconsinMilwaukee, August 1991, p. 111. [ 61CF. Baes, R.E. Mesmer,The Hydrolysisof Cations, Wiley, New York, 1976, p. 103. [7] Standard Methods for the Determination of Water and Wastewater, 16th edn., American Public Health Association, Washington, DC, 1985, pp. 418419,422-425 and 525-532.