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served before analysis or for subsequent reference, they should be placed in tightly sealed containers in order to avoid loss by evaporation. Further details as to the correct procedure in sampling will be found in the United States Treasury Customs Regulations, page 786.

2. DIRECT POLARIZATION

The determination most widely used in the analysis of sugar and sugar products, both in chemical control during extraction and refining and also in the adjustment of commercial transactions, is the direct polarization. The term "direct polarization" is defined as the reading on the saccharimeter of 26.000 g of the sample dissolved in 100 ml of solution at 20° C, with only such substances removed by clarification as impede the passage of light. The value thus obtained is the resultant optical rotation of all optically active substances present in the solution and indicates the percentage of sucrose only in cases where the other constituents have no effect on the rotation. Distinguished from the direct polarization, is the Clerget, or doublepolarization, method, which indicates the true percentage of sucrose. În sugars of high purity, the direct polarization and the sucrose by the Clerget method give results that closely agree, but in low-grade sugars and molasses the differences may become considerable.

The direct polarization is executed in a great variety of ways with respect to the details of manipulation. The following method is the one most conveniently used in commercial work, and if carefully performed, the results are sufficiently uniform and reproducible for the adjustment of sugar trade relations and for the control of sugar manufacture in the sugar industry.

The sample is thoroughly mixed, all lumps being broken up, and weighed very quickly in the weighing dish, great care being taken to prevent moisture change during weighing, by hastening the process as much as needed accuracy permits. (If the polariscope is read to only 0.05° S, it is useless to weigh more closely than 0.015 g). By means of a jet of distilled water, the sample is then washed into a 100-ml sugar flask and dissolved. This is readily accomplished after a little practice. However, if difficulty is experienced, the transference to the flask may readily be made by use of a funnel, the stem of which extends just into the body of the flask. The sugar is then brought into solution by a few minutes shaking, a shaking machine being convenient if large numbers of samples are to be handled. After solution is complete, the clarifying agent is added. The method of clarification depends on the nature and amount of impurities present. The universal principle is to add the minimum quantity necessary to clarify, whatever the agent added. In a large proportion of samples the clarification consists in the addition of 0.5 to 2 ml of basic lead acetate solution. The contents are mixed and the volume completed to 100 ml, the neck of the flask being washed down. If foam appears on the surface of the meniscus, rendering it impossible to adjust the volume accurately, it may be dispersed by blowing upon it a small quantity of alcohol or ether from an atomizer.

To obtain good definition while making up to volume, the bottom of the meniscus should be made to appear dark by a reflection of the finger or of a section of rubber tubing placed a few millimeters below the mark on the neck of the flask. The meniscus, shaded in this way,

should be made tangential to the upper edge of the graduation mark. It is essential that the temperature of the solution during the process be the same as that of the saccharimeter, tubes, and quartz control plates.

The solution is then shaken thoroughly and the entire contents of the flask is poured on a filter in a stemless funnel. The first 25 ml of the filtrate is rejected, since this portion is almost invariably made turbid by the passage of a small amount of the lead precipitate through the filter paper. In addition, the first portions passing through the dry filter paper suffer a change in concentration, as shown by Hardin and Zerban [1]. By rejecting the 25-ml portion, this absorption effect is largely or wholly avoided. During the filtration it should be made an invariable practice to cover the funnels with cover glasses to pre

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FIGURE 32.-Bates saccharimeter and lamp.

vent evaporation of the solution. This important precaution is frequently neglected. The contention that the evaporation is one of the constant errors of this method is not valid. The evaporation is not constant but depends on the accidental conditions of temperature, relative humidity of the atmosphere, and the movement of air in the room. Bates and Phelps [2] have made an exhaustive study of the influence of atmospheric conditions in the testing of sugars. They found for raw sugars, filtered once, that:

1. The increase in the polarization due to evaporation is negligible in ordinary testing for all potential heads (P-Pa) up to 22 mm, and that it is therefore unnecessary to use any precautions to prevent or to correct for evaporation for ordinary atmospheric conditions, provided the duration of the filtration does not exceed 10 or 12 minutes.

DEGREES SUGAR

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2. If the correction for the increase in polarization is desired, it may be obtained from the following equation:

Q=0.00017(P.-P.)T,

where Q increase in polarization in degrees sugar,

(36)

Ps saturation vapor pressure at the temperature of the solution,

Pa saturation vapor pressure at the temperature of the dew point in the air,

T-time of filtration in minutes.

Q should be subtracted from the observed polarization to obtain the true polarization.

3. Practically all increase in polarization, regardless of atmospheric conditions, may be prevented by covering the funnel with a watch glass.

