Abstract
We have used the constant volume apparatus of Van Slyke and Neill in the estimation of potassium in solutions whose inorganic composition approximates that of serum. The potassium is precipitated as the potassium-sodium cobalti-nitrite compound, essentially as in the original method of Kramer and Tisdall. 1 The precipitate is decomposed with sodium hydroxide as suggested by Clausen, 2 ∗ and the nitrogen of the sodium and potassium nitrite is liberated by adding a solution of urea and sulphuric acid. 3 † The amount of nitrogen is then determined in the constant volume apparatus. It is possible to determine from 0.07 to 0.45 mg. potassium, with an error not exceeding 0.01 mg.
One cc. of the potassium solution is measured into a 15 cc. graduated centrifuge tube, containing 1 cc. of water. One cc. of the sodium-cobalti-nitrite reagent is added slowly, drop by drop, and the solution thoroughly mixed after each drop. At the end of 45 minutes, the volume is made up to 5 cc. with water and the contents mixed again and centrifuged for one-half hour at a speed of about 1800 revolutions per minute.
All but 0.3 cc. of the supernatant fluid is pipetted off so as not to disturb the precipitate. Five cc. of water are allowed to run in slowly. The stream is controlled in such a way that the added water is thoroughly mixed with the residual reagent and the precipitate is not disturbed. The procedure is repeated twice, making three washings in all. In the third washing, the water is added in such a way as to gently agitate the upper portion of the precipitate. The tube is centrifuged for 5 minutes.
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