After determining the Loss on drying
, weigh a quantity of undried Polyvinyl Alcohol, equivalent to 6.00 g on the dried basis. Over a period of seconds, transfer the test specimen with continuous slow stirring to about 140 mL of water contained in a suitable tared flask. When the specimen is well-wetted, increase the rate of stirring, avoiding mixing in excess air. Heat the mixture to 90
, and maintain the temperature at 90
for about 5 minutes. Discontinue heating, and continue stirring for 1 hour. Add water to make the mixture weigh 150 g. Resume stirring to obtain a homogenous solution. Filter the solution through a tared 100-mesh screen into a 250-mL conical flask, cool to about 15
, mix, and proceed as directed under Viscosity 911
Wash the tared 100-mesh screen used in the test for Viscosity
with two 25-mL portions of water, and dry at 110
for 1 hour: not more than 6.4 mg of water-insoluble substances is found (0.1%).
Degree of hydrolysis
Transfer about 1 g of Polyvinyl Alcohol, previously dried at 110
to constant weight and accurately weighed, to a wide-mouth, 250-mL conical flask fitted by means of a suitable glass joint to a reflux condenser. Add 35 mL of dilute methanol (3 in 5), and mix gently to assure complete wetting of the solid. Add 3 drops of phenolphthalein TS, and add 0.2 N hydrochloric acid or 0.2 N sodium hydroxide, if necessary, to neutralize. Add 25.0 mL of 0.2 N sodium hydroxide VS, and reflux gently on a hot plate for 1 hour. Wash the condenser with 10 mL of water, collecting the washings in the flask, cool, and titrate with 0.2 N hydrochloric acid VS. Concomitantly perform a blank determination in the same manner, using the same quantity of 0.2 N sodium hydroxide VS.
Calculation of saponification value
Calculate the saponification value by the formula:
)N56.11] / W
in which B
are the volumes, in mL, of 0.2 N hydrochloric acid VS consumed in the titration of the blank and the test preparation, respectively, N is the exact normality of the hydrochloric acid solution, W
is the weight, in g, of the portion of Polyvinyl Alcohol taken, and 56.11 is the molecular weight of potassium hydroxide.
Calculation of degree of hydrolysis
Calculate the degree of hydrolysis, expressed as percentage of hydrolysis of polyvinyl acetate, by the formula:
in which S
is the saponification value of the Polyvinyl Alcohol taken: between 85% and 89% is found.