I 



CHAPTER V. 

 STANDARD CHEMICAL SOLUTIONS. STOCK SOLUTIONS AND SOLUBILITIES 



Preparation of Standard Solutions: 



Reagent grade chemicals and freshly distilled water should be used 

 for all solutions. Weighings should be carried out in the low humidity 

 room, using calibrated weights. Calibrated glassware should be used in 

 all volumetric operations. Standard solutions should be issued in clean, 

 dry Pyrex bottles. Rubber stoppers (cleaned in hot sodium hydroxide so- 

 lution, then washed and dried) should be used for alkali solutions; glass 

 stoppered Pyrex bottles should be used for the other solutions. 



Standard solutions should be issued only by the member of tlie staff 

 assigned to this work or by the person in charge. The accuracy of the 

 solution should be indicated on the label of each container. When an 

 accurately standardized solution is requested by an investigator, the 

 staff member should find out whether extreme accuracy is needed. If not, 

 an attempt should be made to convince the investigator that an approx- 

 imate solution is adequate. 

 A. Standard Acids and Bases: 



The base solutions are standardized with solid potassium acid 

 phthalate while the acid solutions are standardized with the standard 

 bases. Thus, potassium acid phthalate is the reference standard for 

 all acid and base solutions and the best grade of this compound should 



be used. 



The potassium acid phthalate is dried at 110°- 115° for two or three 

 hours and then stored in a desiccator in the low humidity room. Samples 

 are weighed in the Erlenmeyer flasks to be used in the titrations, and 

 the weighings are carried out in the low humidity room. 

 Normal Sodium Hydroxide : 



Saturated sodium hydroxide solution should be prepared for use 

 during the following year and stored in a paraffined bottle closed by 

 a cleaned rubber stopper. The sodium carbonate precipitates, leaving a 

 clear supernatant solution which should be pipetted or carefully decanted 



from the carbonate. 



For each liter of normal sodium hydroxide, dilute 60 ml. of the 

 saturated sodium hydroxide solution to one liter with freshly boiled 

 distilled water and store in a paraffined bottle protected with a soda- 

 lime tube. Adjust as closely as possible to one normal by single runs 

 against potassium acid phthalate. Finally, accurately standardize the 



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