240 ALKALIMETRY. 



duced coherent gold is easily washed ; it is to be dried, 

 ignited, and weighed. 



The excess of gold is removed from the solution by 

 sulphuretted hydrogen, or by boiling with oxalic acid, 

 and the phosphoric acid and lime are then separated 

 and estimated as in No. 13. 



121. ALKALIMETRY. 



The specimens of potashes and soda-ashes met with 

 in commerce contain very variable quantities of foreign 

 substances. The amount of carbonated alkali, upon 

 which their value alone depends, varies between 40 

 and 95 per cent. 



The potashes contain chiefly chloride of potassium, 

 sulphate, silicate, and phosphate of potassa, and carbo- 

 nate, phosphate, ajid silicate of lime. 



The soda generally contains chloride and sulphide 

 of sodium, sulphate, silicate and hyposulphite of soda, 

 and often also hydrate of soda. 



The amount of alkaline carbonate present in the 

 sample, may be determined by several methods. 



I. By the standard solution test, i. e., by exactly neu- 

 tralizing a weighed portion with dilute sulphuric acid of 

 known strength. 



In order to prepare the test-acid, a known quantity, 

 say 70 grms. of concentrated sulphuric acid, are diluted 

 with 600 grms of water. 



5 grms. of pure anhydrous carbonate of soda are 

 weighed, dissolved in hot water, and the solution 

 colored blue with a little tincture of litmus. 



The test-acid is then added to the solution, from a 

 burette, very carefully as the point of neutralization is 

 approached, until the color is just changed to red, and 



