240 PHYSIOLOGICAL CHEMISTRY 



again as above by the addition of the calculated amount of water, 

 or of the calculated amount of hydrochloric acid in case the 

 solution is now too weak. 



Preserve your tenth normal solution for future use. 



Rinse out the pipette and burette thoroughly. 



7. Preparation of a Standard Alkali. Eead over carefully 

 section (6) on the preparation of a standard acid, as the prin- 

 ciples involved are identical with those for the preparation of 

 the alkali. 



A concentrated solution of sodium hydrate will be used as a 

 stock solution. The strength of the NaOH will be found on the 

 bottle. Calculate the amount required to make up 2, iy 2 , or 1 

 liter of N/10 alkali according to the size of the glass stoppered 

 bottle in your desk. 



Measure this amount into the large bottle and add enough dis- 

 tilled water to make the volume up to about 100 c.c. less than 

 the total volume for which you have made your calculation. 



In titration with alizarine red as an indicator it is necessary to 

 run alkali into acid. Accordingly, measure 25 c.c. of the standard 

 N/10 acid furnished into a clean Erlenmeyer by means of a clean 

 pipette. Add 2-3 drops of alizarine red. 



Clean your burette and either dry it or rinse with the solution 

 to be used in it. Where the supply of liquid is adequate, the 

 second method is easier. 



Fill the burette with your alkali, and titrate the standard acid. 

 Make a duplicate titration to check your result. The duplicate 

 titrations should agree within 0.1 c.c. 



If your alkali were exactly N/10, it would require 25.0 c.c. to 

 neutralize the 25 c.c. acid. As you have made it up slightly 

 stronger than this, it will require somewhat less than 25 c.c. 

 alkali to neutralize the 25 c.c. of acid. 



From the amount of alkali used calculate the amount of 

 dilution necessary to make the alkali exactly N/10. 



Example. 25 c.c. N/10 acid is neutralized by 23.2 c.c. alkali. 

 Thus 23.2 c.c. of alkali contains enough NaOH to neutralize 



