ANALYTICAL CHEMISTRY
406
Table IV.
Determinations of Cobalt in Reagent Grade Nickel Salts Cobalt Present, % Polarographic Manufacturer analysis
Salt Ni(0Ac)z NiCh Ni(N0i)r NiSOk Ni(NHSz(S0Sa Ni(meta1)
0.016 0.10 0.05 0.03 0.05 0.01
0.149 f 0.003 0.096 f 0 . 0 0 3 0.0049 f 0.0006 0.0140 f 0.0002 0.0079 i 0.0005 0.0065 =k 0.0009
sodium sulfate, respectively. Tungsten and vanadium must be absent. The reproducibility attainable by the procedure described is illustrated by the fact that 13 analyses of a sample of reagent grade nickel chloride (stated by its manufacturer to contain no more than 0.10% cobalt) gave a mean cobalt content of 0.096%, with a standard deviation of i 2 . 6 % , the extreme values being 0.092 and 0.100%.
In Table IV are given the results obtained when samples of various nickel salts were analyzed by the recommended procedure. Each value in the last column is the mean of at least three results; that given for the chloride was taken from the analyses described in the preceding paragraph, and the interesting figure for the acetate was derived from six analyses of material taken from various parts of a bottle whose seal was intact when it reached the author. LITERATURE CITED
(1) Kolthoff, I. M.,Watters, J. I., ANAL.CHEM.15, 8 (1943). (2) Ibid., 22, 1422 (1950). (3) Lingane, J. J., J. Am. Chem. SOC.67, 1916 (1945). (4) Meites, L.,ANAL.CHEM.27, 1116 (1955). (6) Meites, L., “Polarographic Techniques,” p. 16, Interscience. New York, 1955. (6) Meites, L., hleites, T., Zbid., 23, 1194 (1951). (7) Watters, J. I., Ph.D. thesis, University of Minnesota, 1943. (8) Watters, J. I., Kolthoff. I. M., ANAL.CHEM.21, 1466 (1949). RECZIVEDfor review September 20, 1955. Accepted November 7, 1945. Division of Analytical Chemistry, 128th Meeting, ACS, Minneapolis, Minn., September 1955. Contribution 1322, Department of Chemistry, Yale University.
Rapid Gravimetric Determination of Mercury in Organic Compounds HAROLD F. WALTON and HOWARD A. SMITH’ University o f Colorado, Boulder, Colo.
In a rapid method for determining mercury in certain organic compounds the compound is decomposed by refluxing with hydriodic acid containing iodine. Mercury forms the HgI,-- ion, which is then precipitated and weighed as cupric propylenediamine mercuriiodide, CupnzHgId. Compounds such as methyl mercuric hydroxide, bromide, and iodide and diphenylmercury give results which are precise but about 1% low. Part of this error is caused by incomplete precipitation.
A
REASONABLY fast and accurate method is needed for
determining mercury in organic compounds. Most published methoda depend on oxidation of the compound, either by oxygen in a combustion tube or by such agents aa sulfuric and nitric acids, ammonium persulfate, or potassium permanganate. Combustion with oxygen is long and tedious, though reliable. Some of the wet oxidation methods cannot be used in presence of halogen. A few methods depend on reduction t o metallic mercury, but these generally give low results. Iodine has long been known to attack organic mercury compounds (4, 6). Hydrogen iodide, of course, is a very effective reducing agent and decomposes many substances. A combination of iodine and hydriodic acid was found to attack alkyl and aryl mercuric halides and hydroxides very rapidly, forming the Eltable complex ion HgII--. The next step was to find a way of quantitatively determining mercury in the form of this ion. Spacu and Spacu ( 2 ) precipitate the salt CupnrHgIc (pn = 1,2propanediamine) by adding cupric propylenediamine sulfate and weigh it. They claim an accuracy of 1 part in 1000 for 0.5-millimole quantities of mercury added as mercuric chloride. To explore the possibilities of this method for organic compounds, two things were necessary-a study of the Spacu method for inorganic mercury and a study of the effectiveness of the hydriodic acid-iodine digestion. 1
Present address, E. 1. du Pont de Nemours & Co., Houston, Tex
Spacu and Spacu Method. To 100 to 250 ml. of a solution of a mercuric salt add 2 g r a m of potassium iodide per 100 ml., and make weakly basic with ammonia. Heat to boiling, add an excess of a boiling concentrated solution of cupric propylenediamine sulfate, then cool to room temperature, filter, and wash the precipitate first with a solution containing 1 gram each of potassium iodide and cupric propylenediamine sulfate per liter, then with alcohol and ether. Dry in a vacuum desiccator at room temperature and weigh. Duval and Dat Xuong ( f ) showed that drying a t room temperature was unnecessary, and that the precipitate could be heated to 157” C. without decomposition. They consider this method one of the best gravimetric methods for mercury. The precipitate is eaaily filtered and dried, and its molecular weight, 920.2, gives a very favorable gravimetric factor. EXPERIMENTAL
