ANALYST, JANUARY 1993, VOL. 118 105 Simultaneous Determination of Iodide and Nitrite by Catalytic Kinetics Zhong-liang Zhu and Zhi-cheng Gu* Department of Chemistry, Tongji University, Shanghai 200092, People's Republic of China Nitrite and iodide can be determined simultaneously by a single experiment using their kinetic effect on the colour fading of the iron(i1)-thiocyanate complex in nitric acid solution. The rate of the colour-fading reaction is in proportion to the concentration of iodide and is independent of the concentration of nitrite. The length of the induction period of the indicator reaction is in inverse proportion to the logarithm of the concentration of nitrite and is independent of the concentration of iodide. Under conditions of 1.3 rnol 1-1 HN03, 0.067 moll-' Fe3+ and 2.7 x 10-4 rnol 1-1 SCN-, concentrations of 4.0 x 10-5-1.6 x 10-4 rnol I-' iodide and 4.1 x 10-7-6.6 x 10-6 rnol 1-1 NO2- were determined with mean relative errors of 1.5 and 3.9%, respectively.Keywords: Kinetic catalytic method; simultaneous determination; iodide and nitrite determination Although some methods for the simultaneous determination of two or more catalysts by catalytic kinetics without prior separation have been described, most need two or more kinetic runs under different conditions, such as pH,' tem- perature,2 concentration of reagents,3 etc. Weiser and Pardue4 have developed a method for the simultaneous kinetic determination of catalysts based on differences in the rates of inhibition. A method for the simultaneous determination of Br- and I-, based on their different catalytic behaviours, was carried out recently with a single kinetic run.5 The catalytic action of iodide on the colour-fading reaction of the iron(rr1)-thiocyanate complex was used to determine iodine .b This indicator reaction was characterized by an induction period, the length of which depended on the concentration of nitrite.This kinetic effect has been utilized to determine the concentration of nitrite.7 It has been found that in a certain range, the length of the induction period only depended on the concentration of nitrite. Changes in the NO?_- concentration had no influence on the rate of colour- fading. Only changes in the I- concentration influenced the rate of the indicator reaction, although they did not cause changes in the induction period.Hence it could be used for the simultaneous determination of I- and NOz- in a single kinetic run. So far there has been no report about the simultaneous determination in just one kinetic run using induction periods and reaction rates. This work concentrates on the simul- taneous determination of a mixture using only one kinetic run. Experimental Apparatus and Reagents A Model F7230 spectrophotometer was used to record the changes in absorbance over time. The reaction was performed at 24.0 "C using a Model 501-type thermostatically controlled bath. The data was handled using an IBM PC computer. All of the chemicals used were of analytical-reagent grade, except p-aminobenzene sulfonic acid, which was of primary-standard grade.Sub-boiling distilled water was used. Ammonium iron( 111) sulfate-nitric acid mixture solution. A 0.10 rnol I-' ammonium iron(ii1) sulfate solution was prepared in 2 moll-' nitric acid. Before use urea was added to the nitric acid, in order to remove the nitrous acid, and the acid boiled t o hydrolyse the excess of urea remaining. Potassium thiocyanate solution. A 0.10 rnol 1-1 solution of KSCN was prepared. It was diluted to 0.0050 rnol 1-1 prior to use. * To whom correspondence should be addressed. Potassium iodide standard solution. A 0.10 rnol 1-1 solution of KI was prepared and stored in a brown bottle. The solution was standardized against AgN03 by potentiometric titration and diluted to 5 x 10-4 rnol 1-1 prior to use. Standard