284 PHYSIOLOGICAL CHEMISTRY 



Each student should prepare one standard solution. The class 

 may be divided into pairs, one student of each pair preparing a 

 standard acid, his partner a standard alkali. The solutions pre- 

 pared then can be used jointly by each pair of students. 



(3) Preparation of N/10 acid. Read all of section (3) 

 before beginning your work. Cone, hydrochloric acid is about 

 41% by volume, that is, 100 c.c. contains 41 grams of hydro- 

 chloric acid gas. Calculate the amount of cone, acid required to 

 make 2 liters, ~L l /2 or 1 liter of N/10 acid, according to the size of 

 the large glass stoppered bottle with which you are provided, and 

 measure this amount of acid into your large bottle. Add 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. 



With a clean pipette (see note under Specific Gravity) meas- 

 ure out 25 c.c. of the 'acid into an Erlenmeyer flask. The pipette 

 either may be dry, or it may be rinsed out once with the solu- 

 tion to be measured. 



Add 2-3 drops of alizarine red as indicator, and from a burette 

 (also cleaned as above and either dry or rinsed with N/10 NaOH) 

 run in N/10 sodium hydrate. In reading the burette, read the 

 lowest curve of the meniscus. Have your eye on a level with the 

 meniscus. If the acid were exactly N/10 it would require exactly 

 25 c.c. of the alkali to neutralize it. Make the titration in dupli- 

 cate. Duplicates should agree within 0.1 c.c. From the amount 

 of alkali used in titration calculate the amount of dilution neces- 

 sary to make the acid exactly N/10. 



Example. 25 c.c. acid used. Amt. N/10 alkali to neutralize 

 the acid 27.2 c.c. Thus 25 c.c. of the acid used contains enough 

 hydrochloric acid to neutralize 27.2 of N/10 alkali. To make the 

 acid exactly N/10 it must be diluted in the ratio of 25 : 27.2. 



Measure the total amount of your acid with a large cylinder 

 and calculate the volume of water necessary to dilute it to tenth 

 normal acid. Add this amount of water from a cylinder, and 

 shake thoroughly. Allow to stand for a few minutes, and repeat 

 the titration as above. 



If your acid does not check with the N/10 alkali, correct it 



