but is more irreversible on boron carbide. For further clarifit ation and interpretation of the axnperometric and potentiometric end-pGnt behavior, a series of voltammetr c sweeps in the low current region on platinum were obtained. Each of the curves in Figure 10 represents s solution of different composition in the vicinity of the equivalence point. It should be noted that the so ution potentials measured for these qolutions do not corrcyiond to the 1 otential of zero
current in the cases of excess Mn(II1). This behavior results from a combination of factors including surface oxidation, double layer charging and great irreversibility. Prior oxidation of the working electrode shifts the anodic portion of the curves to greater positive potentials. LITERATURE CITED
(1) Cooke, W. D., Reilly, C. Tu'., Furman, N. H., ASAL. CHEM. 23, 1662 (1951). (2) .DeFord, D. D., private communication, presented at the 133rd National
Meeting, ACS, San Francisco, Calif., April 1958. (3) Fenton, A. J., Furman, N. H., ANAL. CHEM.32, 748 (1960). (4) ~, Lineane. J. J.. Anson. F. C.. Ibid., 28, 1g71 (1956). ' (5) Meier, D. G., Myers, R. J., Swift E. H., J. Am. Chem. SOC.71, 2340 (1949). (6) Selim, R. G., Lingane, J. J., Anal. Chim. Acta 21,536 (1959). (7) Tutundzic, P. S., Mladenovic, S., Anal. Chim. Acta 12, 382, 390 (1955). RECEIVED for review August 8, 1962. Accepted March 11, 1963. Division of Analytical Chemistry, 142nd Meeting, ACS, A4tlanticCity, N. J., September 1962.
Coulometric Titration of Nitrite Using the Dual Inte rmedi(2tes Manga nese(III) a nd Iron(II) RICHARD P. BUCK (and THOMAS J. CROWE Bell & Howell Research Center, Pasadena, Calif. A, constant current :oulometric titration of nitrite has been devised, based on oxidation of nitrite to nitrate in the presence of excess, pregenerated Mn(ll1). Excess Mn(l1l) i s determined b y back-titration wit1 Fe(l1) using a null potentiometric end point. A relative standard deviation of i0.2% i s readily achieved for samples of 62 peq. of nitrite; but ihe deviation increases with decreasing sample size, becoming about =t1.6% a t the 0.3peq. level. A means for minimizing halide interference is described.
Electrolytically generated bln(I1I) for the coulometric titration of Fe(II), As(II1). and o.talic acid was introduced by Tutundzic and Madenovic (13). Current efficiencies for RSn(II1) generation on smooth platinum in sulfuric acid solutions were investigated by Selim and Lingane (11). Further measurements of current efficiencies on platinum and gold were made by Fenton and Furman ( 4 ) , and on wax-impregnated carbon and boron carbide by Buck (2). EXPERIMENTAL
A
SUJIBER of oxidative titrations of nitrite have been reported. The oxidants permanganatcx (j),dichromate ( 8 ) , cerium(1V) (15). bromate r6), hypochlorite ( 7 ) , or manganese(II1) ( 1 4 ) have been recommended. R e actions with the latter four titrants might reasonably be considered as the basis of secondary coulometric titrations. However, certain difficulties in the use of some of the intermediates can be inferred from t h s behavior of the potentiometric titration.. While electrolytic generation of Ce(1V) at high current efficiency is 1)ossible (3, IO), Willard and Young (15 ) demonstrated that, the titration of nitrite is quantitative only if the reaction mixture is warmed to 50" to 60" C. Electrochemical generation of bromate in basic solutions occurs by disproportionation of hypobromite ( I ) , but has not been shown to occur a t all in acidic solutions. Current efficiency for the generation of hypochlorite has not h e m reported and, in addition, its reaction with nitrite is slow ('7). I n contrast: manganese(II1) was shown by U h e l o h d e (14) to oxidize nitrite rapidly at room temperature.
