Determination of Nitrite by Controlled-Potential Coulometry J. E. Harrar Chemistry Department, Lawrence Radiation Laboratory, University of California, Livermore, Calif. 94550
CLASSICAL TITRIMETRIC PROCEDURES for the determination of nitrite in acid solution are complicated by the instability and volatility of nitrous acid and its low rate of reaction with many titrants ( I ) . For example, the recommended procedure for assaying reagent grade nitrite salts (2) is an indirect titration using both permanganate and ferrous ion standard solutions. Reported amperometric methods (3, 4) are also subject to losses of nitrite in acid solution; however, they may be preferable for low concentrations. Constant-current coulometry eliminates the need for standard titrants, but two procedures that have been developed (5, 6) require dual-intermediate back titrations and rather complex techniques. This note describes a simple, accurate method in which nitrite is determined by coulometry at controlled potential. Nitrite is oxidized directly to nitrate at a platinum electrode in pH 4.7 acetate buffer solution. Possible loss of nitrous acid is not a factor in the analysis, and the reproducibility of the platinum electrode surface has a minor effect on the analytical results. EXPERIMENTAL
Apparatus. The details of the construction and operation of the platinum-electrode electrolysis cell assembly are available elsewhere (7). The controlled-potential coulometer system (8) and the technique of predictive coulometry (9) have also been described. The predicted analytical results and the electrolytic rate constant data were both obtained by means of the analog predictor system. Reagents. All chemicals were reagent grade or equivalent purity. Standard solutions of nitrite were prepared from a dried sample of J. T. Baker NaNOz sticks, which was assayed to American Chemical Society specifications (2). The K M n 0 4 titrant was standardized against NBS NazC2O4. The NaNOs salt assayed 100.07Z,with a combined estimate of 0.14z for the relative standard deviation [from the K M n 0 4 solution standardization, the Fe(I1) solution standardization, and the NOs- titration]. All volumetric glassware was calibrated or certified. Micropipets were used to take aliquots of the nitrite standard solutions for coulometric analysis. Procedure. ELECTRODE PRETREATMENT. When the platinum working electrode is used for the first time, contamination is suspected, or the electrode has been anodized above f1.4 V us. SCE, pretreat as follows. Immerse the electrode in boiling concentrated HC1; after 5 min, rinse with water, and place it in the coulometric cell assembly. Subsequent treatment with HC1 is needed only when loss of electrolysis speed or high background currents appear. (1) A. J. Clear and M. Roth, in “Treatise on Analytical Chemistry,” Part 11, Vol. 5, I. M. Kolthoff and P. J. Elving, Ed., Interscience, New York, N. Y . , 1961, pp 276-278. ( 2 ) “Reagent Chemicals,” 4th ed., American Chemical Society Publications, Washington, D. C., 1968, pp 469, 548. (3) J. T. Stock and R. G. Bjork, Microchem. J . , 6 , 219 (1962). (4) J. T. Stock and R. G. Bjork, Talanta, 11,315 (1964). (5) R. P. Buck and T. J. Crowe, ANAL. CHEM., 35,697 (1963). (6) R. Magno and M. Fiorani, Ric. Sci., 38, 119 (1968). (7) J. E. Harrar, AEC Rep., UCRL-50417,Livermore, Calif., March 1968. (8) J. E. Harrar and E. Behrin, ANAL.CHEM., 39, 1230 (1967). (9) F. B. Stephens, F. Jakob, L. P. Rigdon, and J. E. Harrar; ibid., 42, 764 (1970).
With the 1M sodium acetate-1M acetic acid supporting electrolyte in the cell, polarize the working electrode a t 1.lo V us. SCE for 5 min. Replace the cell solution with a fresh portion of the supporting electrolyte and polarize the electrode at $0.95 V us. SCE for 5 min. The background current should be less than 10 PA. If the electrode is not used for several hours, repeat the polarizations at $1.10 and +0.95 V before carrying out a determination. COULOMETRIC ANALYSIS.Place sufficient 1M sodium acetate-1M acetic acid supporting electrolyte in the cell and in the two salt-bridge tubes. With stirring, pipet an aliquot of sample solution containing not more than 4 mg of nitrite into the cell. Electrolyze the solution at +0.95 UP. SCE and measure the integrator readout voltage when the current has decreased to 5-25 PA. The initial current should be 2540 mA/mg NOz-. Determine the background correction by electrolyzing the supporting electrolyte alone for the same length of time as required for the nitrite sample. This correction should not exceed the equivalent of 0.003 mg of nitrite for a 22-min electrolysis. For a determination by predictive coulometry, first carry out a complete electrolysis to ascertain the time at which prediction accuracy is satisfactory. Correct the results for the background consisting of the accumulated charge at the time of prediction and the quantity &/k (9), where ib is the constant background current and k is the electrolytic rate constant, The k value can be estimated with sufficient accuracy from the time of a complete electrolysis (k = 6.9/t0.99~), or it can be determined directly (9). The sum of the accumulated background charge ( i d ) and 2 ib/k should not exceed the equivalent of 0.003 mg of nitrite.
