98 Analyst, February, 1967, Vol. 92, $9. 98-102 The Amperometric Titration of Submillinormal Concentrations of Hexacyanoferrate (111) with Mercury (I) Perchlor ate BY JOHN T. STOCK AND R. J. MERRER (Department of Chemistry, The Universit3. of Connecticut, Stows, Connecticut 06268, U.S.A .) Although quite precise under rigidly controlled conditions, the mercury (I) amperometric titration, a t a rotating platinum electrode, of submillinormal concentrations of hexacyanoferrate(II1) in sodium hydroxide - potassium iodide medium gives results that vary with the concentrations of alkali, iodide and hexacyanoferrate (111). Dependence upon solution composition is small in perchloric acid - potassium thiocyanate medium. In this medium, the titration of 5 x to 1 3 - 3 ~ hexacyanoferrate(II1) is precise and accurate to within k1.5 per cent.N hexacyano- ferrate(II1) is precise to about 5 per cent. The titration of 5 x THE mercury(1) perchlorate potentiometric titration of approximately 0.01 N hexacyano- ferrate(II1) in sodium hydroxide - potassium iodide medium is stated to be satisfact0ry.l The titration can also be carried out in acidified potassium thiocyanate m e d i ~ m , ~ , ~ in which precise results for hexacyanoferrate(II1) concentrations of approximately 0.004 to 0.023 N are r e p ~ r t e d . ~ The present work concerns the amperometric mercury( I) titration of sub- millinormal concentrations of Eiexacyanoferrate(II1) ion by methods similar to those used for iron (I I I) ,4 copper (I I)5 and iodine. EXPERIMENTAL VOLTAMMETRY- The voltammetry of mercury(1) and mercury(I1) in acid thiocyanate and acid thio- cyanate - iodide media has already been de~cribed.~?~ In 0.01 N perchloric acid - 0.5 N potas- sium thiocyanate, the limiting current of hexacyanoferrate(III), measured at a fixed potential within the range +0.1 to -0.1 volt (all potentials are with respect to the saturated calomel electrode, S.C.E.), was found to be proportional to concentrations of this ion when these were not greater than about 8 x 1 0 - 4 ~ .A linear relationship between the current and the concentration of hexacyanoferrate(III), at a fixed potential in the same range, was also obtained in N sodium hydroxide - 0 . 4 ~ potassium iodide. Hexacyanoferrate( 11) is not electroactive in either medium over this potential range.Fig. 1 shows current - voltage curves obtained at various stages in the amperometric titration of 5 x N hexacyanoferrate(II1) in alkaline iodide medium with mercury(1) perchlorate solution. A similar group of curves obtained in acid thiocyanate medium is shown in Fig. 2. AMPEROMETRIC TITRATIONS- All titrations were run at zero potential and at room temperature (in the range 24" to 27" C). End-points in acid media were located by procedures (A) and (B) (below) and by the L-curve m e t h ~ d . ~ 9' In alkaline-media titrations, which were carried out by similar methods, clogging resulted when the tip of the microburette containing mercury( I) perchlorate solution was immersed in the solution being titrated. The tip was therefore placed a few millimetres above the solution and each addition of titrant was rinsed down with four successive drops (a total of 0-2ml) of water.STOCK AND MERRER 99 METHOD REAGENTS- Use analytical-grade reagents and distilled or de-mineralised water throughout.Mercury ( I ) perchlorate, approximately 0.1 N in N Perchloric acid-Prepare, dilute as required with N perchloric acid, and standardise by the dichromate - iodide method, as described by Berka, Vulterin and Zyka.* Store over metallic mercury and shake the solution thoroughly before use. Potassium hexacyanoferrate(II1) , approximately 0.1 N-Dilute as required and standardise iodimetrically . Perchloric acid, approximately 0.02 N. Potassium thiocyanate, approximately N. Use conventional apparatus for amperometric titration at a rotating platinum electrode that is maintained at zero potential.5 Clean and pre-condition the electrode as de~cribed,~ but use potassium hexacyanoferrate(II1) as the substance titrated. The platinum electrode used in the present work was rotated at 600 r.p.m.It then had a sensitivity of 0-0298 pA per micromole of hexacyanoferrate(II1) per litre, measured at zero potential in de-oxygenated 0.5 N potassium thiocyanate - 0.01 N perchloric acid at 25" C. . +0I -0.1 -0.2 -03 Potential, volts Fig. 1. Current - voltage curves at stages in the titration of 5 x N hexacyanoferrate(II1) in N sodium hydroxide - 0 . 