nitrite solution. A 0.02 moll-' solution of NaN02 was prepared and stored in a brown bottle.The concentration was standardized against p-aminobenzene sulfonic acid as follows. About 0.5 g of acid was dried at 120 "C to constant mass, which was determined to an accuracy of +0.0002 g, then the acid was dissolved in 20 ml of NH3-H20, and about 20 ml of HCI and 150 ml of H20 was added, the mixture was titrated with NaN02 by potentiometry.8 The solution was diluted to 4.3 x 10-4 rnol 1-1 prior to use. Procedure A sample solution containing I- and NO2- was placed in a 25 ml standard calibrated flask, 2.0 ml potassium thiocyanate solution was added and diluted to exactly 25 ml (solution A). Both solution A and 0.10 rnol 1-1 ammonium iron(i1i) sulfate-2 moll-' HN03 solution (solution B) were placed in a thermostatically controlled water-bath at 24.0 "C until the temperature remained stable.A 1.0 ml aliquot of solution A was transferred into a cuvette and 2.0 ml of solution B added rapidly. The absorbance change with time at 458 nm, measured against water, was recorded synchronously. A typical pattern for the reaction curve is shown as Fig. 1. Data Processing The linear part of the colour-fading, shown in Fig. 1, was prolonged and intersected the extrapolation of the line at the initiation of the curve, shown as point P. The time correspond- ti Time - Fig. 1 present of I- and NOz- (A-t relation curve) Change in absorbance of FeSCN2+ complex with time in the106 ANALYST, JANUARY 1993, VOL. 118 1.5 1.2 g 0.9 g m 2 0.6 a 0.3 0 1 I 1 I 180 360 540 720 Time/s 1 900 Fig.2 Dependence of A-t relation on [Fe"]. Conditions: HN03, 1.3 moll-1; SCN-, 3.3 x 10-4 moll-I; I-, 1.1 X moll-l; NOz-, 2.3 x 10-6 rnol 1-1; and temperature 25.0 "C. [Fe"]: A, 0.033; B. 0.067; C, 0.100; and D. 0.133 rnol 1-1 1.5 1.2 ! 0.9 e 2 0.6 a 0.3 1.5 I I I 1.2 a, ; 0.9 g 2 0.6 a 0.3 0 180 360 540 720 900 Time/s Fig. 4 3.3 x 3. [HNOs]: A, 1.3; B, 2.0; and C, 2.7 rnol I-1 Dependence of A-t relation on [HN03]. Conditions: SCN-, mol 1-1; temperature, 25.0 "C; others are the same as Fig. 1.2 a, E 0.9 g 2 0.6 a 0.3 I I I I 0 180 360 540 720 900 Time/s Fig. 3 Dependence of A-f relation on [SCN-1. Conditions: HNO?, 1.3 rnol I - I ; Fe3+, 0.067 rnol I-'; I - , 1.6 x 10-3 moll-1; NO2-, 4.6 x 10-"mol I-'; and temperature 20.0 "C. [SCN 1: A , I .3 x 10-4; B, 2.0 x 10F; C, 2.7 x and D, 3.3 x 10-d rnol 1-1 0 180 360 540 720 900 Time/s Fig.5 Dependence of A-t relation on temperature. Conditions: Fe3+, 0.067 mol 1-1; others are the same as Fig. 2. Temperature: A , 21.7; B, 24.6; and C, 27.5 "C ing to point P was defined as the induction period ti. The rate of the colour-fading reaction (Y) was characterized by the slope of the linear part of the curve. Results and Discussion Effect of the Concentration of Fe3+ Ion The initial absorbance and the length of induction period increased with increasing Fe3+ concentration, while the reaction rate decreased (see Fig. 2). Lower concentrations of Fe3+ gave higher sensitivities for iodide determination. Very low concentrations of Fe3+ caused difficulties in the recording of the reaction stage because the coordinated reaction of Fe3+ with SCN- was very incomplete.An excess of Fe3+ ensured that the indicator of the reaction was only FeSCN2+. Hence 6.7 x 10-2 rnol 1-1 Fe3+ was specified in the procedure. Effect of the SCN- Concentration The effect of SCN- concentration on the reaction rate was marked, but the length of induction period remained almost unchanged (Fig. 3). The initial absorbance increased linearly with increasing SCN- concentration. Over the range 1.3 x 10-4-3.3 x rnol 1-1 SCN-, the sensitivity for the determination of I- increased with increasing SCN- concen- tration. A concentration of 2.7 x 10-4 rnol 1 - I SCN- was specified in the procedure. Effect of HN03 Acidity It was found that