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RESULTS AND DISCUSSION
Characteristics of Electrolysis of Nitrite. The results of recent voltammetric studies of the anodic oxidation of nitrite and nitrous acid in various media (IO, 11)have suggested that this reaction might form the basis of a controlled-potential coulometric method. Accordingly, the optimum conditions for the determination were sought, and the data shown in Figures 1 and 2 summarize these experiments. At low pH, the error in the analysis (Figure 1) increases with the amount of time the nitrite solution is left in the cell before electrolysis is initiated. Thus, the negative bias is probably caused by the decomposition of nitrous acid. At the recommended p H of 4.7, no such loss could be detected for a period of 1 hour. Standard solutions of N a N 0 2 in water, containing approximately 1 mg N02-/ml, were also quite stable and remained within 0.1 titer for six weeks. The maximum rate of electrolysis within each usable applied potential range decreased as the p H of the solution was raised. No electrolysis of nitrite was detected in 0.1M NaOH. Confirming the reported voltammetric behavior ( I O , I ] ) , the oxidation of nitrite to nitrate was observed to be totally irreversible and the electrolysis rate to be strongly influenced by the extent of oxidation of the platinum electrode surface. AS
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(10) E. Julien and M. Comtat, Reu. Chim. Miner., 6 , 885 (1969). (11) G. Schmid and M. A. Lobeck, Ber. Bunsenges. Phys. Chem., 73, 189 (1969).
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Table I. Analyses of Standard Solutions of Nitrite by Controlled-Potential Coulometry 1M HOAc-1M NaOAc supporting electrolyte, E = +0.95 V US. SCE Time of electrolNitrite (mg) ysis Re1 std Re1 Taken Found n min dev error 4.016 8 23 0.03 -0.03 4.017 -0.03 5 11 0.05 4.017 4.016a 5 8 0.14 4.017 4.041“ +0.6 -0.05 6 22 0.03 2.009 2.008 6 11 0.05 2.009 2.009= 0.00 6 8 0.07 2.009 2.01Y +O. 3 -0.11 6 22 0.03 0.9474 0.9464 6 22 0.03 -0.17 0.4737 0.4729 0.0882 $0.2 7 20 0.3 0.0884 5 17 0.8 -0.5 0.0219 0.0220 5 4 0.7 -0.5 0.0219a 0.0220 Determination by predictive coulometry.
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Figure 2. Electrolytic rate constant for oxidation of nitrite and background current in pH 4.7 acetate buffer supporting electrolyte
shown in Figure 2, this passivation phenomenon causes the electrolysis rate to reach a maximum and then decrease. This effect is even more pronounced in stirred solution voltammetry (10, / I ) . The maximum rate of electrolysis is still slower, by at least a factor of three, than it would be if the process were controlled solely by mass transfer; thus, a complete electrolysis in the cell employed required 18-22 min at +0.95 V cs. SCE. The maximum rate of electrolysis is highest immediately after the pretreatment with boiling HCI; the electrode then ages in a few days to the condition represented in Figure 2. The recommended pretreatment polarization of the electrode at +1.10 V cs. SCE rapidly stabilizes the background current at the analytical control potential of +0.95 V. However, if the electrode is polarized at still greater anodic potentials ( e . g . , at +1.4 V), the electrode becomes even more passivated, and a complete electrolysis at +0.95 V requires 50-60 min. Measurements were also carried out with a pure 144
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gold working electrode, but no improvement in the rate of electrolysis was found. At corresponding applied potentials, the background current was lower with gold than with platinum, but the electrolysis rate peaked a t the same value. Because the oxidation of nitrite is totally irreversible, the background current at +0.95 V is low, and nitrite is stable in the recommended supporting electrolyte, the precision and accuracy of the analysis are not greatly dependent on the time of the electrolysis or the surface condition of the electrode. However, if desired, the time of electrolysis can be significantly shortened by predictive coulometry (9), since, for a given electrode condition and applied potential, the electrolytic rate parameter is relatively constant. Analytical Accuracy and Precision. Table I illustrates the precision and accuracy obtained in analyzing standard nitrite solutions, by both conventional and predictive controlled-potential coulometry. The conditions of the determination and the low equivalent weight of NO2- allow extension of the procedure to fairly low levels with good results. Interferences. Forty-five species were tested for their effect on the determination of nitrite, with the results summarized in Table 11. Included are species commonly associated with nitrite, those reported to form complexes with nitrite (12), and those expected to be oxidized a t +0.95 V US. SCE. Among the untested substances which are known to be oxidized at this control potential are H202, Ir(III), Ru(III), Pu(III), and the lower oxidation states of osmium. Most of the substances for which a level of interference was found were concomitantly oxidized with nitrite, and their contribution to the charge transferred was a simple additive function of the amount added. In addition, Mn(II), Sb(III), and SCN- poisoned the working electrode--i.e., not only was the rate of electrolysis of nitrite decreased considerably in the presence of these species, but subsequent electrolyses in their absence were also sluggish. The deposition of a n insoluble material on the electrode is the probable cause of this behavior, since this is known to occur during the oxidation of Mn(I1) (13) and Sb(II1) (14). The recommended pretreatment with (12) L. G. Sillen and A. E. Martell, “Stability Constants of MetalIon Comdexes.” 2nd ed., The Chemical Society, London, 1964, pp 164-166. (13) C. 0. Huber and L. Lemmert, ANAL.CHEM., 38,128 (1966). (14j B. B. Baker and W. M. MacNevin, J . Amer. Chem. Soc., 75, 1473 (1953).
