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 solu

tion,

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

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 ofvirus a lo sodo T alquier od Jn toloo add no lograd mory emilhot anoth Courport of Ind

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-leadmonoxy-hexacetate, 3Pb(C2H3O2)2 PbO.3H2O; tri-lead-dioxy-diacetate, Pb(C2H3O2)22PbO.4H,O; 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(C2H2O2),PbO and 3 parts Pb (C2H2O2),.2PbO. At this Bureau it has been found

advisable to use this product for preparing the clarifying solution, as well as for dry lead clarification, by the Horne method [5].

The laboratory method of preparing the solution prescribed by the Association of Official Agricultural Chemists [3] is as follows:

Boil 430 g of neutral lead acetate, 130 g of litharge and 1 liter of water for 30 minutes. Allow the mixture to cool and settle, and then dilute the supernatant liquid to a specific gravity of 1.25 with recently boiled distilled water. Solid basic lead acetate may be substituted for the normal salt and litharge in the preparation of the solution.

In 1909 The International Commission for Uniform Methods of Sugar Analysis, adopted the solution of basic lead acetate of the German Pharmacopoeia, which is prepared by boiling 3 parts normal lead acetate, 1 part lead oxide, and 10 parts of water. Various other proportions of lead acetate and lead oxide for the preparation of the reagent are given in various handbooks.

With the exception of the dry basic lead acetate prepared by a few firms, the samples occurring in commerce and also samples prepared in the laboratory, even by official methods, vary in composition within wide limits. Basic acetates having the highest proportion of Pbo have the greatest clarifying power, but they also combine to the greatest extent the errors accompanying clarification with this reagent. The constitution of basic acetate may be determined chemically by a double analysis of the sample.

(2) ANALYSIS.-Weigh out 10 g of the solid or take a known volume of the solution containing approximately this quantity of the solid substance, and dissolve in water in a 500-ml flask. In general, this will give a milky solution because of the partial hydrolysis of the lead salt. In order to avoid the possibility of the subsequent formation of basic lead sulfate, it is advisable to add a measured volume of normal acetic acid until a clear solution is obtained. Then add the equivalent of 60 ml of normal sulfuric acid, fill to a volume of 501.3 ml, close the flask, shake thoroughly, and allow the precipitate to settle. The extra 1.3 ml is to compensate for the volume of the precipitated lead sulfate, and is added from a burette after filling the flask to the 500-ml mark.

After the precipitate has settled, determine the excess of sulfuric acid by adding a slight excess of barium chloride to 100 ml of the clear solution. Filter the precipitate, ignite, and weigh as barium sulfate. The calculation of the total lead present computed as lead oxide is as follows:

To ascertain the quantity of lead present in the form of Pb(C2H3O2)2 and that in the form of PbO, add to another 100-ml aliquot portion a few drops of phenolphthalein and titrate with a standard caustic alkali solution, taking the necessary precautions to free the solution of carbonic acid. The calculation of PbO is as follows: 5(total ml of normal acid added-ml of normal alkali) 1/2 mol. wt PbO.

Directions for the preparation of activated litharge are given under (i) (1) p. 237 this Circular.

The total normal acid is the sum of the acetic and sulfuric acids. This computation gives the weight of lead present as lead oxide. If this is subtracted from the total lead oxide, the remainder is the lead oxide present in the form of neutral acetate, and this weight multiplied by the factor 1.4574 reduces it to the weight of neutral acetate.

Basic lead acetate has the great advantage of efficiency, but it has also many disadvantages which require the exercise of great caution in its use. In general, the minimum quantity which is necessary to clarify the solution should be used. This quantity is gaged with requisite accuracy by experienced workers. The needed quantities for particular cases cannot be stated, but the following approximate numbers are given as examples: For Java, Peruvian and Cuban "first" sugars, from 0. 5 to 2 ml of the lead solution; molasses sugars, 2 to 4 ml; Philippine III, 3 to 6 ml. Molasses usually requires 6 to 12 ml. Many analysts follow the lead treatment by adding a little alumina cream. The washed alumina cream is used for high-grade samples, the cream with soluble sulfates for low-grade samples.

(3) CORRECTION FOR VOLUME OF PRECIPITATE. The basic lead acetate owes its clarifying action to its ability to precipitate the suspended albuminoids along with other organic impurities. Since the total volume of 100 ml is occupied by the solution and precipitate, the solution alone occupies somewhat less than the volume indicated and is thus correspondingly concentrated. The error in the polarization thus caused has occasioned considerable discussion, and a number of methods have been devised either to correct or to avoid it.

Method of Sachs. [6].-This is practically a direct measurement of the volume of the precipitate. It is described in the SpencerMeade Handbook [7] as follows: Clarify 100 ml of the juice or the dissolved normal weight with the subacetate as usual. Wash the precipitate by decantation, first with cold water and finally with hot water until all of the sucrose is removed. Transfer the precipitate to a 100-ml flask and add one-half the normal weight of cane sugar (of known polarization), dissolve the sugar and dilute the solution to 100 ml; mix, filter, and polarize, using a 400-mm observation tube. The volume of the precipitate is (100P-100P)/P', in which P is the polarization of the sugar taken, and P' the polarization of the sugar in the presence of the precipitate.

Method of Scheibler. [8].-To 100 ml of the sugar solution 10 ml of lead solution is added and the saccharimetric reading taken. A second solution is prepared by mixing the same volumes of the saccharine liquid and lead solution, which is then diluted to 200 ml and polarized.

The Scheibler method may be expressed by the following equations: r=100R/(100-A), where r is polariscopic reading, R the true reading if the solution were continued in 100 ml, and A the volume of the precipitate. Similarly, r1=100R/(200-A). Combining the two equations and eliminating A, we obtain R=rr/(r-r1). A further simplification is due to C. A. Browne, who deduces the expression R=4r1-r. Method of Horne. The following method gives a more direct determination of this volume and is free from the difficulty of determining small differences between large numbers. A solution of the raw sugar is prepared and precipitated in the usual manner. The precipitate is allowed to settle and is washed by decantation, all the