URINE: QUANTITATIVE ANALYSIS. 385 



one hour, then shake up the solution and filter it through a weighed 

 Gooch crucible. 1 



Wash the precipitate of BaSO 4 with about 250 c.c. of cold water, 

 dry it in an air-bath or over a very low flame, then ignite, 2 cool and 

 weigh. 



Calculation. Subtract the weight of the Gooch crucible from 

 the weight of the crucible and the BaSO 4 precipitate to obtain the 

 weight of the precipitate. The weight of SO 3 3 in the volume of 

 urine taken may be determined by means of the following propor- 

 tion. 



Mol. wt. Wt. of Mo', wt. 



BaSO 4 : BaSO 4 : :SO 3 :^r (wt. of SO 3 in grams), 

 ppt. 



Representing the weight of the BaSO 4 precipitate by y and substi- 

 tuting the proper molecular weights, we have the following pro- 

 portion : 



231.7:3;: : 79.5: 4' (wt. of SO 3 in granls in the quantity of urine 



used). 



Calculate the quantity of SO 3 in the twenty-four hour specimen 

 of urine. 



To express the result in percentage of SO 3 simply divide the value 

 of x, as just determined, by the quantity of urine used. 



2. Inorganic Sulphates. Folin's Method. Place 25 c.c. of 



1 If a Gooch crucible is not available the precipitate of BaSCX may be filtered 

 off upon a washed filter paper (Schleicher & Schiill's, No. 589, blue ribbon) and 

 after washing the precipitate with about 250 c.c. of cold water the paper and 

 precipitate may be dried in an air-bath, or over a low flame. The ignition may 

 then be carried out in the usual way in the ordinary platinum or porcelain 

 crucible. In this case correction must be made for the weight of the ash of the 

 filter paper used. 



2 Care must be taken in the ignition of precipitates in Gooch crucibles. The 

 flame should never be applied directly to the perforated bottom or to the sides 

 of the crucible, since such manipulation is invariably attended by mechanical 

 losses. The crucibles should always be provided with lids and tight bottoms 

 during the ignition. In case porcelain Gooch crucibles, whose bottoms are not 

 provided with a non-perforated cap, are used, the crucible may be placed upon 

 the lid of an ordinary platinum crucible during ignition. The lid 'should be sup- 

 ported on a triangle, the crucible placed upon the lid and the flame applied to 

 the improvised bottom. Ignition should be complete in 10 minutes if no organic 

 matter is present. 



3 It is considered preferable by many investigators to express all sulphur values 

 in terms of S rather than SOs. 



26 



