Analysis of Mercuric ion-Anion Mixtures JAMES B. FERNANDEZ, LLOYD T. SNIDER, AND EDWARD G. RIETZ University of Florida, Gainesville, Fla. Mercuric ion and acetone were observed to react, with the production of hydrogen ion and a complex mercury compound. As a result of the complex forination, mercuric ion may be determined alkalimetrically and chloride, bromide, thiocyanate, and iodide ions may be determined argentimetrically. The method described is a more simple procedure than those commonly employed for the analysis of mercuric ion-anion in ix t I Iree.
THE
of mercuric ion. The avei'age deviation from the mean was 1 2 . 4 parts per thousand, the standard deviation was ~ 2 . parts 9 per thousand, and the probable deviation was *2.0 parts per thousand. The effect of excess amounts of chloride ion w m next inveetigated. As indicated by the data of Table I, relatively large concentrations of chloride ion-as much as 10 moles of chloride ion per mole of mercuric ion-had no adverse effect upon the arruracy of the method.
analytical procedure herein described was developed when i t was attempted to perform a thiocyanate determination of mercuric ion remaining in a mercuric chloride-acetone reaction mixture. The thiocyanate method requires the complete absence of halide ion and, for this reason, analyses of mercuric ion-halide ion mixtures are normally preceded by isolation of the mercury as mercuric oxide via alkali precipitation. Application of this procedure failed to effect any precipitation in the solution under consideration; the only reaction observed was a neutralization of the alkali and the appearance of a Plight yellow turbidity which disappeared on standing. Investigation of the literature disclosed that this behavior had been observed as early as 1871 (If?) and that several mercuric ion-acetone complexes have been investigated ( I S , 9, IO, 13, 15). However, no information is available regarding the stoichiometry of alkali, acetone, and mercuric ion. Accordingly, preliminary investigations were undertaken to determine whether any quantitative relationship could be established between mercuric ion in acetone solution and the quantity of added alkali. h standard alkali solution was added to an aqueous-acetonc wlution of mercuric chloride until the solution was definitely alkaline and clear. Standard acid was added to neutralize the excess hase, and the amount of acid necessary to effect various lower pH values was noted. Back-titration to the colorless phenolphthalein end point involved an over-all proportion of 2 moles of hydroxide ion per mole of mercuric ion. On the basis of this stoichiometry and in view of the gradual increase in acidity of a mercuric chloride-acetone solution on standing, the following equation is suggested as a reasonable expression of the reaction.
0
Figure 1. Per Cent Recovery of Mercuric Ion + +
Two other complications Jiere next considered. Obviously, an acidified solution of mercuric chloride would require a mr dification of the method and certain anions because of the stability of their mercury complexes would be expected to preclude the use of this method. The complication of added acid presents no difficulty, provided that the anion of the acid does not interfere. It interfering anions are absent, the solution is adjusted to thr bromocresol green end point (the pH of a mercuric chloride solution) prior to the titration. The anion complication, hoxvever, imposes a limitation on this method. With the exception of chloride and bromide ions, all complex-forming anions introduced erroneous results. In thr case of bromide ion, quantitative results are possible if certain modifications are made. Such modifications are required becausc. the stability of the mercuric ion-bromide complex produces lov results, as indicated by Figure 1. The bromide ion complicatioii may be solved in either of tlvo ways: This ion may be removed nearly quantitatively prior to the analysis, or a correction from Figure 1 may be applied to the results obtained from the unmodified method. The former method is preferred and is described in detail in the experimental section. Although cyanide, bromide, iodide, and thiocyanate ions limit the applicability of the mercuric ion determination, the converw is not true-Le., all of the ions listed, with the exception of thr cyanide ion, may be determined quantitatively in a mercuric ion mixture after the mercuric ion-acetone complex has been formed On formation of the complex and adjustment of pH, the chloridtl,
Determination of Mercuric Ion in Presence of Chloride and Bromide Ion
Equiv. Halide/ Equiv. Hg + in Sample
Hg + -
Hg++ Found Mg. Mg. 1 . 0 (CI-) 250.8 249.7 1 . 8 (CI-) 250.8 251.2 2 . 6 (CI-) 250,s 251.0 280.8 3 . 4 (CI-) 250.9 4 . 2 (CI-) 250.8 251.0 250.8 5 0 (CI-) 250.9 1 . 2 (CI-)a 250.8 250.8 250.8 1 . 0 (Br-)b 240.9 2 . 0 (Br-)b 250.8 250.7 Unless otherwise indicated, niercuric chloride wa8 source of mercuric ion. a Solution prepared b y addition of 1.25 millimoles of mercuric oxide t o nitric acid followed by addition of indicated amount of potassium chloride. b Analysis performed by prior precipitation of bromide as de-crihed in experimental section. +
9.0
From a 0.05 M solution of HgCh, plotted against equivalent Rr/equivalent Hg
These observations were applied to the analysis of a 0.02168 &I solution of mercuric chloride. A mean of 123.8 * 0.29 mg. was found on analyses of seven aliquot samples containing 123.9 mg.