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Table 11. Tolerances of Diverse Substances in the Determination of Nitrite by Controlled-Potential Coulometry 1 mg NO2-, 1M HOAc-1M NaOAc supporting electrolyte, E = +0.95V cs. SCE Amt to Amt to cause 0 . 3 % cause 0 . 3 % Substance Added as re1 error, mg Substance Added as re1 error, mg clNaCl >20 As(1II) NaAsOz 0.005 BrKBr 0.030 Au( 111) HAuC14 0.40 IKI 0.002 Bi(I1I) Bi(N0& >1 .o NH4+ NHiNO3 >450 Cd(I1) CdCI2 >1.5 NzH4 NzH4. HzS04 0.01oa Ce(II1) Ce(ClO4)a 0.010 NH20H ("*OH)?. HzSOa 0.003= Co(I1) COS04 >1.0 0.004 Cr(111) Cr(ClOd3 0.002 NJNaN, NO,NaN03 >1500 Cu(I1) cuso4 >1.0 CNNaCN >O. 5 Fe(I1) Fe(NH4)2(S04)2 0. l o a SCNNHnSCN 0 .0 0 l b Fe(II1) Fe(C10h >10 OCNKOCN 1.5 MI) Hgz(N03)z 0.025 Phthalate KHCsHdO4 >400 Hg(W HgS04 >1.5 0.06 Mn(I1) MnS04 0.002b c204'Na2C204 CH02NaCH02 0.75 Pb(I1) Pb(OAC), >l.OC > 1200 Pd(I1) NaaPdC14 0.007 ClOaHClOi PO43Na2HP04 > lo00 Pt(I1) PtC12 0.020 >6 Pt(IV) NazPtCls >0.5 ~ 0 ~ 3 Na2HPO3 H?P02NaH2PO2 2.0 Rh(1II) RhC13 >O. 5 s04'NasS04 > lo00 Sb(II1) SbC13 0 ,0 0 5 b 0,0200 TU) T12SOd 0,010 so32Na2S03 S 2(NHdzS 0 .OlOa U(VU UO?(C104)* >5 Se0,'H2SeO3 >O. 5 WV) voso4 0.005 >1 o c Ad11 AgsS04 a With preoxidation at +O. 57V us. SCE. * Rate of electrolysis significantly decreased. c Rate of electrolysis significantly increased.
hydrochloric acid was required to restore the working electrode to its normal condition. With SO3?-, N3-, and Pt(II), which are oxidized at $0.95 V L'S. SCE, and with H2PO2-, Pd(II), and Au(III), which are not oxidized, a reaction with the nitrite was indicated by the appearance of a negative bias with increasing amounts of the interfering substance. Several interferences were rapidly oxidized to a stable species at f0.57 V cs. SCE; thus, their tolerance levels may be raised by pre-electrolysis at this potential. However, most of these substances still cannot be tolerated at a very high level. Additional oxidation of the interference takes place at f0.95 V for hydroxylamine and sulfide, and there is evidence that both Fe(I1) and hydrazine chemically reduce the platinum oxide film, increasing the background correction at f0.95 V. Iodide is quantitatively oxidized to I? at +0.57 V, but further to IO3- at $0.95 V ; thus, little advantage is gained by preoxidation. Sulfite is oxidized to SO4*- a t t 0 . 5 7 V, but, because of its slow reaction with nitrite, moderate amounts still cause a significant bias.
A catalytic effect was observed when nitrite was oxidized in the presence of Pb(I1) or Ag(1). No detectable oxidation of Pb(I1) or Ag(1) occurred at +0.95 V us. SCE, and no error was found in the determination at the interference levels indicated in Table 11. However, the time for a complete electrolysis was reduced to 8 min, and, in the case of Pb(II), as with the poisoning effects noted above, the working electrode retained the catalytic activity. Thus, in certain circumstances it appears that it would even be desirable t o include Pb(1J) or Ag(1) in the supporting electrolyte for this determination.
RECEIVED for review September 14, 1970. Accepted October 29, 1970. Work performed under the auspices of the U. S. Atomic Energy Commission. Reference to a company or product name does not imply approval or recommendation of the product by the University of California or the U. S. Atomic Energy Commission to the exclusion of others that may be suitable.
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