Modification of Spacu Method. The cupric propylenediamine sulfate reagent was made by mixing 1 volume of 1,Z-propanediamine (Eastman practical grade, redistilled) with 5 volumes of 1Jf cupric sulfate. The question arose whether this grade of propylenediamine was sufficiently pure for the purpose, for ethylenediamine, a probable impurity, would not be removed by distillation. A quantity of propylenediamine was therefore made by converting the redistilled “practical” amine to the sulfate, recrystallizing this from aqueous methanol, and reconverting to free amine by distilling from sodium hydroxide. Reagent made from this purified amine gave the same analytical results as that made from the redistilled practical amine. Furthermore, the addition of 5% ethylenediamine to the propylenediamine had practically no effect on the analysis. Cupric propylenediamine sulfate reagent which was 6 months old gave the same results as fresh reagent. This is in contradiction to Spacu and Spacu, who say that the reagent must be freshly mixed and should be heated before being added to the ammoniacal mercury solution. This heating was found to be unnecessary. Not only is it unnecessary to wash the precipitate with ether,
V O L U M E 2 8 , NO. 3, M A R C H 1 9 5 6 Table I.
Recoveries of Inorganic Mercury
of Detns.
Taken, Mmole
Found. Mmole (Arithmetic Mean)
Standard Deviation
HgCb
7
HgBrr
5 3 11 3
0.1104 0.2752 0.5504 0.1109 0.2764 0.5528
0.1090 0.2738 0.5499 0.1098 0.2752 0.5516
0.0009 0.0004 0.0005 0.0003 0.0006 0.0007
No.
Salt
6
407
Loss Mmole
% 1.27 0.51 0.09 1.00 0.45 0.22
0.0014 0.0014 0.0005 0.0011 0.0012 0.0012
but it is potentially harmful. Ether that contained peroxide was found to discolor the precipitate, forming a red-brown substance which is probably cuprous mercuriiodide, and to make the results as much as 5% low. The precipitate could be safely dried a t 105’ to 110’ C. and left at 105’ overnight rvithout losing more than 0.3 to 0.5 mg. Accuracy of Spacu Method. To test the accuracy of the method as modified, standard solutions were prepared from mercuric chloride and bromide (analytical reagent grade, once recrystallized from water) and known volumes were pipetted out and diluted to i 5 ml. before addition of the reagent. The results are summarized in Table I. All were low, but not seriously so, and the precision was good. The percentage error fell with increasing amount of mercury, but the error term, in millimoles, was nearly constant. This suggests that the low results were caused by the solubility of the precipitate; experiments in which weighed amounts of dry precipitate were suspended in water and put through the motions of the procedure, then recovered and weighed, supported this view. By increasing the amount of added potassium iodide fivefold, the accuracy n-as slightly increased. Chloride, nitrate, sulfate, phosphate, and acetate anions did not interfere in 100-fold excess, but phosphate and acetateinterfered in higher concentrations, causing low results At least a twofold excess of iron or aluminum could be present without causing error if citrate was added to prevent precipitation of hydrous oxides. Procedure for Organic Compounds. Organic materials were refluxed for 30 minutes or more with a reagent made by mixing 40 grams of potassium iodide, 4 grams of iodine, and 100 ml. of 6 N sulfuric acid. The presence of free iodine in the reagent is essential. Best results were obtained with 20 to 100 mg. of mercury and 10 ml. of reagent. Refluxing was done in a 100-ml. flask with standard-taper joint. If the unknown was a solution, not more than 5 ml. was taken; if it was a solid, it was dissolved in 5 ml. of Cellosolve (diethyl ether of ethylene glycol) before addition of the reagent. Ethylene glycol or ethers of diethylene glycol were suitable solvents. After refluxing, the solution was diluted with water to i 5 to 100 ml. in a beaker and heated to boiling. Solid sodium sulfite was carefully added to reduce the iodine color to a faint yellow. Excess of sulfite later produced insoluble cuprous mercuriiodide; therefore, any excess of sulfite was removed by adding enough potassium iodate to restore a faint yellow iodine color. Ammonia was then added to about pH 7 (by test paper), and 5 ml. of cupric propylenediamine sulfate reagent added to the boiling solution. The solution was cooled as fast as desired (the precipitate is coarsely crystalline even with rapid cooling), and filtered through a sintered-glass or porous porcelain crucible. The precipitate was washed with some 50 ml. of a solution containing 1 gram of potassium iodide and 2 ml. of cupric propylenediamine sulfate reagent in 1 liter of water, then with 25 ml. of 95y0 ethyl alcohol, dried at 105” C. for 15 to 30 minutes, then weighed. The crucibles n-ere cleaned with aqua regia. Drops of a heavy oily liquid were sometimes seen on diluting after refluxing of Cellosolve solutions. These did not interfere in the determinations.
