URINE 



463 



the urine but continues to be excreted in the urine over a long period 

 of time, in some cases for several months. The urine may be examined 

 for arsenic by the following methods. 



i. Marsh and Marsh-Berzelius Method. This method has the advantage of 

 serving as a qualitative and quantitative determination, and is a very delicate test; 

 it is, however, long and tedious. The various steps in the analysis are: (i) the 

 destruction of the organic matter in the urine; (2) treatment with sulphuric acid to 

 drive off excess nitric acid and break up nitro-compounds; and (3) application of 

 independent test to the resultant solution. Proceed as follows: The urine, to 

 which is added one-third its volume of nitric acid, is placed in a casserole or evapo- 

 rating dish and evaporated at 65-7oC. to a syrupy consistency. The mass is 

 then allowed to cool and 5 c.c. concentrated sulphuric acid added, and gentle heat 

 applied. The heating must be done cautiously, or deflagration takes place and 



FIG. 137. MARSH APPARATUS. 



some of the arsenic is sure to be lost. The mass will liquefy and finally darken, 

 indicating organic matter. Cool and add concentrated nitric acid, i c.c., and apply 

 very gentle heat; copious reddish-brown fumes are evolved. Gradually raise the 

 temperature until darkening of the solution occurs, then cool, add i c.c. concentrated 

 nitric acid and again apply gentle heat, and repeat the process until the solution 

 fails to darken. Now raise the temperature until white fumes begin to come off. 

 At this temperature excess nitric acid will have been removed and all nitro-com- 

 pounds broken up. The solution at this point is clear and at most a pale straw color. 

 Cool and add a mixture of 10 c.c. concentrated sulphuric acid and 40 c.c. water, 

 and test for arsenic using a Marsh apparatus. The apparatus (see Fig. 137, above) 

 consists of a wide- mouth flask 250 c.c. capacity fitted with a two-hole stopper. 

 Through one hole is passed the stem of a separatory funnel of 50 to 60 c.c. capacity. 

 Through the other hole a piece of glass tube bent at right angles, which is fitted to 

 a calcium chloride tube, and this in turn to a narrow quartz tube, the distal end of 

 which is drawn to a fine bore and bent up> almost at a right angle. All joints must 

 be air-tight. 



Introduce 30 to 40 grams of arsenic-free granulated zinc into the flask, insert 

 the stopper and through the funnel introduce 50 c.c. dilute sulphuric acid (i part 



