ESTIMATION OF PHOSPHATES AND SULPHATES IN UEINE 173 



i.e. until the filtrate remains clear after the addition of a few drops of hydric 

 sulphate. Then wash Avith hot alcohol, and afterwards with ether. Remove 

 the filter, and place it with its contents in a platinum crucible. Heat to 

 redness. Cool over sulphiu-ic acid in a desiccator ; weigh, and deduct the 

 weight of the crucible and filter ash ; the remainder is the weight of barium 

 sulphate formed. 



Error. — When the experiment is carried out as above there is a slight 

 error from the formation of a small quantity of sulphide of barium. This 

 may be corrected as follows : After the platinum crucible has become cool 

 add a few drops of pure sulphuric acid (H.^SOJ. The sulphide is converted 

 into sulphate. Heat again to redness to drive off excess of sulphuric acid. 



(b) The following is Salkowski's ^ method of estimating the combined 

 sulphuric acid— that is, the amount of SO3 in ethereal sulphates : — 100 c.c. 

 of urine are mixed with 100 c.c. of alkaline barium chloride solution, which is 

 a mixture of two volumes of solution of barium hydrate with one of barium 

 chloride, both saturated in the cold. The mixture is stirred, and after a " s v 

 minutes filtered : 100 c.c. of the filtrate ( = 50 c.c. of urine) are acidified with 

 10 c.c. of hydrochloric acid, boiled, kept at 100° C. on the water-bath for an 

 hour, and then allowed to stand tiU the precipitate has completely settled ; if 

 possible, it should be left in this way for twenty-four hours. The farther 

 treatment of this precipitate ( = combined sulphates) is then carried out as in 

 the last case. 



Calculation. — 233 parts of barium sulphate correspond to 98 parts of 

 H.^SO^, or 80 parts of SO3 or 32 parts of S. To calculate the HjSO^, multiply 



the weight of barium sulphate by = 0-4206 ; to calculate the SO., multiply 



by ° = 0-34335 ; to calculate the S multiply by ^^ = 0-13734. This method 

 233 ^^ 



of calculation applies to the gravimetric estimation both of total sulphates 



and of combined sulphates. 



(c) To obtain the amount of pre-formed sulphuric acid subtract the 

 amount of combined SO3 from the total amount of SO3. The difference is 

 the pre-formed SO3. 



Example : 100 c.c. of urine gave 0-5 gramme of total barium sulphate. 



80 

 This multiplied by = 0-171 gr. = total SO3. Another 100 c.c. of the same 



urine gave 0-05 gr. of barium sulphate from ethereal sulphates ; this multi- 



pUed by °^^ - 0-017 gr. of combined SO3. Total SO3- combined SOj 



Zoo 



= 0-171 - 0-017 = 0-154 gr. of pre-formed SO3. 



' Zeit. physiol. Chem. x. p. 346. This method is a modification of Baumann's 

 original method, ibid. i. p. 7L 