The method was tested with several pure compounds. These were recrystallized solids except for methyl mercuric hydroxide; an aqueous solution of this compound was made from methyl
mercuric iodide and silver oxide, and standardized by adding excess potassium bromide and titrating the potassium hydroxide which was liberated. This gives a very sharp end point ( 9 ) . The results of these analyses are summarized in Tables I1 and 111. Good results were also obtained with methyl mercuric chloride, phenyl mercuric acetate and hydroxide, and o-chloromercuriphenol, using 1-hour digestion times. Five samples of diphenylmercury Tr-ere also heated in sealed tubes q-ith 10 ml. of reagent for 90 minutes at boiling water temperature; the accuracy and precision were not sensibly different from those obtained by refluxing. Probably better results could be obtained with diphenylmercury by digestion in sealed tubes at a higher temperature. Mercurochrome, on the other hand, could not be analyzed by this method. An hour’s digestion not only did not destroy all the Mercurochrome, but also produced tarry products which interfered seriously with subsequent precipitation and filtration. Tarry masses were also obtained with pharmaceutical tinctures containing phenolic compounds. The elapsed time per determination is of the order of 2 hours. The operating time, however, is only 20 to 30 minutes.
Table 11. Recoveries of Organic Mercury Compound
No. of Detns.
CHaHgOH
8
2
6
CHlHgBr CH:HgI CsHaHgCl (CsHs)zHg
2 3 2 4 2 2
3 3
Taken, Mmole
0.3045 0.3045 0.3045 0.3045 0.3045 0.34.4 0.2-0.5
2.0-2.5 0,2000 0.2000 0.2000
Table 111. Compound
Reflux Time, Min.
CHaHgBr
60
CHzHgI
30
Reagent Used, M1. 5
5 5 5 10 5 5
10 5
10 10
Reflux Time Min.’ 30 20 15 10
20 60
30
6Q 60
30 240
iFzI3: % 1.0 0.86 0.3 2.5 0.13 1.03 1.28 0.03 3.5 1.7 1,s
Mmole 0.0030 0,0026 0.0009 0.0074 0.0004 0.0038 0.0037 0.0007 0.0070 0.0034 0 30
Examples of Precision Hg
Taken Mmod
Found, Mmole
Loss, %
0.3579 0.3924 0.2145 0.2592 0.2937 0.5053
0.3542 0.3881 0.2110 0.2560 0,2892
0.97 LO9 1.66 1.23 1.53 0.69
0.5018
ACKNOWLEDGMENT
I t is a pleasure to express thanks to the Ringwood Chemical Corp. and Panogen, Inc., for supplying mercury compounds used in this work. LITERATURE CITED (1) Duval, C., D a t X u o n g , N., Anal. Chim. Acta 5, 4 9 4 (1951). (2) Spacu, G., Spacu, P., 2.anal. Chem. 89, 187 (1932). (3) Waugh, T . D . , Walton, H. F., Laswick, J. A., J. Phys. Chem. 59, 395 (1955). (4) Whitmore, F. C . , “Organic Compounds of Mercury,” Chemical Catalog C o . , N e w York, 1921. (5) Whitmore, F. C., Thorpe, M. A., J. Am. Chem. SOC.55, 782 (1933). RECEIVED for review January 31, 1955. Accepted December 9, 1955. Second Western Regional Meeting, Chemical Institute of Canada, Vancouver, Sept. 11, 1954. The major part of this work was submitted by Howard A. Smith in partial fulfillment of requirements for the degree of master of science at the University of Colorado, 1954.
