XXVI. PURITY

1. CALCULATION

The "coefficient of purity" is the percentage of sucrose in the solids or dry substance of a sugar product and may be expressed by the formula

Purity=

percent sucrose.
percent solids

X100

(131)

It is also known as the "purity quotient" or more generally as the "purity." If accurate methods are used in the determinations, such as the Clerget for sucrose, and drying for solids, the above ratio is known as the "true purity." This value is used in special research and for purposes of checking or comparing products at different stages of manufacture and should always be recorded and referred to as true purity.

Of more general use for purposes of process control in individual factories is the "apparent purity," the value of which is found by dividing the direct polarization of the product in solution by the degrees Brix and multiplying the quotient by 100. Although the apparent purity is only an approximation, it is of great value for comparative purposes because of the rapidity with which the determinations may be made. It is important, therefore, that the analyses for each type of product be performed in exactly the same manner.

The procedure for apparent purity is applied directly to raw or thin juices without weighing or measuring. Solids or massecuites must be dissolved and thick liquors diluted. The solution is diluted with water to a convenient density between 12 and 20 Brix, thoroughly mixed, and allowed to stand in the cylinder until all air has escaped. The removal of air bubbles may be hastened by connecting the cylinder to suction by means of a one-hole stopper and suitable tubing. The degrees Brix is then read using standard hydrometers and thermometers, the reading being corrected to 20° C.

To a portion of the solution (150 to 200 ml) roughly measured, a sufficient amount of anhydrous basic lead acetate is added to clarify. About a teaspoonful of dry diatomaceous earth is added and the whole is thoroughly mixed, and filtered on a rapid filter paper. The clear filtrate may be polarized directly in a 200-mm tube or, if it is deeply colored, in a 100-mm tube. In cane products, varying amounts of invert sugar are present and the negative rotation of the levulose constituent is reduced by the presence of basic lead acetate. The positive rotation of the dextrose is not thus affected, so that the net result is a plus error in rotation.

When the amount of invert sugar is high, the effect of the excess lead may be obviated in either of two ways: (1) After addition of the dry lead salt and shaking, dry powdered oxalic acid is added, a little at a time, until the leaded solution is faintly acid to litmus. The whole is then thoroughly mixed and filtered. The clear filtrate is polarized as above. (2) The filtrate from the solution treated with lead alone (100 ml in a 110-ml flask) is treated with dilute acetic acid until the reaction of the solution to litmus paper is slightly acid. Dilution to the 110-ml mark is completed with water, and the solution is mixed and polarized. The polarization is corrected by adding one-tenth of the observed reading.

In beet products little or no invert sugar is normally present. A solution of basic lead acetate (sp gr 1.25, 20°/20°) is commonly used for clarification instead of the dry salt. To 100 ml of the solution (after determining the Brix) contained in a 100-110 ml flask is added the proper amount of lead solution from a burette. The solution is mixed, a drop or two of ether being added to the flask to disperse the foam, if present, and the volume is completed to 110 ml with water. The solution is then filtered and polarized and the reading corrected by the addition of one-tenth of its observed value. The apparent purity of a sugar solution may be calculated by means of the formula

Apparent purity=Factor direct polarization.

(132)

A formula for obtaining the "factor" to be used in the second term of the above equation was originally elaborated by Cassamajor and was based upon the Mohr cubic centimeter at 17.5° C and the normal weight, 26.048 g. An equation based upon the modern units, milliliters at 20° C and the normal weight of 26.00 g as published by Osborne [1], was derived as follows:

Let

D= true sp gr of solution at 20°/20° (table 109).

D' apparent sp gr of solution at 20°/20° (table 114, p. 632).
B =degrees Brix of solution, then

D' D+0.001 (D-1).

=

Factor=

26.00X100X1.1
99.72XBD'

28.680
BD'

(133)

This equation includes the correction for one-tenth dilution and by its use a table of factors was calculated for each 0.1 Brix from 0 to 25, which may be applied in eq 132.

