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:

5[ml H2SO, normality factor–

wt. BaSO4
1/2 mol. wt. BaSO

1/2 mol. wt. PbO

or 5.578 ml H2SO1Xfactor-wt. BaSO,
5.578

0.1167

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

washings being poured through a weighed Gooch crucible. As little of the precipitate as possible is transferred to the filter. When the washing is completed, the precipitate is transferred to a weighed picnometer, which is filled to the mark and weighed. The difference between the first and final weighings gives the total weight of the lead precipitate.

The density of the precipitate is found in the following way:
Let c weight of precipitate transferred in decanting,

A weight of water in picnometer when filled,
B=weight of water and precipitate in picnometer,

C=weight of precipitate in picnometer found by difference be-
tween second and third weighing of Gooch crucible,

C+W total weight of precipitate, and

Density=C/A—(B–C).

The total volume of the precipitate is then its total weight divided by its density, thus

Volume=W/D=C+/[^— (B—C)].

If care is taken to avoid any considerable loss of precipitate during the decantation, the determination may be shortened by neglecting the small quantity of precipitate lost in this way. The washed precipitate may be transferred directly to the picnometer, which is filled and weighed. The picnometer is then emptied directly upon a weighed Gooch filter. The volume of the precipitate is then the weight of water displaced in the picnometer by the precipitate, or A-(B–C). If the weight of the precipitate lost in the decantation does not exceed a few percent, the shorter method is satisfactory.

Horne dry-lead method.-A method intended to avoid rather than correct for the effect of the volume of the lead precipitate has been proposed by Horne [5]. Dissolve the sample in water and make up to 100 ml before adding the clarifer. Add a minimum quantity of dry basic lead acetate until sufficient clarification is obtained. Or, as in careful work with a lead solution, add the solid in successive small amounts until precipitation is almost complete. It is evident that it is necessary to stop short of complete precipitation because an excess of the solid, which does not produce a corresponding precipitate, serves to swell the volume of solution and a corresponding error is introduced. Horne has been able to show that by this method the volume of the solution is very approximately that indicated.

(4) EFFECT ON SUCROSE.-It is often erroneously stated that basic lead acetate has no effect on the rotation of sucrose. The experiments of Bates and Blake [9] show that errors in rotation caused by excessive amounts of basic lead acetate solution are of equal importance to the other errors in saccharimetry. These authors have found, figure 35, that an excess of 2 ml causes a diminution of polarization of 0.10° S; 1 ml, 0.12° S; 2 ml, 0.11° S; 3 ml, 0.09° S. The rotation reaches a minimum value when an excess of 1 ml is present. It returns to its initial value when 6 ml in excess has been added and continues to increase linearly with the amount of lead solution added. If as much as 50 ml is present, the rotation is then increased by a whole degree Ventzke. This source of error is avoided if the minimum quantity of lead solution necessary to clarify is added.

SUGAR

DEGREES

(5) EFFECT ON LEVULOSE.-By the action of basic lead acetate on levulose, the direct polarization may be considerably disturbed. This effect may occur from two causes. A soluble combination of lead and levulose may be formed which has a lower specific rotation than levulose or the lead-levulose compound may be actually precipitated from solution. The result in either case is an increase in dextrorotation or a higher polarization. Prinsen Geerligs has shown that basic acetate of lead precipitates levulose when the same solution contains salts which are capable of producing insoluble compounds with lead.

FIGURE 35.-Influence of lead acetate on normal sugar solution.

The combinations between lead and levulose are very easily broken up by a slight acidification. Acetic acid is sufficiently effective, but many other acids have been used for this purpose. Sulfur dioxide, tannic acid, and, as is frequently claimed, a solution of alum is acid enough to decompose this rather loose combination. In any case, only a slight excess of acid should be present.

(d) BASIC LEAD NITRATE (HERLES SOLUTION)

Herles solution [10] is prepared by dissolving 100 g of solid NaOH in 2 liters of water, and a second solution is prepared by dissolving 1 kg of neutral lead nitrate in 2 liters of water. Upon mixing equal volumes of the two solutions, basic lead nitrate is precipitated, the reaction being expressed by the equation

2Pb(NO3)2+2NaOH=Pb(NO3)2.Pb(OH)2+2NaNO3.

The precipitate is washed free of soluble impurities and mixed with water to a cream for use in clarification. The clarification may also be performed by forming the basic lead nitrate in the solution to be polarized. This is done by first adding a measured quantity of the above lead nitrate solution, 1 to 10 ml, according to the darkness of the sample, and then, after mixing, adding an exactly equal quantity

of the alkaline solution. An excess of alkali must be avoided. The mixture is then shaken and made to volume. The latter procedure gives the better clarification but introduces a considerable quantity of soluble salts, which may affect the polarization. The defects of the basic nitrate are, in general, those of the basic acetate. The volume of the precipitate is even greater because of the bulk of the solid clarifier. The precipitation of reducing sugar is even more marked than in the case of the basic acetate.

(e) HYPOCHLORITE (ZAMERON METHOD)

The hypochlorite solution is prepared by grinding 625 g of dry bleaching powder in a mortar with 1 liter of water. The mass is squeezed out in a sack and the extract filtered through paper. The filtered solution, about 700 to 800 ml of about 18° Baumé, is preserved in a stoppered dark-glass bottle in darkness. To perform the clarification, a few milliliters of the hypochlorite solution is added to the sugar solution and then a few milliliters of neutral lead acetate solution. The reagent usually causes a slight rise in temperature so that the solution should be readjusted to the temperature of the polariscope before making to volume. This method of clarification is very effective, and if no great excess of the reagent is employed, the reducing sugars are unaffected. The volume of the precipitate, which is increased because of the presence of insoluble lead chloride, is the main fault of this method.

(f) HYDROSULFITE

Sodium hydrosulfite is prepared by the reaction of zinc, sodium bisulfite, and sulfuric acid according to the formula

2NaHSO3+Zn+H2SO4=ZnS2O4+Na2SO4+2H2O.

The zinc hydrosulfite is changed to the sodium compound by the reaction

ZnS2O4+Na2CO3=Na2S2O4+ZnCO3.

The sodium hydrosulfite is salted out from solution by means of sodium chloride and dehydrated by warming with strong alcohol. The compound is then dried in a vacuum at 50° to 60° C. This substance is produced commercially under the names of Blankit and Redo. It is frequently used in sugar manufacture for bleaching massecuites and, in dissolved form, as a wash for whitening sugar in centrifugal machines. To prepare a solution for polarization, a quantity of alumina cream is added and then a few crystals of hydrosulfite, 0.1 to 1 g, according to the color of the solution. The solution is made up to volume, shaken thoroughly, and filtered. As the clarified solutions occasionally redarken, they should be polarized immediately. The clarifying action, according to Weisberg [11], is due to free sulfurous acid and nascent hydrogen. The reduction by the latter leaves compounds which may be reoxidized and cause a redarkening of the solution.

Another hydrosulfite derivative (sodium sulfoxylate-formaldehyde) known as Rongalite accomplishes a permanent clarification but it is slower and less effective than Blankit.

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