XXVI. METHODS FOR THE ANALYSIS OF BAKING POWDER AND

BAKING POWDER CHEMICALS.-PROVISIONAL.

All the processes hereafter described, except determination of acidity, may be employed in the analysis of baking powders, and all the processes, except determination of carbonic acid, in the analysis of cream of tartar and its substitutes. The sample under examination is entirely removed from the package, carefully mixed, and passed through a sieve without grinding.

1. Total Carbon Dioxid.

Make the determination by the absorption method, employing any apparatus which gives accurate results when checked with pure calcite. Whatever apparatus is chosen, the tubes and materials used for absorbing and drying the carbon dioxid may be varied according to the preference of the analyst. According to the amount of absorbent employed vary the weight of sodium or calcium carbonate from 0.25 to 1 gram, and use about twice as much baking powder. The corrections for temperature and pressure given with the Heidenhain apparatus may ordinarily be disregarded.

(a) METHOD USING KNORR'S APPARATUS.

(1) DESCRIPTION OF APPARATUS.

This apparatus (fig. 9) has only ground-glass joints, and may be quickly made ready for use or taken to pieces and packed away. On the other hand, it is inflexible and must be carefully handled, and has the additional disadvantage that broken parts can not readily be replaced. Therefore it is of more value for occasional determinations than for a long series.

(2) PREPARATION OF REAGENTS.

(1) The potassium hydroxid solution usually employed for absorbing carbon dioxid has a specific gravity of about 1.27. Many analysts, however, prefer a solution having a specific gravity of about 1.55.

(2) The calcium chlorid and soda lime employed should be finely granulated and freed from dust with a sieve.

(3) DETERMINATION.

Close

Place some of the baking powder in a perfectly dry distilling flask. the flask with a stopper carrying the tube connecting with the absorption apparatus and also with the funnel tube. Weigh the tubes in which the carbon

a The small calcium chlorid tube shown in the cut attached to the potash bulb F is usually replaced by a second Liebig bulb with sulphuric acid. Better results are obtained if the same drying tubes are used before and after the potash bulb. Many analysts prefer to replace the bulb F and attached calcium chlorid tube by two U-tubes filled with sifted soda lime. When the second tube shows a material increase in weight it is placed first, and the first tube refilled and placed in the second position.

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dioxid is to be absorbed and attach them to the apparatus. In case two Liebig bulbs are employed, one for potassium hydroxid and the other for sulphuric acid to absorb the moisture given up by the potassium hydroxid solution, weigh them separately. If two soda-lime tubes are employed it will be found advantageous to weigh them separately and fill the first tube anew when the second tube begins to increase in weight materially. Nearly fill tube B with hydrochloric acid (sp. gr. 1.1) and place the guard tube Cin position. Then start the aspirator at such a rate that the air passes through the Liebig bulb at about the rate of two bubbles per second. Open the stopper of the funnel and allow

FIG. 9.-Knorr's apparatus for the determination of carbon dioxid; A, distilling flask fitted to condenser by a ground-glass stopper. B, Reservoir containing acid. C, Soda-lime tube fitted to acid reservoir by a ground-glass stopper. D, Condenser. E, Liebig bulb filled with sulphuric acid. F, Liebig bulb with a solution of potassium hydroxid for the absorption of carbon dioxid and followed by a calcium-chlorid tube. An additional guard tube filled with soda lime should follow the tube F, though not shown in the cut.

the acid to run slowly into the flask, care being taken that the evolution of gas shall be so gradual as not to materially increase the current through the Liebig bulb. After all the acid has been introduced continue the aspiration and gradually heat the contents of the flask to boiling, the bulb in tube B being closed. While the flask is being heated the aspirator tube may be removed, although many analysts prefer when using ground-glass joints to aspirate during the entire operation. Continue the boiling for a few minutes after the water has begun to condense in D, then remove the flame, open the valve in tube B, and allow the apparatus to cool with continued aspiration. Remove the absorption tubes and weigh. The increase in weight is due to carbon dioxid.

(b) METHOD USING HEIDENHAIN'S APPARATUS.

(1) DESCRIPTION OF APPARATUS.

This apparatus was originated by G. J. Mulder, recommended and improved by Kolbe, Stolba, and Fresenius," and modified by H. Heidenhain. As shown in fig. 10, drawn on a scale of 1: 12, it consists of

A. A cylinder filled with soda lime to free the air from carbon dioxid. A thick layer of cotton prevents soda-lime dust from being carried over.

B. Glass cock to regulate the air current, which finds resistance at C.
C. A capillary contraction.

D. Funnel tube of peculiar shape. The funnel is cylindrical, three-fourths of an inch wide and 4 inches long, and is reduced to half its width at the bottom, so as to make a neck for a perforated rubber stopper.

E. A glass tube is tightly fitted into the perforated rubber stopper, allowing the stopper to be taken out and replaced by the glass tube.