4 ~ potassium iodide. Percentage equivalent of mercury( I) perchlorate added: curve A, 0 ; curve €3, 50; curve C, about 100; curve D, 150 f U C t! 3 U +03 + 0 2 +O.I 0 -0.1 4 2 Potential, volts Fig. 2.Current - voltage curves a t stages in the titration of 5 x 1 0 - 6 ~ hexacyanoferrate(II1) in 0 . 0 1 ~ perchloric acid - 0-5 N potassium thio- cyanate. Percentage equivalent of mercury( I) perchlorate added: curve A, 0 ; curve B, 50; curve C, about 100; curve D, 150 PROCEDURE- ( A ) Transfer 50 ml of 0.02 N perchloric acid and 50 ml of N potassium thiocyanate to the titration cell. Insert the platinum electrode and salt bridge, de-oxygenate with a stream of nitrogen, then stop the gas stream. Inject 0.01 N potassium hexacyanoferrate(II1) so that the amount introduced is about 30 per cent. of that contained in the sample solution. After 2 minutes, note the current reading, P, then inject the sample solution. Read the current after a further 2 minutes, then titrate with 0-01 to 0.1 N mercury(1) perchlorate until the current has fallen nearly to zero.Allow an interval of 1 minute between a titrant addition and the reading of the current. Find the end-point graphically as the intersection of the linear portion of the titration curve and the line: current = P. (B) Proceed as in ( A ) up to the stopping of the gas stream. Note the residual current, R, then at once inject the sample and titrate as in ( A ) . Find the end-point graphically by producing the linear portion of the titration curve to cut the line: current = R.100 STOCK AND MERRER: AMPEROMETRIC TITRATION OF [ArtaZyst, Vol. 92 RESULTS TITRATIOK IN ALKALINE IODIDE MEDIA- N hexacyanoferrate(II1) ion in sodium hydroxide - potassium iodide media.The actual normality of the titrant was 0.0932. Although the results are fairly precise, they are noticeably influenced by the com- position of the medium. Such an effect is not necessarily intolerable in routine titrimetry. Table I gives the results obtained in triplicate titrations of TABLE I EFFECT OF HYDROXIDE AND IODIDE CONCENTRATION IN THE TITRATION OF 10-4 N HEXACYANOFERRATE(III) Hydroxide concentration, 0.1 0.5 1.0 1-5 2.0 4.0 1.0 1.0 1.0 N Iodide concentration, 0.4 0.4 0.4 0.4 0.4 0.4 0.1 0.6 0.9 N Apparent mercury( I) normality Procedure ( A ) 0.088 * 0.002 0-089 & 0.002 0*093,& 0.001," 0.100 * 0.001 0.102 & 0.005 0.128 0-002 0.087 * 0.001 0.105 f 0.002 0.192 0.004 * 30 runs. L-curve 0.087 f 0.002 0.086 f 0.002 0.088, j: 0.002," 0.098 & 0.001 0.100 & 0.006 0.126 0.008 0.085 f 0.001 0.101 & 0.003 0.185 f 0.003 1 Procedure (B) 0.088 0.002 0-087 f 0.003 0-090, & 0.001,* 0.100 & 0.002 0.101 f 0.005 0.128 & 0.007 0.086 & 0.001 0.102 & 0.002 0.185 5 0.003 However, the results obtained in N sodium hydroxide - 0.4 N potassium iodide are accurate only at a hexacyanoferrate(II1) concentration of about 10-4~ (Table 11).An alkaline iodide medium cannot therefore be recommended for the mercury( I) titration of submillinormal concent rations of hexacy anof errat e (I I I) ion. TABLE I1 TITRATION OF HEXACYANOFERRATE(III) IN N SODIUM HYDROXIDE - 0.4 N POTASSIUM IODIDE Hexacyano- ferrate (I 11) concentration, (1.N 10 20 50 100 200 500 1000 Number of runs 6 4 9 30 3 3 6 Apparent mercury(1) normality L-curve I h 3 Procedure ( B ) 0.116 f 0.030 0.097 f 0.009 0.108 & 0.012 0.103 & 0.006 0.098 & 0-008 0.102 & 0.008 0.093, & 0.001 0.088, f 0.002, 0.090, * 0.001, Procedure ( A ) 0.092 & 0.004 0.089 & 0.004 0.093 & 0-004 0.078 & 0.002 0.077 5 0.002 0.077 & 0.002 0.081 & 0.001 0.082 4: 0.002 0.081 f 0.001 0.085 