Apparatus and Reagents. Currents constant t o 0.1% were obtained from a n electronically controlled constant current supply described elsewhere (2). Generation times were measured with a precision timer Model S-10 of the Standard Electric Time Co. The sensitive null potentiometric end point has been described (2). The titration vessel using a bright platinum wire indicator and low-resistance calomel reference cell was a water-jacketed 150-ml. lipless beaker fitted with a plastic cap into which the electrodes, K2S04-saturated salt bridge, gas dispersion tube for deaeration with prepurified nitrogen, and isolation tubes terminating in fritted glass disks were inserted. -4 solution volume of 75 ml., prior to sample addition, was used. Platinum foil and carbon rod generator electrodes were described previously ( 2 ) . A Teflon-covered magnetic stirring bar was used. The stock solution was 0.4F MnSOa, 0.12F Fe2(S04)3,and 4F H2S04; and was prepared according to the procedure of Fenton and Furman (4). The stock solution should be freed from reducing impurities b y boiling with a few crystals of potassium permanganate and filtering. A stock 0.03F sodium nitrite solution was prepared from C.P. material and
standardized against potassium permanganate previously standardized against sodium oxalate. Samples of sodium nitrite, ranging from 0.3 peq. to 62 peq. were determined coulometrically. X calibrated 1-nd. volumetric pipet and a series of calibrated micropipets ranging from 500 to 50 lambda were used for sample addition. Procedure. T h e procedure is based upon oxidation of nitrite t o nitrate in t h e presence of excess, pregenerated hln(II1). This step is necessary because nitrous acid is easily lost from acidic manganous sulfate solutions prior t o and during generation of Mn(II1). Excess N n ( I I 1 ) is backtitrated with electrolytically generated Fe(I1). A carbon or platinum internal generator electrode may be used. Carbon is preferred because the range of current densities yielding high generation efficiencies is n-ider. The solution must be thoroughly deaerated to prevent loss of Fe(I1) during the backtitration. However, nitrogen flow muqt be stopped when the sample is added or low results are obtained, presumably due to loss of intermediate S O n . The potentiometric titration curve for the reaction of Nn(II1) and nitrite spans a more limited potential range than that of the Fe(I1) titration. Latimer (9) reports a formal potential E" = -0.94 for the nitrite-nitrate couple compared n-ith E" = -0.68 for the Fe(I1)-Fe(TI1) couple in sulfuric acid media ( 1 2 ) . An end point potential of 900 mv. 1's. S.C.E. was used throughout. This potential is above the platinum oxide formation potential in this medium so that oxide layer is not reduced during the titration, thus removing a source of potential drift. Use of less positive preset potentials corresponding to a significant concentration of Fe(II), causes fleeting end points because of reduction of nitrate, in addition to oxide layer reduction. Initially, the end point potential is V O L . 35,
NO. 6 , M A Y 1963
697
established by incremental generation of Mn(II1) or Fe(I1) until a null potential is obtained. Then, excess Mn(II1) is generated, the sample added under the surface, and the indicator potential brought back to its preset value with Fe(I1). Care must be taken to avoid generation of excess Fe(II), since the electrode response is slow during subsequent generation of Mn(II1). Stabilization of the electrode potential is achieved most rapidly by approaching the preset potential from the positive potential side, corresponding to excess RIIn(II1). Prior t o the first titration, the indicator electrode should be conditioned b y shorting to the generator anode and Mn(II1) generated for a t least 30 seconds a t the 10-ma. rate. RESULTS A N D DISCUSSION
A relative standard deviation of 0.2% of the amount present is readily achieved for samples of 62 peq. or greater; but the deviation increases with decreasing sample size, becoming about 1.6% at the 0.3-peq. level. Many analyses above 12 peq. yield more accurate results than the generation efficiency permits. Apparently, a tendency for high results due to less than 100% Mn(II1) generation is compensated b y some unavoidable volatilization of nitrous acid during sample addition. Results of typical analyses are shown in Table I. Interferences and Precautions. Other oxidizable materials interfere i n t h e nitrite determinations. Among these are chloride, bromide, and iodide. Results for t h e determination of samples containing 6 peq. of nitrite in various mixtures with potassium chloride are shown in Table 11. Analyses are within experimental error when the chloride level is 0.003F. The errors increase a t higher chloride levels and result from oxidation of the chloride during sample addition. This can be verified by generating Mn(II1) for a known time and back titrating with Fe(II), after a chloride-containing sample has been titrated. The accumulated chloride concentration should not be allowed to increase above 0.018’. High results from chloride can be minimized by including in the solvent mixture, 0.01F Ag;PS08. Analyses of nitrite in 0.15F chloride are shown in Table 11. Higher concentrations of silver are not tolerable because silver is reduced on the cathode during the generation of Fe(I1). The end point should be purposely overrun and a second amount of Mn(II1) generated corresponding to approximately 10% of the original amount. Any plated silver is rapidly removed and the titration finished in the usual way. A t 0.01F AgXOS, the amount plated is small and can be conveniently oxidized in a few seconds a t the 10-ma. 698
ANALYTICAL CHEMISTRY
Table 1.
