Part 22 (2/2)

_Pota.s.sium Iodide Solution_--Dissolve 150 g. of pota.s.sium iodide in water and make up to one liter.

_N_/10 _Pota.s.sium b.i.+.c.hromate_--Dissolve 4.903 g. of C. P. pota.s.sium b.i.+.c.hromate in water and make the volume up to one liter at the temperature at which t.i.trations are to be made.

The Committee calls attention to the fact that occasionally pota.s.sium b.i.+.c.hromate is found containing sodium b.i.+.c.hromate, although this is of rare occurrence. If the a.n.a.lyst suspects that he is dealing with an impure pota.s.sium b.i.+.c.hromate, the purity can be ascertained by t.i.tration against re-sublimed iodine. However, this is unnecessary in the great majority of cases.

_Standardization of the Sodium Thiosulfate Solution_--Place 40 cc. of the pota.s.sium b.i.+.c.hromate solution, to which has been added 10 cc. of the solution of pota.s.sium iodide, in a gla.s.s-stoppered flask. Add to this 5 cc. of strong hydro-chloric acid. Dilute with 100 cc. of water, and allow the _N_/10 sodium thiosulfate to flow slowly into the flask until the yellow color of the liquid has almost disappeared. Add a few drops of the starch paste, and with constant shaking continue to add the _N_/10 sodium thiosulfate solution until the blue color just disappears.

DETERMINATION--Weigh accurately from 0.10 to 0.50 g. (depending on the iodine number) of the melted and filtered sample into a clean, dry, 16-oz. gla.s.s-stoppered bottle containing 15-20 cc. of carbon tetrachloride or chloroform. Add 25 cc. of iodine solution from a pipette, allowing to drain for a definite time. The excess of iodine should be from 50 per cent to 60 per cent of the amount added, that is, from 100 per cent to 150 per cent of the amount absorbed. Moisten the stopper with a 15 per cent pota.s.sium iodide solution to prevent loss of iodine or chlorine but guard against an amount sufficient to run down inside the bottle. Let the bottle stand in a dark place for 1/2 hr. at a uniform temperature. At the end of that time add 20 cc. of 15 per cent pota.s.sium iodide solution and 100 cc. of distilled water. t.i.trate the iodine with _N_/10 sodium thiosulfate solution which is added gradually, with constant shaking, until the yellow color of the solution has almost disappeared. Add a few drops of starch paste and continue t.i.tration until the blue color has entirely disappeared. Toward the end of the reaction stopper the bottle and shake violently so that any iodine remaining in solution in the tetrachloride or chloroform may be taken up by the pota.s.sium iodide solution. Conduct two determinations on blanks which must be run in the same manner as the sample except that no fat is used in the blanks. Slight variations in temperature quite appreciably affect the t.i.ter of the iodine solution, as acetic acid has a high coefficient of expansion. It is, therefore, essential that the blanks and determinations on the sample be made at the same time. The number of cc. of standard thiosulfate solution required by the blank, less the amount used in the determination, gives the thiosulfate equivalent of the iodine absorbed by the amount of sample used in the determination.

Calculate to centigrams of iodine absorbed by 1 g. of sample (= per cent iodine absorbed).

DETERMINATION, TUNG OIL--Tung oil shows an erratic behavior with most iodine reagents and this is particularly noticeable in the case of the Ha.n.u.s reagent which is entirely unsuitable for determining the iodine number of this oil since extremely high and irregular results are obtained. The Hubl solution shows a progressive absorption up to 24 hrs.

and probably for a longer time but the period required is entirely too long for a chemical determination. The Wijs solution gives good results if the following precautions are observed:

Weigh out 0.15 0.05 g., use an excess of 55 3 per cent Wijs solution. Conduct the absorption at a temperature of 20-25 C. for 1 hr.

In other respects follow the instructions detailed above.

SAPONIFICATION NUMBER (KOETTSTORFER NUMBER).

PREPARATION OF REAGENTS. _N/2 Hydrochloric Acid_--Carefully standardized.

_Alcoholic Pota.s.sium Hydroxide Solution_--Dissolve 40 g. of pure pota.s.sium hydroxide in one liter of 95 per cent redistilled alcohol (by volume). The alcohol should be redistilled from pota.s.sium hydroxide over which it has been standing for some time, or with which it has been boiled for some time, using a reflux condenser. The solution must be clear and the pota.s.sium hydroxide free from carbonates.

DETERMINATION--Weigh accurate about 5 g. of the filtered sample into a 250 to 300 cc. Erlenmeyer flask. Pipette 50 cc. of the alcoholic pota.s.sium hydroxide solution into the flask, allowing the pipette to drain for a definite time. Connect the flask with an air condenser and boil until the fat is completely saponified (about 30 minutes). Cool and t.i.trate with the _N_/2 hydrochloric acid, using phenolphthalein as an indicator. Calculate the Koettstorfer number (mg. of pota.s.sium hydroxide required to saponify 1 g. of fat). Conduct 2 or 3 blank determinations, using the same pipette and draining for the same length of time as above.

MELTING POINT.

APPARATUS--_Capillary tubes_ made from 5 mm. inside diameter thin-walled gla.s.s tubing drawn out to 1 mm. inside diameter. Length of capillary part of tubes to be about 5 cm. Length of tube over all 8 cm.

_Standard thermometer_ graduated in tenths of a degree.

_600 cc. beaker._

DETERMINATION--The sample should be clear when melted and entirely free from moisture, or incorrect results will be obtained.

Melt and thoroughly mix the sample. Dip three of the capillary tubes above described in the oil so that the fat in the tube stands about 1 cm. in height. Now fuse the capillary end carefully by means of a small blast flame and allow to cool. These tubes are placed in a refrigerator over night at a temperature of from 40 to 50 F. They are then fastened by means of a rubber band or other suitable means to the bulb of a thermometer graduated in tenths of a degree. The thermometer is suspended in a beaker of water (which is agitated by air or other suitable means) so that the bottom of the bulb of the thermometer is immersed to a depth of about 3 cm. The temperature of the water is increased gradually at the rate of about 1 per minute.

The point at which the sample becomes opalescent is first noted and the heating continued until the contents of the tube becomes uniformly transparent. The latter temperature is reported as the melting point.

Before finally melting to a perfectly clear fluid, the sample becomes opalescent and usually appears clear at the top, bottom, and sides before becoming clear at the center. The heating is continued until the contents of the tube become uniformly clear and transparent. This temperature is reported as the melting point.[33] It is usually only a fraction of a degree above the opalescent point noted. The thermometer should be read to the nearest 1/2 C., and in addition this temperature may be reported to the nearest degree Fahrenheit if desired.

CLOUD TEST.

PRECAUTIONS--(1) The oil must be perfectly dry, because the presence of moisture will produce a turbidity before the clouding point is reached.

(2) The oil must be heated to 150 C. over a free flame, immediately before making the test.

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