These data show the change to be practically linear over the range 10° to 30° C, and have been combined into the following equation [4, p. 7]:

8=-0.000184+0.0000063 (t-20° C)

3. COMBINATION OF CORRECTIONS FOR QUARTZ-WEDGE

SACCHARIMETER AND SUGARS

(31)

If a proper temperature correction is to be applied to a polarization, it is necessary to add algebraically all the corrections applicable to the conditions under which the polarization is being made, or to determine the correction experimentally.

(a) SUCROSE

In the ordinary testing of sucrose, the solution is made up to volume and read at the same temperature, which in general is not the standardization temperature. It is desirable, therefore, to know the variation with temperature in the saccharimeter reading of a near normal sucrose solution.

Among the factors that require consideration are the changes in the quartz-wedge system due to changes in temperature, changes in the specific rotation of sucrose, in the volume of solution, in the volume of the flask, and in the length of the tube used. Some of these act in opposite directions and thus partially cancel or compensate for each other.

By a summation of the best known values of the separate coefficients which enter into the correction factor, we obtain the value 0.0309 per degree centigrade for a normal sucrose solution (100° S).

Owing to its importance, this over-all temperature coefficient has been measured experimentally by a number of investigators. Their results are given in table 7, based upon a normal solution of sucrose. TABLE 7.-Saccharimeter temperature coefficient for sucrose

The average value obtained is 0.0303 for a normal sugar solution (100° S). The value varies slightly for different instruments, probably in part because of slightly different differential expansions of the scale and wedge mountings.

This correction factor is predicated upon the instrument and all apparatus, flasks, tubes, etc., having been originally calibrated at 20° C but used at some other temperature between 20° and 30° C. It is important and necessary, for the proper application of this temperature correction, that the solution be made up to volume at the same temperature as that at which it is to be read in the saccharimeter and that the entire saccharimeter also be at that same temperature.

903232 O 50-8

Under these conditions the polarization in sugar degrees at 20° C (S20) of an approximately normal sucrose solution made up to volume and polarized at the same temperature (t° C) is given by the following:

S20 S+S, 0.0003 (t-20° C).

=

(32)

This equation is sufficiently accurate for all ordinary polarizations, irrespective of type of tube or scale.

(b) APPLICATION OF TEMPERATURE CORRECTION BY MEANS OF A QUARTZ CONTROL PLATE

This correction is most conveniently and satisfactorily applied by means of a standardized quartz control plate. The proper use of such a plate gives an over-all correction which includes not only the above temperature correction but also corrects any accidental or irregular variations, such as those due to minor residual temperature differences in the quartz wedges, etc.

The procedure in using a quartz control plate is quite simple and is as follows:

When making a series of polarizations of sugar solutions on the saccharimeter, also make a series of readings on the standard quartz control plate. Observe the temperature. From the table of sugar values for the standard plate find the sugar value corresponding to the observed temperature. The difference between the observed reading of the plate and the sugar value obtained from the table is applied as a temperature correction to all polarizations made during that series.

This is the procedure which has long been recommended by this Bureau whenever making polarizations where a standard 20° Č constant-temperature room is not available.

Because of its convenience and accuracy this practice was introduced by the Bureau into the various Customs laboratories at a very early date. Figure 23 is a facsimile copy of a table showing the sugar values for various temperatures of a quartz control plate, which was issued in 1907 as a part of the certificate of test for that plate. Prior to that date similar tables were issued and are still being issued whenever requested in connection with the certificates for quartz control plates. (See this Circular, p. 553, Fee Schedule 423e; Circular C44, Polarimetry, 2d edition (1918), p. 111, Fee Schedule 44d; Circular C44, 1st edition (1914), p. 96, Fee Schedule 44d).

These tables are based upon the measured value of the plate at 20° C and the saccharimeter temperature coefficient 0.0003, as defined above. They are calculated from the equation

SS20 [1+0.0003 (t-20° C)]

(33)

and are valid, of course, only under the conditions stated above, namely, that the solution be made up and polarized at the same temperature.

(c) CORRECTIONS FOR USE IN TROPICAL COUNTRIES

Temperature correction by the method outlined above is frequently utilized and gives excellent results in tropical countries, and it is

Degrees Sugar Degrees

Sugar Degrees Sugar Degrees Sugar Centigrade Value Centigrade Value Centigrade Value Centigrade Value

FIGURE 23.-Facsimile of certificate for quartz control plate 226-BS-1907.

preferable to adopting a different standardization temperature, such as 25° or 27.5° C, as sometimes suggested.

