Determination of Small Amounts of Potassium Evaluation of Cobaltinitrite Precipitate Using Ceric Sulfate BERNARD KLEIN
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RIENDEL JACOBI, Chemical Laboratory, Beth-El Hospital, Brooklyn, N. Y.
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of 0.2 M silver nitrate is added dropwise with shaking; and 0 . 5 ml. of sodium cobaltinitrite reagent is then added dropwise, allowing each drop to be thoroughly mixed before adding the next. The mixture is allowed to stand at room temperature for 10 to 20 minutes and then centrifuged a t high speed for 15 minutes. The supernatant liquid is drawn off by means of a bent capillary pipet,germitting about 0.3 ml. to remain. The preci itate is washe with 5.0 ml. of distilled water, without disturfing the precipitate, by running the wash liquid down the sides of the tube as the tube is slowly rotated. The residual liquid and wash water are mixed by swirling and the tubes are centrifuged for 5 minutes. The supernatant liquid is once more drawn off and the process is repeated until the supernatant liquid is clear and colorless. Three washings are usually sufficient. The packed precipitate is decomposed by blowing 1.5 ml. of 1 N sodium hydroxide onto it and heating in a boiling water bath for 10 minutes with occasional shaking. The suspension is cooled in ice water and 2.5 ml. of 1 N sulfuric acid are blown in to dissolve the precipitated cobalt and silver hydroxides. A small drop of ferroin is added and the solution is titrated at once with ceric sulfate, The end point is a blue color throughout the solution lasting at least 2 minutes. The end point is sharp and easily discerned.
HE volumetric determination of the cobaltinitrite com-
plex appears to have been neglected as a rapid, accurate, and precise method for the estimation of small amounts of potassium. The best known is that of Kramer and Tisdall (6) who precipitated the dipotassium sodium cobaltinitrite, decomposed i t with alkali, oxidized the liberated nitrite with permanganate, and back-titrated with standard sodium oxalate. The disadvantages of this method were pointed out by Brown, Robinson, and Browning ( 2 ) who used ceric sulfate instead of permanganate. Recently, Weichselbaum, Somogyi, and Rusk (8) have g-iven the experimental conditions for the preparation of the potassium silver cobaltinitrite first announced b y Burgess and Kamm ( 3 ) . Their method gave a precipitate of constant composition between certain carefully defined limits. Potassium was then calculated either by determining the nitrite content gasometrically, according to Kramer and Gittleman ( 6 ) ,or by a Volhard titration of the silver present. More recently, Kaye (4) presented a method whereby the nitrite content of the sodium potassium cobaltinitrite is oxidized with excess ceric sulfate and back-titrated iodometrically. The method presented here was used with success in the mineral analysis of bile and other biological materials, the results of which will appear elsewhere shortly. Potassium silver cobaltinitrite, K&gCo(NOa)e, was precipitated, the nitrite liberated with hot alkali, acidified, and immediately titrated with ceric sulfate directly, using ferroin. The procedure is satisfactory for the determination of potassium in amounts rmging from 0.1 to 0.2 mg. The error does not exceed 2 per cent. This method lends itself especially to the rapid analysis of biological materials.
Calculation Mg. of K in sam le ml. of ceric sulfate used sulfate used in blani
