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The defects of hydrosulfite as clarifiers are, in addition to the frequent redarkening, their effect on reducing sugars, the possible separation of finely divided sulfur, and their ineffectiveness in discharging the color of caramel bodies. Bryan [12] states that the rotation of dextrose is lowered by hydrosulfites and finds evidence of the formation of a laevo oxysulfonate. No immediate effect is observable upon sucrose or fructose, but sucrose is apparently inverted by a prolonged action. These clarifiers have not come into general use in analytical work, but nevertheless they are unique in that they produce no volume error.

(g) BONE CHAR

In cases where neither alumina cream nor lead subacetate is capable of producing a clear solution, recourse may be had to bone black. Bone black, for analytical purposes, may be prepared by treating the granular material used in sugar refining with a slight excess of hydrochloric or nitric acid until all of the mineral matter is dissolved. The treated char is washed with boiling water, dried at 120°, and finely powdered and bottled. The more completely the material is freed from mineral matter, the more effective is its action for analytical purposes. Bone black probably owes its clarifying action to the very large surface which is caused by its porosity.

The most serious error accompanying clarification with bone char is caused by its tendency to absorb sugar and thus give abnormally low readings. For this reason, most official methods of clarification exclude bone black as an agent. It is difficult to make a correction for the amount of sugar absorbed, because it varies with the composition and concentration of the sample and the condition of the bone char. In order to avoid the error arising from the absorption of sugar, the absorption coefficient may be determined under the approximate conditions of the analysis or the solution may be made up to volume and filtered through a column of bone black, the first third of the filtrate being rejected.

4. REFERENCES

[1] G. H. Hardin and F. W. Zerban, Louisiana Planter 73, 388 (1924). [2] F. J. Bates and F. P. Phelps, Bul. BS 10, 537 (1914) S221.

[3] Official and Tentative Methods of Analysis of the Association of Official Agricultural Chemists, 5th ed., p. 490 (1940).

[4] R. F. Jackson, Bul. BS 11, 331 (1914) S232.

[5] W. D. Horne, J. Am. Chem. Soc. 26, 186 (1904).

[6] F. Sachs, Z. Ver. deut. Zucker-Ind. 30, 229 (1880).

[7] G. L. Spencer and G. P. Meade, A Handbook for Cane-Sugar Manufacturers and Their Chemists, 7th ed., p. 224 (1929).

[8] C. Scheibler, Z. Ver. dent. Zucker-Ind. 25, 1054 (1875).

[9] F. J. Bates and J. C. Blake, Bul. BS 3, 105 (1907) S52.

[10] F. Herles, Z. Zuckerind. Böhmen 13, 559 (1888); 14, 343 (1889); 21, 189 (1896).

[11] J. Weisberg, Centr. Zuckerind. 15, 975 (1906).

[12] A. H. Bryan, Bul. Bur. Chem. No. 116.

VIII. CLERGET METHOD

1. INTRODUCTION

The direct polariscopic reading of a sugar solution is the resultant rotation of all optically active substances present, and is conse

quently a correct measure of the sucrose only when the other substances present have no effective rotatory power. If other optically active substances are present, the direct polarization must be supplemented by a second observation in which the rotations of these substances are kept constant while that of sucrose is subjected to a change which can be measured and is known to be an exact function of the quantity of sucrose. This change is brought about by the hydrolysis of sucrose to invert sugar. The change of rotation of the normal solution of pure sucrose is known as the Clerget divisor. The divisor is not a constant but its numerical value is influenced by concentration, temperature, and impurities. The hydrolysis or inversion can for analytical purposes be effected by either the enzyme, invertase, or by hydrochloric acid.

In its simplest form applicable to the ideal case, where nothing but the rotation of sucrose is altered, the Clerget formula is

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in which P and P' are the direct and invert polarizations, respectively, of the normal solution; C, the basic value of the Clerget divisor at 20° C; ac, the change in the value of the divisor with concentration of substance; and b, its change for each degree rise in temperature.

The Clerget formula is frequently applied in the above form without assurance that the fundamental condition is fulfilled, namely, that the rotations of all substances except sucrose remain unaltered in the two polarizations. In general, it is true that for samples containing high percentages of sucrose and small quantities of invert sugar the method yields reliable results and that for low-grade products it yields results which are sometimes sufficiently accurate for the purposes at hand.

