Voltammetric Determination of Nitrate and Nitrite Ions Using a Rotating Cadmium Disk Electrode Sir: Voltammetric procedures are described in the chemical literature for the quantitative determination of NO3- and NOz- in aqueous media based on their electrocatalyzed reductions at mercury electrodes in the presence of various di- and trivalent cations (1-13). The mechanisms of these electrochemical reactions involve (i) the direct reduction of a complex formed between the cation and the nitrogen species or (ii) the enhancement of the voltammetric wave for the electroreduction of the cation when the reduced form of the cationic species is rapidly oxidized by NO3- or NOz- transported to the surface of the electrode by convective-diffusional processes. The uncatalyzed reductions of NO3- and NOz- a t mercury electrodes are sufficiently irreversible that the large cathodic current due to evolution of HZ obscures any cathodic wave for NO3- or NOz-. Direct electrochemical reductions of NO3- have been studied a t a variety of other electrode materials, including Cu, Ti, Zn, Pb, Sn(Hg), Cu(Hg), Cr, and Pt (14-20). For none of these electrodes in dilute solutions of NO3- can analytically useful cathodic waves be resolved from the wave due to evolution of Hz. Well developed anodic waves for the oxidation of NOz- to NO3- a t platinum electrodes are observed in the pH range 0.5-8 (21, 22). The process is strongly influenced by the extent of oxidation of the electrode surface (23). A quantitative method for determining N02- by oxidation at a platinum electrode in a n acetic acid-acetate buffer was reported (23) in which controlledpotential coulometry was employed. We describe here cathodic current-potential curves recently obtained with a rotating cadmium disk electrode (RCDE)in weakly acidic solutions of NO3- and NOz-. At low rotational velocities and bulk concentration, the cathodic currents are limited by convective-diffusionalmass transport.
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EXPERIMENTAL The cadmium disk electrode was constructed by Pine Instrument Company of Grove City, Pa., of reagent grade cadmium from Mallinckrodt Chemical Works. The area of the cadmium disk was 0.456 cm2, The electrode was rotated by a synchronous rotator, Model PIR, from Pine Instrument Company. The surface of the cadmium electrode was polished to a mirror finish with a 600 grit Buehler Handimet strip followed by 30 pm, 6 pm and 1 Fm Buehler AB Metadi Diamond on nylon lapping cloth. Before each experiment, the disk surface was polished with 0.3 pm Buehler AB Polishing Alumina on Buehler Microcloth. Following insertion of the RCDE in the solution of supporting electrolyte, it was potentiostated a t -1.50 V us. SCE for 5 min, and H2 was evolved. The three-electrode potentiostat used is described in Ref. 24. Current-potential (I-E) curves were recorded with a Plotamatic Model 815 X-Y recorder from Bolt, Beranek and Newman of Palo Alto, Calif. The electrolysis cell was of all-glass construction with a volume of 100 ml. The counter electrode was a platinum spiral placed in a side-arm of the cell separated from the test solution by a fritted glass disk and filled with supporting electrolyte solution. The reference electrode was a Beckman SCE placed in a side arm connected to a Luggin capillary by a ground-glass stopcock. The side arm and capillary were filled with the supporting electrolyte solution. All potentials were measured and are reported in V vs. SCE. All chemicals used were from commercially available reagent grade stock. Water was triply distilled with deionization following the first distillation. The second distillation was from alkaline permanganate solution. Solutions were deaerated with purified nitrogen (99.99770) from Air Products.
