66 LABORATORY MANUAL OF MICROBIOLOGY 



Add 0.7 gram of mercuric oxide, 0.1 to 0.3 grams of crystalline 

 copper sulphate, and 10 grams of anhydrous sodium sulphate or 

 powdered potassium sulphate. 



Add 30 to 35 cubic centimeters of concentrated sulphuric acid. 

 Thoroughly moisten the whole sample with acid before beginning 

 to heat. 



Place the flask on the digestion shelf and heat slowly until 

 frothing ceases. Avoid a very high flame; do not aUow the flame 

 to touch the flask above the part occupied by the liquid. 



Now raise the heat (avoid a very hot flame) until the acid boils 

 rapidly. 



Digest for some hours after the liquid clears — until the cool 

 liquid is no longer yellow but blue-white in color. 



In case the contents of the flask are likely to become solid 

 before digestion is complete, cool, and add 10 cubic centimeters 

 more of sulphuric acid. 



When digestion is complete, cool, and add about 300 cubic 

 centimeters of water. Shake until the mixture is thoroughly in 

 solution. Be sure that none of the digested material remains 

 caked to the sides of the Kjeldahl flask. 



Recool, add a teaspoonful of powdered pumice or a small 

 amount of zinc powder (0.5 gram) to prevent bumping. 



Add 25 cubic centimeters of an 8 per cent sodium thiosulphate 

 solution in order to precipitate the mercury. 



Add 80 to 100 cubic centimeters of the concentrated sodium 

 hydroxide. (The stock solution of alkali should be prepared 2 

 days or more before it is to be used in order that the sodium car- 

 bonate may precipitate out. Avoid the deposit in the bottom of 

 the alkali.) Enough alkali should be added to the acid digest to 

 make the solution react strongly alkaline. A few strips of litmus 

 paper may be added in order to test the reaction. The alkali 

 should be poured slowly down the sides of the flask, connected 

 at once to the condenser, and shaken. 



See that the rubber stopper flts snugly in the flask. Now mix 

 the contents thoroughly by shaking. 



Just before connecting the flask have a very low flame burning 

 on the distillation shelf. After the alkaU and acid mixture are 

 well mixed, raise the flame. 