Of the two commonly employed hydrolytic agents, invertase is the superior because of its highly selective action on the sucrose group and because it is without effect on the rotations of other substances occurring as impurities in sugar samples. Its disadvantages are its relatively high cost, the considerable labor required for its preparation, the uncertainty that the preparation has retained its activity, and, except under certain conditions, the long time required for the completion of the hydrolysis.

Hydrochloric acid, on the other hand, involves negligible expense and is capable of completing the hydrolysis in any desired period of time by merely regulating the temperature of reaction. However, it is not selective but hydrolyzes any glycosidic group. Moreover, it influences the rotatory power of invert sugar and many other impurities occurring in natural products.

2. ACID METHODS

(a) BASIC VALUES OF THE CLERGET DIVISOR

In devising the method, Clerget [1] in 1849 took 50 ml of a normal sucrose solution in a 50- to 55-ml flask, added 5 ml of "pure and fuming" hydrochloric acid, and after mixing, placed the flask in a water bath so regulated that 10 minutes were required to raise the temperature of the solution to 68° C. Upon attaining this temperature the flask was removed, cooled rapidly to 20°C, and the solution polarized.

The invert reading was multiplied by 11/10. The percentage of sucrose was calculated by the formula

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in which D is the algebraic difference between the two corrected polarizations and t is the centigrade temperature.

Browne [2] modified the Clerget method by allowing the solution to invert overnight at room temperature and making it to a volume of 55 ml at the completion of the inversion. It was pointed out that as the result of three factors a contraction in volume of about one-third of a milliliter occurred in the original method. The diminution in volume is caused, first, by the contraction which all sucrose solutions undergo during inversion and which for 13 g of sucrose in 55 ml is about 0.25 ml; second, the elevation of temperature caused by dilution of 5 ml of concentrated hydrochloric acid; and third, by the evaporation of water during the inversion. The advantage claimed by the author is that the invert solution, being but slightly diluted, can be observed with practically the same precision as the solution for direct polarization and with no great multiplication of errors, as is the case in methods in which the invert solution is more highly diluted. This advantage is lost, however, if the invert solution in the alternative methods is observed in a 400-mm column.

Browne found the percentage of sucrose to be expressed by the formula

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in which the parenthetical term multiplied by 0.01 in the denominator is the correction of the divisor for concentration of sugar. If the concentration of sucrose is diminished from 26 to 25 g, the parenthetical term becomes roughly 5.19 and the concentration coefficient 0.052 for 1 g of sucrose. This value is considerably lower than the prevailing value 0.0676, and quite at variance with the revised value, 0.0794, of Jackson and McDonald discussed in a later paragraph.

Jackson and Gillis [3] showed that under the conditions prescribed by Browne inversion is complete in 8 minutes at 60° C.

In 1888 Herzfeld [4] devised the modification of the Clerget method which has remained in use to the present day. He observed the direct polarization in the usual manner, employing the normal weight of sucrose. He then took the half-normal weight (13 g) in 75 ml of solution, added 5 ml of hydrochloric acid (38 percent or 1.188 specific gravity), and warmed the solution to 67° to 70° C in from 2 to 3 minutes. The solution having attained the prescribed temperature, he kept it as near 69° C as possible for 5 minutes, when it was quickly cooled, made to a volume of 100 ml at 20° C, and polarized at the same temperature. Being that of a half-normal solution, the reading was multiplied by 2. Under these conditions he found the Clerget formula to be

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The basic value of the Clerget divisor in the above formula, 142.66, is defined as the algebraic difference between the rotation of the normal pure sucrose solution at 20° C (i. e., +100) and twice the rotation of 13 g of inverted sucrose in 100 ml at 20° C, corrected to 0° C by the term +0.5t. The actually measured value, free from the uncertainty of the value of the temperature coefficient, is 132.66 at 20° C.

The basic value of the divisor has been the subject of numerous investigations with different results. Herzfeld, in his original publication, adopted the value 132.66 at 20° C, but apparently the actual measurement was made by Dammüller [5]. Later investigators have invariably obtained higher values: Walker [6], 132.78; Tolman [7], 132.88; and Steuerwald [8], 133.05. Jackson and Gillis computed that by the Herzfeld procedure the destruction of invert sugar caused a lowering of the rotation by 0.15 to 0.20° S, which, deducted from their recalculated value, 133.18, left a resultant rotation of -33.03 to -32.98.