0.002 0-084 0.002 0-084 f 0.002 TITRATION IN ACID THIOCYANATE MEDIA- The normality of the mercury(1) perchlorate solution used for titrations in acid thio- cyanate media was 0.1017.Table I11 lists the results obtainedin triplicate runs at a hexacyano- ferrate(II1) concentration of N. Although it needs to be fairly high, the concentration of thiocyanate can be varied without noticeable effect upon precision or accuracy.Runs made in 0.01 N perchloric acid - 0.5 N potassium thiocyanate, which gave optimum results at a hexacyanoferrate concentration of N, showed that this medium is also satisfactory for the titration of both higher and lower concentrations of this ion (Table IV). Procedure ( A ) is precise and accurate to within 1.5 per cent. for hexacyanoferrate(II1) concentrations of from 5 x to 10-3 N, or to within 1 5 per cent. for concentrations down to 5 x 10-6 N. Procedure (B) is a little less precise than procedure (A) and is less accurate. However, procedure ( B ) becomes satisfactory if it is also used for the actual standardisation of the titrant. The results obtained by the L-curve method are decidedly inaccurate and are generally more erratic than the results given by procedures ( A ) and (B).Procedure (B) is useful when the sample is presented as a highly dilute solution. The sample is made 0.01 N in perchloric acid, de-oxygenated, made 0-5 N in potassium thiocyanate,February, 19671 SUBMILLINORMAL CONCENTRATIONS OF HEXACYANOFERRATE(III) 101 TABLE I11 TITRATION OF N HEXACYANOFERRATE(III) IN ACID THIOCYANATE MEDIA Thiocyanate concentration, 0-05 0.2 0.25 0-3 0.5 0.5 0.5 0.5 0.8 1.0 1.5 3.0 N Perchloric acid concentration, 0.02 0.02 0.02 0.02 0.01 0.02 0.05 0.08 0.02 0.01 0.01 0.01 N TITRATION OF Hexscyano- ferrate( 111) concentration, PN 1 5 10 50 100 1000 Apparent mercury(1) normality Procedure ( A ) 0.098 f 0.002 0.098 f 0-002 0.095 f 0.002 0.099 -+ 0*002* 0.101 & 0.001 0.100 f 0.001 0.099 f 0.001 7 0.101 f 0.001 0.100 f 0.001 0.101 f 0.002 0.100 f 0.001 * 6 runs.t Medium decomposc :d. L-curve 0.094 f 0.002 0.090 f 0.002 0.090 f 0.004 0.091 f 0*002* 0.097 f 0.001 0.096 f 0.001 0.092 f 0.002 t 0.100 f 0.001 0.094 & 0.002 0.094 f 0.001 0.097 f 0.001 Procedure '(B) 0.093 f 0.003 0-092 f 0.002 0.092 f 0.002 0-093 f 0.002* 0.099 f 0.001 0.096 f 0.001 0.094 f 0.001 t 0.103 f 0.001 0.097 f 0.001 0.096 5 0.001 0.098 f 0-001 TABLE IV HEXACYANOFERRATE(III) IN 0.01 N PERCHLORIC ACID - 0.5 N POTASSIUM THIOCYANATE Number of runs 3 6 6 6 10 6 Apparent mercury(1) normality f 1 Procedure ( A ) L-curve Procedure ( B ) 0.0145 & 0.0009* 0-0094 f 0.0006* 0.0101 -+ 0.0004* 0.0102 f 0*0004* 0.0087 f 0-0005* 0.0092 f 0.0004* 0.0098 & 0*0002* 0.0078 f 0*0003* 0.0086 & 0*0004* 0.1011 f 0.0004 0.0967 f 0.0004 0.1005 f 0.0009 0.1005 & 0.0011 0.0953 f 0.0019 0.0980 f 0.0016 0.1028 0.0011 0.0990 f 0.0007 0.1002 f 0.0004 * Titrant diluted 10-fold.de-oxygenated for 5 to 10 minutes longer, and then titrated. A separate 0.01 N perchloric acid - 0-5 N potassium thiocyanate solution is used for residual-current determination. Three results thus obtained with N hexacyanoferrate(II1) were precise and accurate to within 5 per cent. With the availability of a suitable ultramicroburette, the volume of hexacyanoferrate(II1) solution required for titrationlo could probably be reduced by a factor of at least 50. DISCUSSION Titrations in alkaline medium (Tables I and 11) give apparent mercury(1) normalities that are high when the concentrations of hydroxide, iodide and hexacyanoferrate( 111) are high, high and low, respectively.The normalities are low when the concentrations of hydroxide or iodide are reduced, or when the concentration of hexacyanoferrate(II1) is in- creased. These trends suggest the occurrence of side reactions that cause the destruction of the substance being titrated and of the titrant. Accurate results in the titration of N hexacyanoferrate(II1) in N sodium hydroxide - 0.4 N potassium iodide probably arise from mutual compensation of these sources of