54 LABORATORY MANUAL OF MICROBIOLOGY 



PREPARATION OF STANDARD SOLUTIONS 



A great many methods have been described for the standard- 

 ization of solutions. Only a brief outline will be given of certain 

 of these methods; for detailed directions the reader is referred to 

 some of the recent textbooks on volumetric analysis. 



Normal Sulphuric Acid Solution 



A normal solution of sulphuric acid is one-half the molecular 

 weight of H2SO4 in grams, diluted to 1 liter with distilled water. 

 Since the molecular weight of sulphuric acid is (2 + 32 + 64) 98, 

 then 49 grams, one-half of 98, is the amount necessary for each liter. 



1. In order to secure 49 grams of H2SO4, it requires 49 divided 

 by 1.80 (specific gravity of concentrated sulfuric acid), or 27.2 

 cubic centimeters of chemically pure acid. To be sure that 

 sufficient acid has been used, measure out about 27.5 cubic 

 centimeters of acid. 



2. Place the acid in a 1,000-cubic centimeter Erlenmeyer flask, 

 containing about 500 cc. of water, mix well and transfer to a 

 volumetric flask; make up to 1,000 cubic centimeters with water 

 and mix carefully. 



3. From this mixture remove 10-cubic centimeter portions, 

 accurately measured in a 10-cubic centimeter pipette, and 

 place in weighing bottles which have been thoroughly cleaned, 

 dried in an oven, cooled, and weighed. 



4. One cubic centimeter of chemically pure ammonia is added 

 to each weighing bottle to neutralize the sulphuric acid. 



5. The water and excess of ammonia is then evaporated in an 

 oven at 100°C. and the ammonium sulphate remains behind. 

 If the chemicals are pure, 100 cubic centimeters of the solution 

 should give 49 grams of sulphuric acid. In 10 cubic centimeters 

 of the solution there should be 0.49 gram of H2SO4. 



H2S04:(NH4)2S04::98:132 

 49 : X ::98:132 

 X =0:66 



If the solution is exactly normal, there should be 0.6600 gram of 

 (NH4)2S04 formed from 10 cubic centimeters. In case the 

 amount of (NH4)2S04 formed is too great, its factor is determined 

 by dividing 0.6600 into the weight of ammonium sulphate found. 