Table I.
0.5 I.o 1.5 EQUIVALENT BR-/EQUIVALENT H g + +
Present
899
900
ANALYTICAL CHEMISTRY
Table 11. Determination of Anion in Presence of -Mercuric Ion Equiv. H g + + Equiv. Anion
Anion Present liig.
1 .o
2.0 4.0 1.0 2.0 3.0 1.0 1.0 2.0 a
88.65 C1126.9 C1126.9 C1199 8 Br199 8 B r 199 8 B r 317.3 I-a 158 3 S C N 168.3 SCX-
Anion Found 41g . 88.76 126.9 127.0 199.8 199.8 199.8 317.8 168.1 158.3
Determined grarimetrically. All other analyses by Mohr method.
bromide, and thiocyanate ions may be determined by the Rlohr method and the iodide ion may be determined gravimetrically as silver iodide. Typical results of such anion analyses are shown in Table 11. In the opinion of the authors, the method herein described permits a much more convenient means for the determination of these anions than the previously published procedures (4-8, 14, 16). The latter methods require the removal of mercuric ion from the reaction mixture prior to analysis, and are accordingly more tedious and difficult. The determination of chloride and bromide ions in mercuric ion solutions by the Mohr method is as readily accomplished as their determination in the presence of the alkali metal ions. This similarity permits a new and interesting standardization of silver nitrate. Under the conditions described, mercuric chloride may be used as a primary standard. It presents an advantage over sodium chloride because of its higher equivalent weight, 135.76 as compared with 58.45 for sodium chloride. Application of this method to the standardization of silver nitrate yielded results which differed no more than 2 parts per thousand from the values obtained by the Mohr standardization with sodium chloride as described by Pierce and Haenisch (11). EXPERIMENTAL
Determination of Mercuric Ion in Presence of Chloride Ion. A 25-ml. sample, 0.02 t o 0.2 M in mercuric ion (as the chloride or nitrate), was adjusted with 1 M potassium chloride if necessary t o assure that chloride ion rras present in amount at least equivalent t o mercuric ion. (The chloride ion is required to prevent precipitation of mercuric oxide if the solution requires adjustment to the bromocresol green end point.) If the solution was not already at the bromocresol green end point-i.e., if it contained added acid-it was adjusted to this end point with carbonate-free sodium hydroxide or sulfuric acid. In the event that the solution was highly acidic, the adjustment was made with 1 A- sodium hydroxide until the end point was nearly attained. The final adjustment was then made with 0.1 A’ sodium hydroxide or sulfuric acid as required. A 26-ml. portion of acetone was then added and standard 0.1 LVsodium hydroxide was added to attain the red phenolphthalein end point. The yellow precipitate of mercuric oxide which first formed dissolved in a few minutes and the completely clear solution was back-titrated with 0.1 sulfuric acid to the colorless Dhmolnhthalein end Doint. The volume of the added alkali after deduction of the hasekquivalent of the added acid and the acetone blank represents the amount of mercuric ion present. The acetone blank is determined by mixing 25 ml. of acetone with about 25 ml. of water, followed by titration to the red phenolphthalein end point. Commercial acetone (Carbide and Carbon) was found to be entirely satisfactorv for analytical purposes and consistently provided a blank of about 0.1 ml. of 0.1 iV sodium hydroxide. The sodium hydroxide solution used in all determinations mas carbonate-free, and was standardized against primary standard grade potassium acid phthalate using phenolphthalein as an indicator. The sulfuric acid solution was standardized against the sodium hydroxide solution. Two observations should be noted a t this point: The proportion of acetone is not critical, except in so far as the lower limit should be in the proportion of 0.45 ml. of acetone per ml. of water a t the end point, and care should be exercised to assure that the colorless phenolphthalein end point has been attained. While