F. Evolution flask, ordinarily of 150 ce capacity, for foaming liquids of 300 cc capacity.

G. Return condenser, simply a glass tube of one-fourth of an inch bore, around which a small lead pipe is wound. The tube following the condenser contains a few pieces of calcium chlorid to retain the bulk of the moisture. It is refilled when contents are liquefied.

H. U tube filled with coarse calcium chlorid.

K. Filled at I with a 3-inch long column of pumice stone impregnated with copper sulphate completely dehydrated at 150° C. The remainder of the tube is filled with fine calcium chlorid.

L. Cock to close the apparatus when not in use.

M. First absorption tube about one-half inch in diameter and 5 inches long, filled mainly with soda lime, with a little calcium chlorid at the side at which the air current enters.

N. Second absorption tube of same size as M, filled half with soda lime and half with calcium chlorid. Place the side containing calcium chlorid toward the end of the apparatus where the air current leaves.

O. Guard tube containing calcium chlorid toward N and soda lime toward P. P. Indicator tube trapped with glycerin.

R. Safety bottle to receive water which may be sucked back from the aspirator.

S. The aspirator, which is a Mariette's bottle of about 4 liters capacity.

(2) PREPARATION OF REAGENTS.

Use calcium chlorid dehydrated at 200° C., not fused. Grind it coarsely in a coffee mill and sift through No. 18 wire gauze to remove the extremely coarse and through No. 30 wire gauze to remove the very fine. Prepare a large quantity of such calcium chlorid at the beginning and use this for the tubes K. M, and N. The reason for this is that the current of air must leave the weighed tubes with the same content of moisture as it entered them, which only can be attained if the absorbent in K and N is of the same nature and quality.

Grind and sift the soda lime for the weighed tubes in the same way. It

@ Fresenius, Quantitative Analysis, 1: 449; 2: 308, German edition.

J. Amer. Chem. Soc., 1896, 18: 1.

An excellent method for the preparation of soda-lime is given by Benedict and Turner, J. Amer. Chem. Soc., 1899, 21: 396.

should not be too dry, as it must not absorb moisture to a higher degree than calcium chlorid. The tubes M and N should hold about 20 grams, making M's capacity for carbon dioxid almost 1 gram and N's capacity for moisture 0.2

gram. M should be refilled when its weight has increased 0.75 gram and N after an increase of 0.1 gram in weight.

Use best rubber for all connections, applying a trace of castor oil as lubricator. For connections of the weighed tubes use rubber tubing boiled in weak lye, washed and dried. Apply also a little castor oil, which is thoroughly wiped

off again before connecting the tubing.

Before using the apparatus fill H and K with carbon dioxid in order to saturate the alkalinity of the calcium chlorid, and exhaust after several hours.

(3) DETERMINATION.

Weigh the tubes M and N, taking precaution that they are of the same temperature as the air in the balance room. Shortly before weighing open the tubes for a moment to allow equalization of air. Note thermometer and barometer. Connect tubes with the apparatus and make sure that all joints are tight by closing A at the bottom, opening all cocks, starting the aspirator, and observing P, in which the liquid must soon come to a standstill. Then disconnect the aspirator, close B, remove F, put in the substance (use about 1 gram of sodium carbonate or calcium carbonate, or about 2 grams of baking powder), connect F, and start the cooler. Introduce acid and water through D, lifting E slightly, and allowing only small quantities of the acid and water to enter at the time. Use only carbon dioxid free water. Light the burner, heat to boiling, and reduce the flame to keep the liquid just at the boiling point. If no more air passes P, start the aspiration. When water stops running, open B carefully and adjust the outflow of the aspirator by raising or lowering the syphon to half the safe speed.

(In order to find the allowable rapidity of the air current, proceed as follows: Charge the apparatus exactly as for an analysis, leaving out the carbonate. Begin to aspirate at the rate of about 50 cc per minute. After 2 liters have been aspirated weigh the tubes. If they have lost in weight, repeat the experiment with 40 cc per minute, and so on until the weight of the tubes remains constant. If the work has been done with due precaution, the first tube must have lost just as much as the second has gained. Do not exceed the safe speed thus found.)

After M has become cool increase the current to full safe speed and aspirate altogether 3 liters, continuing boiling to the end of aspiration. After the tubes have assumed the temperature of the balance room open for a moment, weigh, note again thermometer and barometer, and apply correction according to the following formulæ :

in which

— (A2—A1) ×T and + (B-B1) ×B

A' is the temperature at first weighing in °C,
A is the temperature at second weighing in °C,
B' is the air pressure at first weighing in mm,

B2 is the air pressure at second weighing in mm,

and T and B are constants found as follows:

G=weight of empty tubes.
F=weight of fillings.

The specific gravity of glass=2.7.

The specific gravity of filling=2.0.

The specific gravity of brass=8.5.

Change of weight of 1 ce air with 1°=0.0000039 gram.

Change of weight of 1 cc air with 1 mm pressure=0.0000015 gram.