RESULTS AND DISCUSSION Current-potential curves showing a cathodic wave due to the presence of NO3- were obtained with the RCDE in 0.01M HC1 and are presented in Figure 1. Only the curves obtained during the cathodic portion of the cyclic scan of disk potential, E d , are shown. The anodic and cathodic limits for the cyclic scan were -0.85 and -1.30 V, respectively. For E d > -0.85 v, an anodic current due to oxidation of the cadmium disk electrode was observed. If the limit of the anodic scan was significantly more anodic than -0.85 V, the value of the limiting current, I],for the cathodic wave due to N o s - decreased with successive scans. The limiting cathodic process for Ed < -1.30 V is the evolution of Hz. For the supporting electrolyte solution, 0.01M HC1 was found to be optimal. For concentrations greater than O.OlM, the wave for evolution of Hz was shifted to more positive potentials, and it interfered with the determination of I, for N o s - . At concentrations of HC1 less than O.OlM, the rate of the electroreduction of NO3- was so slow that the value of current was not limited by convective-diffusional processes. The value of I1 measured with a rotating disk electrode for a faradaic process is predicted by Equation 1. where n = faradays/mole of reaction; F faraday constant; A = area of the disk electrode; v = kinematic viscosity of the solution; w = angular velocity of electrode rotation; and Cb = bulk concentration of electroactive species. (24) D . T. Napp, D. 481 (1967).
C. Johnson and S. Bruckenstein, Anal. Chem., 39,
ANALYTICAL CHEMISTRY, VOL. 45, NO. 11, SEPTEMBER 1973
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3 50 300
250
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200
(PA)
150 100
50
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-1 2
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Figure 1. /-E curves for reduction of NOS- at RCDE 0.01M HCI; w = 41.9rad/sec; Ed scan rate = 1.0 V/min; concentration of NaN03: (1)O.OmM. (2) 0.100mM,(3)0.200mM, (4)0.300mM, (5) 0.400mM,and (6),0.500mM
t
i
I
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I
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Figure 2. Concentration study 0.01M HCI; w = 41.9 rad/sec; Ed scan rate = 1.0V/min; / I measured at Ed = -1.10V on cathodic scan; 0 = NaN03 and A = NaN02
The plot of 11,measured at Ed = -1.10 V, for the electrochemical reduction of NO3- at the RCDE is plotted us. @(NO3-) in Figure 2. The plot is linear for Cb(N03-) < 0.35mM with a zero intercept in agreement with Equation 1 for the rotational velocity used. For @(NOS-) > 0.35mM, the irreversibility of the cathodic process is such that the current a t Ed = -1.10 v was not on the convective-diffusion limited portion of the wave. Current-potential curves obtained a t the RCDE in 0.01M HC1 containing NOz- are similar in shape to those
1980
Figure 3. / I - W ~ /plot ~ for reduction of NO3- and NOz- at RCDE 0.01M HCI; Ed scan rate = 1.0 V/min; / I measured at Ed = -1.10 V on cathodic scan: 0 = 0.lOlmMNaN03 and A = 0.100mM NaNOz
obtained for NO3-, A plot of I , us. Cb(NOz-) measured a t Ed = -1.10 V is shown in Figure 2. The plot is linear for @(N02-) < 0.35mM with a zero intercept in agreemenc with Equation 1. The cathodic waves for Cb(NOz-) > 0.35mM are distinctly irreversible. Figure 3 contains plots of I , us. u1I2 measured a t Ed = -1.10 V for the electrochemical reduction of NO3- and NOz- a t the RCDE in 0.01M HC1. From the shapes of the plots, it is evident that Equation 1 applies only for low angular velocities of electrode rotation. The cathodic waves for NO3- and NO2- are highly irreversible for ~ 1 ' 2 > 10 (rad/sec)1'2 and the current measured a t Ed = -1.10 V is not in the region of the limiting-current plateau. The use of Ed < -1.10 v was not feasible due to evolution of Hz. On the basis of the results reported here, we are optimistic that rapid procedures for the accurate determination of NO3- and NOz- in aqueous solutions can be developed based on voltammetry with cadmium electrodes Many species are expected to interfere in the deterrnina. tion including oxygen and transition metal cations whiclcan be reduced a t the electrode and separations are re. quired. We are presently investigating the use of ion ex, change resins in forced-flow liquid chromatography for tht removal of interferences and the separation of NO3- ana NOz- in mixtures. A coulometric cadmium electrode if being used to detect NO3- and NOz- in the chromato graphic effluent.
Ronald J. Davenport Dennis C.Johnson Department of Chemistry Iowa State University Ames, Iowa 50010 Received for review January 12, 1973. Accepted June 11 1973.
ANALYTICAL CHEMISTRY, VOL. 45, NO. 11, SEPTEMBER 1973
