Determination of Mercury and Bromides with 1,lO-Phenanthroline Ferrous Sulfate AT. E. II4LLI WITH GEO. M . SMITH Vanderbilt University, .brashrille, Tenn. Physical Properties of Precipitate. Ferroin tribromomercuriat e is a dark red crystalline substance, sparingly soluble in water, insoluble in potassium bromide solution, hydrobromic acid solution, diethyl ether, methyl acetate, and ethyl acetate, but soluble in ethyl alcohol and acetone. I t can be heated to 275" C. without decomposition, and dries to constant weight a t 110' C. The formula weight is 1476.6; the gravimetric factor for mercury i,q 0.2726. The precipitate is somewhat soluble in water a t 25" C. (3.6 nig. per 100 ml. of water) and sparingly soluble (0.5 mg. per 100 ml.of water) a t 5 O C. Because only 0.1 mg. dissolves in 100 ml. of 0.002 d l hydrobromic acid, all precipitates were washed with this solution; 250 mg. of precipitate, washed with 0.002 M hydrobromic acid, dries to constant Lveight in 1hour a t 110" C. If this GRAVIMETRIC DETERMINATION OF RIEHCURY washing is followed by methyl acetate and ether, the precipitate Reagents. Ferroin, 0.025 -If solution of l , l O - p h e n a ~ i t I i r ~ ~ l i r i ~dries ~ completely in 10 to 15 minutes. ferrous sulfate, supplied by the G. Frederick Smith Chemical Co., Effect of Bromide Ion Concentration. Quantitative precipitaC'olunibus, Ohio. tion occurs in concentrations of excess bromide 0.002 M or Potamium bromitlc, 2% aqueous solution. greater. If excess bromide exceeds 0.03 M , results are always 1Iercuric nitrate, crystals, free from halide ions. Solutions wcre standardized against standard potassium thiocyanate soluhigh, probably because some ( C12H8S2)3FeHgBrlforms. Ferroiri tion, using saturated ferric alum solution as the indicator. Potasbromide docs not precipitate in bromide concentrations as great sium thiocyanate was standardized against standard silver nitratr as 2.0 JI. Quantitative results cannot be obtained in high solution, using ferric alum as the indicator. bromide concentrations, as the tetrabromo compound apparThe rations investigated for interference were added as nitrates. The anions were added as sodium or potassium salts. ently is not quantitatively insoluble. Furthermore, in high Principles. The chemistry of this method, and of the volumetbromide concentrations, cadmium interference is greater because ric methods for mri~curyand bromide, is stated in the following of the precipitation of its bromide comples. eq uatiorw: Effect of Excess Ferroin. I n bromide concentrations as great as H g + T 4 2Br- -+ HgBr2 (very weakly dissociated) (1) 0.002 AI, complete precipitation of ferroin tribromomercuriatcb occurs if the concentration of excess ferroin is a t least 0.0001 .I[. HgBrp Br- +HgBrj(2) Larger exce~sesme not objectionable, but are not needed. 2HgBrS(C12HaKn):jFe + + --+(C11HaS?)SFr(€IgBr:~)2 (3) Quantities of Mercury Determined. Quantities of mercury ironi 1.00 t o 66.00 mg. have been determined gravimetricall>The product of Iteaction 3 is an insoluble red compound, ferroin (T:tI)Ie1). tribromomercuriate, which can be filtered, washed, dried, a n d neighed as such. Composition of Precipitate. The composit.ion of [lie prccipitate, Table 1. M e r c i i r y Determination the product of Equation 3, !vas established by determining the quantity of mercuric ion required t o precipitate a known quantity l f e r c u r y Presrnt, Mercury Found, Mg. Mg. of t'eimin, by weighing the precipitate formed by a known quan1.00 0.98 tit!. of mcLrcuric ion, and 11)- determining iron and bromine in the .< nn A a7 10.00 10.10 preciiiitate. 16,50 l H E 1,lO-phenanthroline ferrous complex, as the sulfate, was prepared and described bj- Blau ( 1 ) in 1808. Walden, Hamn e t t , and Chapman ( 1 2 ) and others (IO,11, IS-16j described it,s use as a high potential, reversible osidimetric indicator. The formation of the highl>- colored complex has been used for the colorimetric determination of iron (3,64). Rlau ( 1 ) noted that the complex, called ferroin by Gleu (4), formed insoluble salts with chloroplatinate and picrate. Smith ( 9 ) prepared insoluble perchlorates and periodates. Feigl and 3Iiranda ( 2 ) used f'erroin to detect cadmium by forming a precbipitate with CdI,--. nnd noted also that HgI,-- und Bi1,formed precipitates \vith f'erroin. These properties suggested its u e e in the quantitative tictermination of mercury.
+
+
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~IERCLT,Y REQL-IREI) To I'RECIPITATE I
