ESTIMATION OF PHOSPHORIC ACID.
To urine which contains the phosphates as acid phosphates, a solution of uranium acetate or nitrate is added until an excess of the reagent first becomes appreciable. If the nitrate be used free nitric acid is formed, and causes a part of the precipitated uranium phos phates to redissolve. To prevent this, in practice a little sodium ace tate is added to the urine before titration with uranium nitrate. As an indicator 'a little tincture of cochineal is employed. This yields a green precipitate in presence of a uranium salt in excess. Instead of the cochineal fluid a solution of potassium ferrocyanide, 1 in 10, may be used. This reagent deposits a deep-brown precipitate with a mere trace of uranium salt. This test, however, is less sensitive in the presence of acetate of soda than in simple watery solutions. Hence it is necessary to use a definite quantity of the salt, and to take care that the proportion is maintained in preparing the titra tion fluid.
The solutions required for this process are: 1. Solution of Acetate of Socla.—One hundred grams of acetate of soda are dissolved in 800 c.c. of water, 100 c.c. of a 30-per-cent. solu tion of acetic acid are added, and the mixture is then made up to a litre. Five cubic centimetres are employed with 50 c.c. of urine.
2. Cochineal Tincture.—A cold infusion is made of a few grams of cochineal in a quarter of a litre of a fluid composed of three-fourths parts of water with one of alcohol, and the solution filtered for use.
3. Solution of Uranium, aritle.—About 20.3 gm. of commercial uranium oxide, purified and well dried, is dissolved in pure acetic acid, or in the smallest possible quantity of nitric acid, and the prep aration is then made up to a litre. Of the mixture 1 c.c. indicates 5 mgm. of phosphoric acid.
4. A Solution Containing a Definite Quantity of Phosphoric Acid.— Fifty cubic centimetres should contain precisely 0.1 gm. PA. The preparation is made by dissolving 10.085 gm. of neutral phosphate of soda in a litre of water. The commercial salt should be crystal lized from solution to obtain it free from chlorine, so that no precipi tate forms with nitrate of silver and nitric acid. The crystals are then placed on paper in a funnel, the neck of which is stopped with glass wool, and allowed to dry there until the mother liquor is no longer found to adhere to them. A known weight is then taken and
rubbed up in a mortar and a portion of the powder submitted to a gentle heat in a platinum crucible, and finally incinerated. Two hun dred and sixty-six grams of sodium-pyrophosphate, cor respond to 716 gm. Consequently that quantity of the dried crystals which, when incinerated, yields 266 gm. corre sponds to 71G gm. of pure phosphate of soda.
Titration Process.
Fifty cubic centimetres of the phosphatic solution (4) are meas ured in a flask, 5 c.c. of the solution of acetate of soda (1) and a few drops of cochineal tincture (2) are added, the mixture is boiled, and the uranium solution (3) is gradually supplied until the mixture becomes slightly but permanently green on shaking. In the process a high temperature should be maintained, to promote the formation of uranium phosphates. When ferrocyanide of potassium is used as the indicator, the addition of the uranium solution is suspended when the precipitate ceases to form. The fluid is again heated and a drop is tested by adding to it a drop of ferrocyanide in a porcelain capsule. The further supply of uranium solution is regulated by the earliest appearance of a brown color in the specimens successively tested.
The uranium solution is now diluted according to the quantity found to be necessary, as above, in such proportion that 20 c.c. shall just suffice for the titration of 50 c.c. of the phosphoric acid solution.
Now, 50 c.c. of the phosphoric solution represents 0.1 gm. and consequently 20 c.c. of the diluted uranium solution also corre sponds to 0.1 gm. The titration process is repeated with the urine in precisely the same manner as before : 50 c.c. are taken, 5 c.c. acetate of soda and a little cochineal added, and the mixture heated and the terminal reaction sought.
Every cubic centimetre of the uranium oxide solution employed in titration represents 5 mgm. Hence the phosphoric acid contained in 55 c. c. of urine may be calculated by multiplying the number of cubic centimetres of uranium oxide solution used by 0.005. The result is the quantity of phosphoric acid in grams contained in 50 c. c. of urine. It is advisable in each case to make two such investigations and to take the mean of their results.