Titrations of Standard Nitrite Samples
peq.
Found, (av.), req.
62.05 30.80 12.50 6.205 3.080 1.241 0.6205 0.310
62.04 30.88 12.49 6.200 3.083 1.231 0.6172 0.309
62.05 30. SO 12.50 3.081 1.241 0.6205 0.310
62.04 30.93 12.37 3,075 1.231 0.6115 0.3087
Taken,
Yo. of samples
Rel. std. dev.
Std. dev.
CARBON QENERATOR ANODE
3
f O .10
f0.16 0.29 0.42 0.30 1.10 1.13 0.89 1.55
0.090 0.052 0.0185 0.034 0,014 0.0055 0.0048
11
4 6 6 4 4 7
PLATINUM GENERATOR ANODE
5
7
6
1
4 4 5
0.053 0.15 0.079
0.085 0.49 0.63
o.oi2 0.0054 0.0021
0.97 0.87 0.67
...
Table II. Titrations of Standard Nitrite, Chloride Mixtures
Mole ratio Cl-/NOt1
10 50 100
(Microequivalents of nitrite taken 6.137) KO*C1- in found, s o . of sample wq. samples 0.003F 6.179 6 6.220 4 0.03F 0.15F 7.16 2 0.3F 8.16 2
TITRATIONS WITH
Mole ratio X-/N0250 50
0.01F
Rel. error +0.68
+1.35 $16.7 33
+
SILVER NITRATE INCLUDED I S SOLVENT 5fIXTURE
X- in sample 0.15F X- = C10.15F X- = Br-
rate. Nitrite-bromide mixtures may also be titrated when silver is present. The precipitation step, as with chloride, appears to be rapid compared with halide oxidation. However, the null potentiometric end point is not stable as the end point is approached, due to slow, but significant loss of hln(II1). It is likely that the precipitated bromide is slowly attacked by Mn(II1). Manganese(II1) is a powerful oxidant, and trace-reducible impurities in the intermediate reagents and salt bridge solutions or the presence of agar in contact with the solution lead to potential drift corresponding to loss of Mn(II1). With proper preparation of reagents as described above, the null potential is very stable and solutions containing Mn(II1) a t 900 mv. have been observed to drift no more than 10 mv. while standing overnight. LITERATURE CITED
(1) Arcand, C . M., Swift, E. H., A 3 . x ~ . CHEW.28, 440 (1956). (2) Buck, R. P., Zbid., 35, 692 (1963)
6.120 6.159
-0.28 +0.36
8
3
(3) Fenton, A. J., Furman, N. H., Zhid.
29, 221 (1957). (4) Zhid., 32, 748 (1960). (5) Kolthoff, I. M., Belcher, R., “Volumetric Analysis,” Volume 111, p. 69, Interscience. New York. 1957. (6) Ihid., p. 526. (7) Ihid., p. 591. (8) Ihid.. w. 182. (9) Latiger, W., “Oxidation Potentials,” D. 93, Prentice-Hall, New York, 1952. (10) Lingane, J. J., Langford, C. H., Anson, F. C., Anal. Chim. Acta 16, 165 (1957). (11) Selim. R. G.. Linmne. J. J.. Zhid.. ’ 21, -536 (1959). (12) Swift, E. H., “A System of Chemical Analvsis.” w. 542. Prentice-Hall. Xew York, 1946.(13) Tutundaic, P. S., Mladenovic, S., Anal Chim. Acta 12. 382, 390 (1955). (14) Ubbelohde, A. R. J.’ R., ‘J. Chem. SOC.1935, 1505. (15) Willard, H. H., Young, P., J . Am. Chem. SOC.55, 3260 (1933); 51, 139 (1929); 50, 1379 (1928). ’
I
-
I
RECEIVEDfor review January 25, 1963. Accepted March 11, 1963. Division of Analytical Chemistry, 142nd Meeting, ACS, Atlantic City, N. J., September 1962.
