test procedure of purity

35. Procedure

35.1 Weigh 3 g to 5 g of the sample into a tared low-form,

65-mm diameter glass weighing dish fitted with a cover. Dry to

constant mass at 105 °C 6 1 °C in either a gravity or a

mechanical convection oven. Weigh at the end of an initial 2 h

heating period, then continue with 30-min heating periods until

the change in mass during a 30-min heating period is not more

than 0.10 %. If there is an increase in mass of the sample

during one or more drying periods, record the lowest mass

observed as the mass for use in the calculation of moisture

content. Calculate the loss in mass as the percent of moisture in

the sample.

35.2 Weigh 3 g 6 0.1 g of the sample, in the “as-received”

condition, to the nearest 0.001 g and transfer to a 400-mL

beaker.

35.3 Add 150 mL of ethanol (80 %) that has been heated to

between 60 °C and 65 °C, and immediately place the beaker in

a constant-temperature water bath maintained at 60 °C to

65 °C. The level of the water in the bath should be somewhat

higher than the level of the liquid in the beaker. Cover the

beaker as completely as possible with a lid that will permit

mechanical stirring. Lower a mechanical stirrer almost to the

bottom of the beaker, and stir for 10 min at a rate suitable to

provide good agitation without spattering material on the walls

of the beaker above the liquid level.

35.4 Stop the stirrer. Allow the undissolved matter to settle

with the beaker still in the bath, and then decant the hot

supernatant liquid as completely as possible through a tared,

fritted-glass filtering crucible.

35.5 Add 150 mL of ethanol (80 %), at 60 °C to 65 °C to the

beaker and proceed in accordance with 32.3 and 32.4.

35.6 After decanting the supernatant liquid as completely as

possible, transfer the insoluble matter to the crucible with the

aid of ethanol (80 %) at 60 °C to 65 °C in a wash bottle, being

careful to scrape all insoluble matter from the lid, the stirrer,

and the beaker. A total of about 250 mL of ethanol (80 %) will

normally be required to transfer the insoluble matter to the

crucible and to further wash the insoluble matter in the

crucible. During the operations prescribed in this paragraph,

apply suction only while filtration is in progress. Make every

effort to avoid drying-out of the filter cake. If fines appear to

pass through the filter, use only gentle suction.

35.7 Wash the residue in the crucible with 50 mL of ethanol

(95 %) at room temperature, and finally with several portions

of ether at room temperature (Note 7). Without permitting

suction to continue longer than necessary, place the crucible in

a beaker or weighing bottle on the steam bath until no odor of

ether can be detected.

NOTE 7—Thorough washing with ether is necessary to remove ethanol

completely from the insoluble matter. If ethanol is not completely

D1439 ? 22

6

removed before oven drying, it may not be completely removed during the

oven drying.

35.8 Place the crucible in an oven at 105 °C 6 1 °C for 1 h.

Stir the contents of the crucible with a dissecting needle or thin

rod (preferably of smooth-surfaced metal) to break up the cake

and facilitate complete drying. Again, dry at 105 °C 6 1 °C for

1 h. Place the crucible in a desiccator. Cover it with a flat glass

plate, weighing bottle cover, or other suitable cover to mini-

mize absorption of moisture from the atmosphere in the

desiccator; and cool to room temperature (at least 30 min).

Weigh the uncovered crucible as rapidly as possible.

35.9 Dry the crucible for additional 1 h periods until the

change in mass during a 1-h drying period does not exceed

0.003 g. If increases in mass are observed after such additional

drying periods, record the lowest mass observed as the mass of

the crucible plus dry sodium carboxymethylcellulose.

36. Calculation

36.1 Calculate the percent sodium carboxymethylcellulose,

S, on the dry basis as follows:

S 5 ~A 310000!/~B ~100 2 C!!

(9)

where:

A

= mass of dried residue, g,

B

= mass of specimen used, g, and

C

= moisture in the specimen as received, %.