In 1920 Jackson and Gillis [3] published the results of a careful series of measurements in which they inverted sucrose solutions at 60° C instead of 70° C in order to avoid the destruction of invert sugar. They made their measurements at a somewhat higher concentration than the half-normal solution in order to utilize such standard quartz plates as were available, thus eliminating the errors of graduation of the saccharimeter scale. They were consequently obliged to correct their observed rotations for the higher concentration of sugar, using for this purpose the then prevailing value 0.0676 per g departure from 13 g of sucrose per 100 ml. Recent experiments have shown that this coefficient is considerably too low and that a recalculation of the Jackson and Gillis value for the rotation of the half-normal weight is necessary. Thus their originally announced value, -33.25, now

becomes -33.18 at 20° C.

Subsequent to the publication of the Jackson and Gillis measurements, but previous to the appearance of the German translation of the same article [9], Herzfeld published a posthumous work of Schrefeld [10], whose experiments were made in 1910 to 1912. Schrefeld dissolved the half-normal weight of sucrose in 75 ml of water and added 5 ml of concentrated hydrochloric acid (sp gr 1.19). Inserting a thermometer in the sugar solution, he immersed the flask in a water bath having a temperature of 70° C, and agitated it until in 21⁄2 to 24 minutes the solution had reached a temperature of 67° C. From this moment he allowed it to remain in the bath for exactly 5 minutes, during which time the temperature gradually rose to 69.5° C. The flask was removed and cooled rapidly to 20.0° C and the solution made up to 100 ml. Polarization measurements were made at 20° C. As a mean of seven concordant polarizations, Schrefeld found the rotation of the half-normal solution multiplied by 2 to be -33.00. This value has not been officially adopted by the International Commission for Uniform Methods of Sugar Analysis but has been widely used and has been verified by Browne. Zerban and coworkers under similar conditions found -32.97; Spengler, Zablinsky, and Wolf [11], -33.02; and Jackson and McDonald, -32.99. Thus the basic value, 133.00 at 20° C, for inversion under the Schrefeld conditions must be considered a well-established constant. It has been adopted officially by the Association of Official Agricultural Chemists.

Jackson and McDonald have experimentally corroborated the recalculated value -33.18 of Jackson and Gillis and have extended their measurements to include values obtained after inversion at several other temperatures. The Arrhenius formula, 37, evaluated by Jackson and Gillis, and discussed further on page 133 permits a calculation of the velocity constant of inversion at any desired temperature in the presence of 0.7925 N hydrochloric acid. With the aid of this equation, Jackson and McDonald [15] calculated the time required for 99.99-percent hydrolysis at 49° and 35° C, respectively, and measured the rotation of the half-normal solution. They found the value 33.25 for both temperatures.

Many analysts advocate inversion at room temperature as a safe means of avoiding decomposition of invert sugar. Formula 37 enables us to calculate the velocities of inversion and times required for 99.99-percent inversion at the temperatures which may be expected in uncontrolled laboratories. These periods of time vary considerably with small changes of temperature, as is shown in table 10. While the velocities of many reactions double themselves with a rise of 10 degrees in temperature, the velocity of inversion of cane sugar increases more than fourfold between 20° and 30° C. Thus room-temperature inversion is safe only if such variations of temperature as inevitably occur are known to the analyst and are considered in calculating the time required for complete hydrolysis. It appears from table 10 that 24 hours is insufficient for hydrolysis at 20° C, that 16 or 17 hours for overnight inversion at 30° C is excessive, and that serious decomposition of invert sugar can result. Evidently room-temperature inversion must be carried out with considerable discretion.

TABLE 10.-Time required for inversion of sucrose at room temperature by 0.7925 N hydrochloric acid

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For room-temperature inversion the value of the negative constituent of the Clerget divisor adopted by the Association of Official Agricultural Chemists is -33.20. Jackson and McDonald [15] have measured this constant with care by inverting the half-normal weight of sucrose in a thermostat which maintained a constant temperature within a few hundredths of a degree. The times of inversion were calculated by formula 37 for temperatures varying from 20° to 25° C. The mean value found for the rotation of the half-normal solution was -33.29.

In recapitulation, table 11 shows the values of twice the rotation of the inverted half-normal solution. All of these measurements except the first one at 70° C were made with care to avoid the destruction of invert sugar after the completion of the inversion. Jackson and Gillis showed that when sucrose is inverted by the Herzfeld method

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