error. The spontaneous destruction of hexacyanoferrate( 111) in alkaline iodide media was noted by Burriel-Marti, Lucena-Conde and Arribas- Jimen0.l In the present work, measurements of the limiting currents of N hexacyanoferrate(II1) in N sodium hydroxide - 0.4 N potassium iodide made 1, 10, 20 and 30 minutes after injection of hexacyanoferrate(III), were found to be in the ratio 100: 91 : 84: 78, respectively.As from 10 to 15 minutes must elapse in the amperometric titrations of submillinormal concentrations of hexacyanoferrate(III), the effect is an obvious source of error. At high ionic strength, the formal potentials of the hexacyanoferrate(II1) - hexacyano- ferrate(I1) and iodine (or tri-iodide) - iodide couples are probably not very different.l1,l2 An incipient liberation of iodine will be favoured by increasing the concentrations of potassium102 STOCK AND MERRER iodide and sodium hydroxide. The kinetic aspects of the hexacyanoferrate - iodide reaction in neutral solutions have been extensively studied.The moderately slow reaction is speeded by increasing the iodide concentration or the ionic strength, and by the presence of metallic p1atin~m.l~ In alkaline medium, iodine rapidly passes into hypoiodite and iodide ions. Hypoiodite then disproportionates at a measurable rate into iodate and iodide ions,14 so that the over-all effect is loss of hexacyanoferrate(II1). In the present work, the introduction of approximately N hexacyanoferrate(II1) in N sodium hydroxide - 0.4 N potassium iodide did not change the limiting current or the volume of titrant needed. When made alkaline] mercury(1) perchlorate disproportionates and a precipitate of mercury(I1) oxide and metallic mercury is the final re~u1t.l~ Although this effect may not occur when hexacyanoferrate(II1) is titrated in the presence of much iodide, it should become significant as the iodide concentration is reduced.The erratic behaviour at comparatively high hexacyanoferrate(II1) concentrations (Table 11) may be caused by high local concen- trations resulting from larger titrant additions. N hexacyanoferrate(II1) ion in 0.01 N perchloric acid - 0.5 N potassium thiocyanate was found to be unchanged after ageing for 30 minutes. High titrant normalities are therefore not to be expected. Disproportionation of mercury(1) ion in thio- cyanate medium is po~sible,~ and may account for the somewhat low titrant normalities observed at thiocyanate concentrations of less than 0-5 N. This work was carried out with the partial support of the United States Atomic Energy Commission (Contract AT(30-1)-1977). M potassium iodate into The limiting current of 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. REFERENCES Burriel-Marti, F., Lucena-Conde, F., and Arribas- Jimeno, S., Analytica Chim. Acta, 1954, 10, 301. Tarayan, V. M., and Arutyunyan, A. A., Izv. Akad. Nauk Armyan. SSR, Fiz-Mat. Estestven. Lucena-Conde, F., and Bellido, I. S., Talanta, 1958, 1, 305. Stock, J. T., and Heath, P., Analyst, 1965, 90, 403. Stock, J . T., Ibid., 1966, 91, 27. -, Ibid., 1966, 91, 280. --, “Amperometric Titrations, ” Interscience Publishers, a division of John Wiley and Sons Berka, A., Vulterin, J., and Zyka, J., Chemist-Analyst, 1963, 52, 122. Kolthoff, I. M., and Sandell, E. B., “Textbook of Quantitative Inorganic Analysis,” Third Edition, Stock, J. T., 09. cit., p. 108. Willard, H. H., and Manalo, G., I n d . Engng Chem. Analyt. Edn, 1947, 19, 462. Laitinen, H. A., “Chemical Analysis : An Advanced Text and Reference,” McGraw-Hill Book Co. Spiro, M., and Ravno, A. B., J . Chem. SOG., 1965, 78, and references cited. Morgan, K. J., Q. Rev. Chew. SOG., 1954, 8, 123. Lingane, J . J . , “Analytical Chemistry of Selected Metallic Elements,” Reinhold Publishing Corp., Received August 16th, 1966 i Z’ekh. Nauki, 1950, 3, 651. Inc., New York, London and Sydney, 1965, chapter 1, p. 8. Macmillan Co., New York and London, 1952, p. 595. Inc., New York, Toronto and London, 1960, pp. 290 and 393. New York, 1966, p. 79.