the higher the concentration of HN03 the shorter the induction period and the more rapid the reaction rate.When the concentration of HN03 was more than 6 mol 1-1, the fading reaction of the Fe3+-SCN- complex was completed in about 1 min without any catalyst. When the concentration of HN03 was lower, the time of the reaction was too long. The effect of different HN03 concentrations are shown in Fig. 4. The effect of HN03 concentration on both the reaction rate and the induction period was marked. The sensitivity for the determination of I- was increased with increasing HN03 concentration, but the sensitivity for NO2- determination was markedly decreased. Moreover, the rate of the blank reaction also increased with the increase of HN03 concentration. So as a compromise between sensitivity and time, the concentration of HN03 had to be between 1.0 and 2.0 rnol 1-1.Hence 1.3 rnol 1 - 1 HN03 was specified in the procedure. Effect of Temperature It can be seen from Fig. 5 that the reaction was accelerated and the induction period shortened with an increase in tempera- ture. Also the sensitivity for the determination of 1- was increased and the sensitivity for the determination of NO2- reduced. So different temperatures would have to be used when I- and NOz- are present in different proportions. The 24 "C temperature selected is restricted by the conditions of the experiment. According to the Arrhenius equation, the -Inv-T-1 plot (as shown in Fig. 6) was a straight line between 19.5 and 29.5 "C. The regression equation was obtained by least-squares fitting: -Inv = bl/T + a l (1) where bl = 4.54 x 103 and a l = 0.12.The activation energies can be calculated from the slope of the regression curve of '-lnv-l/T. In this work it was 3.77 x l o 4 J mol-1.ANALYST, JANUARY 1993, VOL. 118 ;_350/ 250 150 107 I I I I I -6.2 5.2 3.30 3.32 3.34 3.36. 3.38 3.40 3.42 1/T x 103 K Fig. 6 Effect of thc tempcraturc on reaction rate (v) and induction period (t,). A. -Inv-l/C and B. -lnt,-l/T. All conditions are the same as Fig. 5 +\ \ - I I I 1.2, 1 0 180 360 540 720 900 Time/s Fig. 7 De endence of A-t relation on [I-]. Conditions. HN03, 1.3 10-6 moll-1; and temperature, 24.0 "C. [I-]: A, 0; B, 8.00 x 10-5; C, 1.07 x and D, 1.33 x rnol 1-1 moll-1; Fe-+, P 0.067 rnol l - l ; SCN-, 2.7 x 10-4 moll-1; NO2-, 4.6 x 1.2, 0 0.3 - I I 1 I I 0 180 360 540 720 900 Time/s Fig.8 Dependence of A-t rclation on [NOz-]. Conditions: I-, 1.1 x 10-4 moll-1; others are the samc as Fig. 7. [NOz- 1: A, 5.8 x 10-7; B , 1.2 x 10-6; C, 2.3 x 10-6; D, 4.6 x 10-6; and E, 9.3 x 10-6 rnol I-' The -Intl-T-l plot over the same temperature range was also linear (Fig. 6). The regression equation is -Inti = b2/T + a2 (2) where bz = -6.82 x 103 and a2 = 0.86. Effect of I- and NO2- on Each Other When the NO2- concentration was kept constant changing the I- concentration only caused a change in the slope of the curve, it caused no change in ti (Fig. 7). On the other hand, changing the NO2- concentration gave rise to the changes in the length of induction period, and not to changes in the slope (Fig. 8). So it was possible to determine these two ions simultaneously.Calibration Graphs The calibration graph for the determination of the I- is shown in Fig. 9. The regression equation was v = sIcI- + kl (3) where s1 = 15.05 and k l = 8.07 x 10-4. I I I I 0 0.5 1 .o 1.5 2.0 [l-]/lO4 mol I-' 1.0 I Fig. 9 Calibration graph (v-[I-]). Conditions: HN03, 1.3 rnol 1-1; Fe3+, 0.067 mol 1-1; SCN-, 2.7 X 10-4 rnol I-'; and temperature, 24.0 "C 550 ', 1 Ln[ NO2-] Fig. 10 Calibration graph (4-ln[NOz-]). HN03, 1.3 rnol 1-1; Fe3+ 0.067 mol 1-1; SCN-, 2.7 x 10-4 mol 1-1; and temperature, 24.0 "C Table 1 Analysis of mixtures of I- and NO2- I-/lOs rnol 1-l N02/107 moll-l Added 16.00 14.67 13.33 12.00 10.67 9.33 8.00 6.67 5.33 4.00 Found 15.9 14.3 13.3 11.8 10.6 