The formula of Rice [2] omits the one-tenth dilution factor and is expressed as follows:

Factor=

26.00X100 99.718Xsp grX Brix

(134)

By means of this equation, Rice calculated a table of factors in increments of 0.1 extending from 0 to 25 Brix. This table (table 146, p. 702) may be used for calculating apparent purity from experimental values of the Brix and polarization.

A convenient table of purity values, expanded from Horne's table as calculated by means of Rice's equation 134 is given by Meade [3]. Here the purity may be read directly from the Brix and polarization of the solution. This table is arranged in intervals of 0.2° S from polarization equals 15 to polarization equals 87.0.

It has already been pointed out that in the determination of "true purity" the sucrose is obtained by the Clerget method and the solids by drying, while in the determination of "apparent purity" the direct polarization and Brix are employed. The above methods are the ones in general use.

However, it is occasionally convenient to use the values for solids. obtained from the refractive index in conjunction with either the percentage of sucrose or the polarization. Thus, there are six possible combinations. Noel Deerr [4] suggests the use of the terms "true

purity," gravity purity," and "refractive purity," when the percentage of sucrose is used as the numerator of eq 131, and solids by drying, by Brix spindle, or by refractometer, respectively, as the denominator. If the polarization is used as the numerator, terms are qualified by the expression "polarization."

2. REFERENCES

the

[1] S. J. Osborne, Methods of Analysis, p. 214 (The Great Western Sugar Co., Denver, Colo. 1920).

[2] E. W. Rice, Facts About Sugar 22, 1066 (1927).

[3] G. L. Spencer and G. P. Meade, Cane Sugar Handbook, p. 494-551 (John Wiley & Sons, Inc., New York, N. Y., 1929).

[4] Noel Deerr, Cane Sugar, p. 492 (Norman Rodger, London, 1921).

XXVII. PACKING OF SUCROSE [1, 2, 3]

1. INTRODUCTION

The volume occupied by a given weight of sugar is a matter of importance, both in relation to the operation of the centrifugal station and the bagging of the sugar. It is the sum of the volumes of the actual crystals and of the voids between them, and depends on their size and shape as well as on the way in which they are packed together. A study of the physical factors involved in the packing of sugar has been made by Drinnen [1]. His results are also valuable in that they indicate the way to an improved technique, which is desirable for further study of the problem.

2. CRYSTAL DIMENSION

Sugar crystals were classified into various fractions in accordance with their size, using a set of standard Tyler sieves. The dimensions of the sieved fractions were estimated under the microscope by an eyepiece micrometer. Since the smallest dimension largely determines the sieve in which the crystal remains, this dimension should be the reference size for crystal measurements, provided that the crystals do not depart very greatly from the cubical shape. In table. 48 are shown the results from the examination of 50 crystals from each sieve fraction.

3. VOLUME OF CRYSTALS AND VOIDS

Two methods were used for this determination, (a) displacing the volume of air in a known weight of the sugar by means of a liquid in which sugar is insoluble and measuring the volume, and (b) weighing a known volume of sugar and calculating the volume of the crystals from the weight and specific gravity of the sugar. The former appears

to be the more accurate method. Average percentage figures obtained are shown in table 49.

TABLE 49.-Volume of sucrose crystals and of interstitial voids as related to sieve mesh size

Grain counts were taken for the various fractions on oven-dried sugar, per gram from each fraction, as follows:

TABLE 50.-Weight of crystals in relation to average grain size (screen fractions)

This is a very difficult matter to estimate, and the findings of Pellet, summarized by Thieme [3], are given in table 51.

TABLE 51.-Relation between weight and surface area of crystals for various crystal sizes (screen fractions)

[1] L. Drinnen, Proc. Queensland Soc. Sugar Cane Tech., 9th Conference, 1938, p. 199-202.

[2] Technical Communication No. 7, Bureau of Sugar Experiment Stations, Brisbane, Australia, 1938.

[3] J. G. Thieme, Studies on Sugar Boiling, translated by O. W. Willcox, p. 26-27 (Facts About Sugar, New York, N. Y., 1928).