ESTIMATION OF PHOSPHATES AND SULPHATES IN UKINE 209 



Method (Salkowski). 100 c.c. of urine are taken in a beaker. This is 

 boiled with 5 c.c. of hydrochloric acid as before. 



Chloride of barium is added till no more precipitate occurs. 



The precipitate is collected on a small filter of known ash, and washed 

 with hot distilled water till no more barium chloride occurs in the filtrate 

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

 acid. Then wash with hot alcohol, and afterwards with ether. Remove 

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

 redness. Cool over sulphuric 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 1: ecome cool 

 add a few drops of pure sulphuric acid (H 2 S0 4 ). The sulphide is converted 

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



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

 sulphuric acid that is, the amount of SO, 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 few 

 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. 011 the water-bath for an 

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

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

 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.,S0 4 , or 80 parts of SO 3 or 32 parts of S. To calculate the H,S0 4 , multiply 



OQ 



the weight of barium sulphate by - = 0-421 ; to calculate the S0 3 , multiply 



2oo 



Qf\ QO 



by ou = 0-343 ; to calculate the S multiply by . = 0-137. This method 

 233 *" 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 S0 3 from the total amount of SO. ( . The difference is 

 the pre-formed S0 3 . 



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



ftfl 



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

 2oo 



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



OA 



plied by -5!i = 0-017 gr. of combined SO,. Total SO, - combined SO.. 

 2oo 



-0-171-0-017 = 0-154 gr. of pre-formed SO,. 