Einsporn and Schönrock [4] have made an elaborate study to find by calculation what the corrections would be if the polarization is carried out at the tropical temperatures of 25° and 30° C. Their final values are identical with those which this Bureau has always given for these temperatures in its tables of temperature corrections accompanying quartz control-plate certificates. (See fig. 23.)

If the solution is made up to volume at 20° C, but is polarized at another temperature, all apparatus being at that temperature, equation 32 is no longer applicable. Under these conditions the temperature coefficient of the normal sucrose solution alone is given by Schönrock as 0.000469, while that of the saccharimeter, as previously shown, is 0.000148. If a glass tube is used, we have as the total temperature correction

The polarization in sugar degrees at 20° C (S20) of the near normal solution of sucrose is therefore given by

S20 S+S,0.000609 (t-20° C).

(34)

Equation 34 obviously holds only while the number of grams of sugar in 100 ml of the solution remains unchanged.

(d) SUGAR MIXTURES

The coefficient 0.0003, in eq. 32, having been determined for the normal weight of sucrose, should be applied with considerable care. It has, however, long been a widespread practice to apply it to all sorts of saccharine products, with the result that a polarization, supposedly accurate, may contain errors of appreciable magnitude. If the solution does not read approximately 100° S, the correction to give the reading at 20° C should not be obtained by multiplying 0.03 by the difference in temperature-a common practice. In general, no great error will be introduced by following this procedure provided the polarization is above 85° S. Nevertheless, the better and safer practice is to solve eq 32, thereby correcting for the difference in sucrose concentration from the normal solution.

An even more widespread practice has been to apply eq 32 to sugar polarizations without regard to the associated impurities. This is particularly true of raw sugars and molasses which contain appreciable quantities of invert sugar. Of the constituents of invert sugar, the dextrose has a negligible temperature coefficient, while the levulose has a very large coefficient, and in such a direction that the positive rotation of the mixture tends to increase upon elevation of temperature. It is therefore manifestly erroneous to apply a pure sucrose temperature coefficient to a mixture unless all the substances, except sucrose, are present only in such small quantities that the error introduced is negligible.

For the temperature correction of the better grades of raw sugar, the temperature formulas 32 and 34 give results which are sufficiently accurate; but if they are applied to low-grade products, errors are

introduced. Raw sugars may be considered as mixtures of pure sucrose and cane molasses. To correct the whole mixture for the effect of temperature change, it would be necessary to apply the resultant coefficient obtained by combining the separate coefficients for sucrose and for molasses, taking into consideration the relative quantities of the two and the constitution of the particular molasses which contaminates the sample. C. A. Browne [5] has done this and shows that the temperature coefficient varies in almost direct proportion to the content of molasses. It is in general incorrect to apply this computed correction because of possible individual variation in the constitution of the molasses. Accordingly Browne advises the omission of the temperature correction for low-grade samples. He has computed an average coefficient for large numbers of samples of raw sugar which may be determined according to the polarization. Thus for cane products the coefficient is 0.0015. It is seen from this that at 80° the coefficient becomes zero and for lower grades it becomes positive instead of negative. If individual variations did not occur, this correction would be a useful one; but as it stands, it merely serves to show the possible error of applying eq 32 to low-grade sugar. For beet products the coefficient is 0.0006.

4. THERMOSTATS

(a) WATER

The satisfactory control of temperature in polarimetric work is an important and troublesome subject. For accurately making up solutions to volume at a desired temperature, water thermostats which give extremely close regulation are to be preferred. Many types which are entirely satisfactory are available from apparatus dealers. These consist essentially of metal or glass tanks suitably insulated on the outside and nearly filled with water. The cooling is accomplished by circulating ice water through an immersed coil, and the temperature regulation is maintained by immersed lamps or heaters operated intermittently by means of mercury relays connected to mercury thermoregulators. The water is kept in constant motion, insuring an even temperature throughout, by means of electric turbines or stirrers. For circulating constant-temperature water through the jackets of polariscope tubes, refractometers, etc., use is made of small electric-driven pumps.

(b) AIR

At the National Bureau of Standards most polariscopic measurements are made in a constant-temperature room maintained at the standard temperature of 20° C. Three such rooms are available. The largest of the three, used for both routine and research work, is approximately 10 by 20 feet. The walls and ceiling are insulated with thick corkboard. The temperature is maintained by the automatic intermittent operation of a large ammonia compressor located in the attic room above, the direct-expansion ammonia coils being mounted on the side walls of the constant-temperature room. Satisfactory temperature control is accomplished through the use of a bimetallic regulator.

The saccharimeters are placed within the room on a table of convenient height. For special work the instruments are enclosed in