When all or a part of the filtrate is returned to the filter, the concentration is increased, since this filtrate takes up the solution already

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(Ps -Pa) IN MM OF HG

CHANGE IN POLARIZATION PER MINUTE=

FIGURE 33.-Curves showing the effect of evaporation on the polarization of raw

sugars.

adhering to the filter paper and which has become concentrated by evaporation after the level of the solution in the funnel has fallen. Bates and Phelps show graphically in figure 33 the change in polarization per minute when normal solutions of raw sugars are poured back after filtration.

Before filling the polariscope tube, it is rinsed two or three times with the filtered solution. During the rinsing the filtration cylinder should be given a rotary motion to stir its contents. This stirring brings the solution to a uniform density, thereby permitting a sharp focusing of the eyepiece of the saccharimeter. Care should be exercised to avoid errors due to the physical condition of the tube.

The accepted reading of the saccharimeter scale should be the average of at least three settings of the end point, and it is a great advantage to use both eyes in making the observations.

Direct polarization by this method gives a value which is uniformly reproducible. It does not, however, represent the percentage

of sucrose in raw sugars, because it does not take into account other optically active substances which are usually present.

3. CLARIFICATION

(a) GENERAL

All methods of clarification at present available have accompanying disadvantages which necessitate great precautions in order to minimize their effect.

The choice of a clarifying agent for polariscopic work depends largely on the color of the sample to be tested. Agents in most frequent use are alumina cream, basic lead acetate, and decolorizing carbon. In less frequent use, but having some advantages in special cases, are neutral lead acetate, basic lead nitrate, alum, sodium hydrosulfite, and sodium hypochlorite.

(b) ALUMINA CREAM

Alumina cream is a suspension of aluminum hydroxide Al(OH), in water. It is prepared by precipitation from alum or aluminum sulfate solution by means of ammonia. The precipitate is washed free of soluble salts or left unwashed, depending on the use to which it is to be put.

In case the precipitate is to be washed, it is advisable to add the ammonia in slight excess. The washing of the precipitate may be conveniently carried out by suspending the mixture in parchmentpaper bags in a vessel of water, changing the water in the vessel frequently, or it may be washed in the usual way on a filter, provided caution is used to prevent the precipitate from becoming dry. The washing is continued until a portion of the wash water tested with barium chloride shows only traces of dissolved sulfates. The washed alumina cream may be used either as the sole clarifier for high-grade samples, if its action is sufficiently effective, or it may be used in conjunction with basic lead acetate. When used with lead, it increases the clarifying action of the basic lead acetate and permits the use of a smaller quantity than would otherwise be necessary. When the washed alumina cream is used alone, the only error introduced is caused by the volume of the precipitate of aluminum hydroxide. If only a few milliliters is used, the volume of the dry solid is small, and for all ordinary purposes, negligible.

If the alumina cream contains an excess of alum and other soluble sulfates, its use is recommended by many as an aid to clarification by basic lead acetate in the analysis of very impure products where a large quantity of lead acetate is required. The alumina cream then fulfills several purposes. It precipitates the excess of lead as lead sulfate. It adds its own clarifying effect and tends to furnish a slightly acid solution, which decomposes some of the compounds formed by lead with some sugars, notably levulose.

Impure saccharine products usually contain large quantities of dissolved inorganic salts, so that the addition of a clarifier containing a relatively small quantity of soluble salts is not seriously detrimental. The method of preparation of the Association of Official Agricultural Chemists [3] is as follows: Prepare a cold saturated solution of alum in water. Add ammonium hydroxide with constant stirring

until the solution is alkaline to litmus, allow the precipitate to settle, and wash by decantation with water until the wash water gives only a slight test for sulfates with barium chloride solution. Pour off the excess of water and store the residual cream in a stoppered bottle.

(c) BASIC LEAD ACETATE

(1) PREPARATION.-Basic lead acetate is the clarifying agent most extensively used. It is formed by the chemical combination of normal

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FIGURE 34.-Isothermal equilibrium between lead acetate, lead oxide, and water at 25° C.

lead acetate, Pb(C2H3O2)2 with litharge, PbO. Jackson [4] in a study of the equilibrium in the system lead acetate, lead oxide, and water, has shown that four compounds capable of existing in the solid phase are neutral lead acetate, Pb(C2H3O2)2.3H2O; tetra-leadmonory-hexacetate, 3Pb(C2H3O2)2 PbO.3H2O; tri-lead-dioxy-diacetate, Pb(C2H3O2)22PbO.4H2O; and lead hydroxide, Pb(OH)2. The basic lead acetate of commerce is a mixture of the two basic acetates, 3Pb (C2H3O2)2. PbO and Pb(C2H3O2)2.2PbO. The reagent known as Horne's dry lead has been found to be quite uniform in composition. It consists of a mixture corresponding to 4 parts of 3Pb (C2H3O2)2PbO and 3 parts Pb (C2H3O2)2.2PbO. At this Bureau it has been found

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