9.52 8.06 6.82 5.40 3.90 Relative error (%) -0.6 -2.5 -1.0 -1.7 -0.6 2.0 0.7 2.3 1.2 -2.5 Added 4.10 5.79 8.19 11.58 16.38 23.17 32.76 46.33 65.53 32.76 Found 3.95 5.31 7.93 11.6 17.0 24.9 33.4 50.1 64.1 32.7 Relative error (YO) -3.5 -8.3 -3.2 0.1 3.8 7.5 1.9 8.1 -2.2 -0.2 The calibration graph was rectilinear over the range 4 X 10-5-2 x 10-4 rnol 1-1.The length of induction period was inversely proportional to the logarithm of the NOz- concen- tration by experiment. The relationship between ti and In[N02-] is shown in Fig. 10. The regression equation was ti = s21ncNo2- + k2 (4) where s2 = -95.3 and kz = -875. The calibration graph was rectilinear over the range 5 x 10-7-9 x 10-6 rnol 1 - 1 . The correlation coefficient (Y) for both of the regression equations was 0.9998. Determination of I- and NO2- in Mixtures Several synthetic sample solutions containing 4.0 x 10-5-1.6 x 10-4 rnol 1-1 I- and 4.1 x 10-7-6.6 x 10-6 moll-1 NOz- were determined.The results are listed in Table 1. The maximum relative errors for T- and NO2- were 2.5 and 8.3%,108 ANALYST, JANUARY 1993, VOL. 118 respectively. The values for the mean relative standard deviation were 1.5 and 3.9% , respectively. The precision for the determination of N02- was less than for I-. One of the important reasons for this was that the effect of fluctuations in temperature on the sensitivity of the determination of NO2- and I- was different. For the determination of I- [From eqn. (l)], the following equation is obtained: dlnv/dT = b l / P or dv/dT = blv/T2 ( 5 ) dci-ldv = l / ~ l (6) From eqn. (3), we get Combining eqns.( 5 ) and (6), we have dci-/dT = blV/(SI P ) If cI- = 1.1 x 10-4moll-1, T = 297.2 K and v = 3.64 x 10-3, then we get dcl- = 1.2 X 10-5 dT. If the fluctuation in the temperature (AT) = 0.1 "C then Ac- = 1.2 x 10-6 mol 1-1. So the relative error is 1.1%. Similarly, for the determination of NO2-, the precision can be obtained from eqn. (2) dlnti/dT = b 2 / P or dti/dT = b2ti/72 (7) dlncNo2-/dti = 1/s2 (8) From eqn. (4), we get Combining eqns. (7) and (8), we have dhcNo2-/dT = b2ti/s2T2 or dcNO2-/cNo2- = b2ti/s2T2dT. When T = 297.2 K, ti = 318.7 s and AT = 0.1 "C, then AcNO2-/cNo2- = 2.6%. So the error in the results for the determination of NO2- would be larger than those for the determination of I- with the same temperature fluctuation. Mechanism of the Reaction The I- does not act as a catalyst on the decomposition of the thiocyanate-iron(ir1) complex.Although it accelerates the reaction in the presence of NO2-, the true catalyst is an oxidation product of I-. It has been found that iodine cannot significantly accelerate the reaction without the presence of N02-. The true catalyst may be iodine(1). The oxidation of I- to iodine(1) is a slow reaction, NO2- can accelerate this reaction, which is why there is an induction period in the catalytic reaction and the length of the induction period depends on the N02- concentration. Owing to the complexity of the mechanism, some steps of the reaction are still unclear. This work was supported in part by National Science Foundation of the People's Republic of China. References Yatsimirskii, K. B., and Raizmann, L. P., Zh. Anal. Khim., 1963, 18, 829. Wolff, C. M., and Schwing, J . P., Buff. SOC. Chim. Fr., 1976, 679. Worthington, J . B.. and Pardue, H. L., Anal. Chem., 1970,42, 1157. Weiser, W. E., and Pardue, H. L., Anal. Chem., 1986,58,2523. Yonehara, N., Yamane, T., Tomiyasu, T., and Sakamoto, H., Anal. Sci.. 1989, 5 , 175. Iwasaki, I . , Utsumi. S., and Ozawa, T., Buff. Chem. SOC. Jpn., 1953, 26, 108. Utsumi, S . , Okutani, T., Sakuragawa, A., and Kenmotsu, A . , Buff. Chem. SOC. Jpn., 1978, 51, 3496. Chinese Standard for Chemical Reagents 1984 (GB 1261-77), Chinese Standard Press, Beijing, 1986, p. 34. Paper 2/01549H Received March 24, 1992 Accepted October 8, I992