1 ml of distilled water at 4° C, the temperature of its maximum density, (d=1.00000) weighs 1 g in vacuo, densities are usually referred to water at 4° C. The standard temperature adopted for sugar analysis is 20° C, and density measurements are usually made at this temperature. The density of a sugar solution at 20° C referred to water at 4° C is indicated as follows: d200. Unless otherwise stated, densities refer to weights reduced to vacuo and are termed "true" or "absolute" densities.

The term "specific gravity" is used to express the relative masses of equal volumes of the substance in question and water, each substance being at a definitely stated temperature. For example, specific gravity at d20 means the specific gravity of the substance at 20° C referred to water at 20° C as unity. Specific gravity is frequently referred to as relative density.

2. DEGREES BRIX (OR BALLING)

The system of graduating hydrometers on the basis of the percentage by weights of sucrose in a pure sucrose solution was first devised by Balling [1]. The Balling scale was subsequently recalculated by Brix [2], and the instrument is now generally referred to as the Brix hydrometer, and the readings are indicated by the term "degrees Brix." The original hydrometers were calibrated at 17.5° C, but at present most Brix hydrometers are standard at 20° C.

3. DEGREES BAUMÉ

The Baumé hydrometer is widely used in the sugar industry, particularly in connection with the sale of molasses and sirups. The original instrument was devised by Antoine Baumé [3], a French chemist, in 1768. Baumé described his method of graduating the hydrometer in L'Avant Coureur in 1768 and repeated them in the several editions of his Elements de Pharmacie.. In the eighth edition of this work, published in Paris in 1797, he stated that he constructed his instrument in this way:

For the hydrometer for liquids heavier than water he prepared a solution of salt containing 15 parts of salt by weight in 85 parts of water by weight. He described the salt as "very pure" and "very dry" and stated that the experiments were made in a cellar in which the temperature was 10° Réaumur (12.5° C). The zero on the scale indicated the point to which the instrument sunk in distilled water and the 15-mark the point to which it sunk in the 15-percent salt solution.

With a pair of dividers the space between 0 and 15 was divided into 15 equal parts, and degrees of the same size were continued above 15.

Although the degrees on the Baumé scale are entirely arbitrary and bear no obvious relation to the density of the liquid, the instrument met with speedy acceptance by workers in the sugar as well as other industries. With the general use of the Baumé scale, other investigators found difficulty in following the original directions for calibrating the instrument, and the values of the scale divisions have been variously reported. This led to the introduction of a number of different so-called Baumé scales.

Chandler, [4] in a paper read before the National Academy of Sciences at Philadelphia in 1881, gave an admirable review of the origin and history of the Baumé scales in use up to that time. He gave the details of 23 different scales for liquids heavier than water. This subject of Baumé scales is also discussed in Bureau Circular C59.

In view of the uncertainty which existed because of the number of different scales, Bates and Bearce [5] devised a new Baumé scale for sugar solutions. This has three features that commend it for use in sugar work.

1. It is based upon the density values of Plato, which are considered the most reliable.

2. It is standard at 20° C, the most widely accepted temperature for sugar work.

3. It is based on the modulus 145, which has already been adopted by the Manufacturing Chemists Association of the United States, by the National Bureau of Standards, and by all American manufacturers of hydrometers.

In constructing this table, Bates and Bearce reduced the Plato density values do to specific gravities at 20°/20° C in order that zero degrees Baumé correspond to zero percentage of sucrose and zero degrees Brix.

The relation between specific gravity 20°/20° C and degrees Baumé (Bates and Bearce) is as follows:

The Bates and Bearce table of equivalents is now in general use; table 109, p. 614.

4. DETERMINATION OF DENSITY

(a) BY MEANS OF HYDROMETERS

Hydrometers are seldom used for great accuracy, since the usual conditions under which they are used preclude such special manipulation and exact observation as are necessary to obtain high precision. It is, nevertheless, important that they be accurately graduated to avoid, as far as possible, the necessity for instrumental corrections. To obtain this end, it is necessary to employ certain precautious and methods in standardizing these instruments.

The hydrometer should be clean, dry, and at the temperature of the liquid before immersing.

The liquid in which the observation is made should be contained in a clear, smooth, glass vessel of suitable size and shape.

In order that a hydrometer may correctly indicate the density of a specified liquid, it is essential that the liquid be uniform throughout and at the standard temperature.

To insure uniformity in the liquid, stirring is required shortly before making the observation.

In making the determination, the hydrometer is slowly lowered into the liquid slightly beyond the point where it floats naturally and then is allowed to float freely.