definite and unambiguous, the phenolphthalein end point is not as definite as that observed in an ordinary alkalimetric determination and acid should be added until the solution is completely colorless. As a precaution, the authors found it advisable to add one mole drop of indicator at the apparent end point. On occasion the additional indicator produced an evident red color in a previously colorless solution. Determination of Mercuric Ion in Presence of Bromide Ion. A 25-nil. sample containing 250 8 mg. of mercuric ion and 1 to 2 equivalents of bromide ion was mixed with excess 1.0 J4 silver nitrate solution to assure precipitation of silver bromide. An C X C ~ S Sof 1 1 W potassium chloride was then added to assure coniplete precipitation of silver ion and t o provide that chloride ion was present in amount a t least equivalent to mercuric ion. The solution vias filtered through a sintered-glass crucible and the filtrate treated as described above. Determination of Chloride, Bromide, or Thiocyanate Ions in Presence of Mercuric Ion. A 25-ml. samole containing various amounts of mercuric ion and approximatdy 0.1 N in uhalide or thiocyanate ions was mixed with 10 ml. of acetone in the case of halides and with 20 ml. of acetone in the case of t,hiocyanate. The solution was taken slightly beyond the red phenolphthalein end point with approximately 1 aV sodium hydroxide. A 1-ml. portion of chromate indicator was added and the halide or thiocyanate determined by the Mohr method. The indicator solution ?vas 1 AI in potassium chromate and 1 M in sodium bicarbonate. Determination of Iodide Ion in Presence of Mercuric Ion. &4 solution containing iodide ion and varying amounts of mercuric ion from mercuric nitrate was mixed with 25 ml. of acet,one and heated if necessary to dissolve any precipitate of mercuric iodide. The solution was made alkaline with 0.1 ‘V sodium hydroxide and 4 ml. of 6 M ammonium hydroxide was added. Excess 0.1 N silver nitrate was then added to assure completeness of precipitation. After digestion for 2 hours, the precipitate was removed by filtration on a n-eighed sintered-glass crucible and washed successively with an acetone-ammonium hydroxide solution, water, 0.01 N nitric acid, and finally with water. The acetoneammonium hydroxide solution was prepared by mixing 5 ml. of acetone, 100 ml. of water, and 5 drops of 6 AT ammonium hydroxide. The washed precipitate was dried a t 100” for 4 hours, and cooled in a desiccator for 1.5 hours prior to weighing. CONCLUSIONS
Mercuric ion may be determined in the presence of chloride ion and acetone by alkalimetric procedures. Bromide ion interferes, but mercuric ion may be determined by the method of this paper by near-quantitative removal of bromide ion prior to the analysis or by application of a correction based upon the amount of bromide ion present. The method is of no value in the determination of mercuric ion in the presence of iodide, cyanide, or thiocyanate ions. Chloride, bromide, and thiocyanat,e ions may be determined in mercuric ion mixtures by the Mohr method after the mercuric ion-acetone complex has been formed. Iodide ion may be determined similarly by gravimetric procedure. LITERATURE CJTED (1) Auld, S. RI., and Hantrsch, A., Ber., 38,2683 (1905). (2) Biilmann, E., Ibid., 35,2584 (1902). (3) Hofmann, K. A . , Zbid., 31, 2783 (1898). (4) Kohn, hl., 2. anorg. Chem., 59, 108 (1908). (5) Ibid., p. 271. ( 6 ) Kohn, M . , and Ostersetrer, A,, Ibid., 80,218 (1913). (7) Koszegi, D., and Tomori, N., 2. anal. Chem., 100,257 (1935). (8) Lang, R., Z . anorg. allgem. Chem., 142, 229 (1925). (9) Lasserre, A.. J . pharm. chim., (6) 22, 264 (1905). (10) Oppenheimer, C., Ber., 32,956 (1896). (11) Pierce, TT. C., and Haenisch, E. L., “Quantitative Analysis,” 3rd ed., pp. 303-4, K e w York, John Wiley & Sons, 1948. (12) Reynolds, J. E., Ber., 4 , 4 5 3 (1871). (13) Reynolds, J. E., Chem. N e w s , 23, 217 (1871). (14) Rupp, E., and Lehmann, F., P h a r m . Ztg., 52, 1020 (1907). (15) Sand, J . , and Gender, O., Ber., 36,3703 (1903). (16) Uzumasa, Y . , and Osaki, &I.,J . Chem. SOC.J a p a n , 62, 489 (1941). RECEIVED June 14, 1950. Presented in part at the 117th Meeting of the AxERIcAN CHEMICAL SOCIETY, Houston, Tex. Authors James B. Fernandes and Lloyd T. Snider are members of the Student Affiliate Section, AMERICAS CHEMICAL SOCIETY at the University of Florida.