The scale reading should not be made until the liquid and hydrometer are free from air bubbles and at rest.

In reading the hydrometer scale, the eye is placed slightly below the plane of the surface of the test liquid; it is raised slowly until the surface, seen as an ellipse, becomes a straight line. The point where this line cuts the hydrometer scale should be taken as the reading of the hydrometer.

The liquid should be at nearly the temperature of the surrounding air, as otherwise its temperature. will change during the observation, causing not only differences in density but also doubt as to the actual temperature. When the temperature of observation differs from the standard temperature of the instrument the observed reading differs from the normal reading by an amount depending on the difference in temperature and on the relative thermal expansions of the instrument and the particular liquid. If the latter properties are known, tables of corrections for temperature may be prepared for use with hydrometers at various temperatures. Such tables should be used with caution, and only for approximate results when the temperature differs much from the standard temperature or from the temperature of the surrounding air. Such temperature corrections are given in tables 110 and 111, pp. 624-25. Further information on the use and testing of hydrometers is given in Bureau Circular C16, The Testing of Hydrometers.

In case the sample is too dense for direct determination, the following dilution method of the AOAC [6] may be employed.

Dilute a weighed portion with a weighed quantity of water or dissolve a weighed portion and dilute to a known volume with water. In the first instance, the percentage of total solids is calculated by the following formula:

Percentage of solids in the undiluted material=W'S/w, in which

S percentage of solids in the diluted material,

=

W weight of the diluted material, and

wweight of the sample taken for dilution.

When the dilution is made to a definite volume, the following formula is to be used:

Percentage of solids in the undiluted material=VDS/w, in which

V volume of the diluted solution,

D-specific gravity of the diluted solution,

S percentage of solids in the diluted solution, and
w=weight of the sample taken for dilution.

A table for the comparison of density, degrees Brix, and degrees Baumé, etc., is given in table 109, p. 614.

In applying the dilution method to low-grade products, such as final molasses, the results obtained are higher than when direct methods are used. This is due to the fact that when diluted with water the solutions of the dissolved salts and other impurities contract by amounts different from solutions of pure sucrose.

(b) BY MEANS OF PICNOMETERS

(1) GENERAL.-For more accurate determinations of the density of liquid sugar products, such as sirups and molasses, some form of picnometer or specific gravity bottle is used. The procedure is as follows: Carefully clean the picnometer by filling with a solution of potassium dichromate in concentrated sulfuric acid, allowing to stand for several hours, emptying, thoroughly washing with water, and, finally rinsing with alcohol. Dry the instrument in an air bath, cool and weigh. Fill the picnometer with recently boiled distilled water which has previously been cooled to 18° or 19° C, insert the stopper or thermometer, being careful to prevent the introduction of air bubbles,

and place the filled instrument in a water thermostat held at 20° C. Allow to remain in the thermostat for a sufficient time to reach the temperature of 20° C. Adjust the volume by removing the excess water which has exuded from the capillary stem, fit the ground-glass cap in place, and remove the instrument from the bath; wipe dry with a clean cloth and after allowing to stand for 15 to 20 minutes, weigh. Reduce the weight in air of the contained water to the weight in vacuo. Obtain the volume of the picnometer by dividing the weight in vacuo of the water content by the density of water at 20° C, 0.9982343. After emptying and drying the picnometer, fill it with the sample in question, and ascertain the weight of the contained sample at 20° C. Reduce the weight in air of the contained sample to the weight in vacuo. Divide the weight of contained sample in vacuo by the volume of the picnometer to obtain the true density, d20.

E

-B

(2) METHOD OF NEWKIRK.-The accurate determination of the density of blackstrap molasses is difficult due to the high viscosity of the material and to the presence of included and dissolved gases. Newkirk [7] has designed a special picnometer for making the determination (fig. 42). It consists of a bottle, C, fitted with an enlargement at the top, B, ground optically flat and closed by another optical flat, A. An expansion chamber, D, is ground to the bottle and fitted with a vacuum connection, E. To avoid loss of water due to evaporation under reduced pressure, the connecting tube is fitted with a stopcock, F, so that when the proper vacuum has been reached the apparatus can be closed off from the vacuum source. In using the picnometer, the expansion chamber, after lubrication of all joints with molasses, is placed on the bottle. The molasses to be analyzed is allowed to flow into the bottle and into the expansion chamber until the latter is about one-third full. The vacuum line is then connected and the pressure reduced until the gas expands into visible bubbles. The apparatus is then closed off by turning the stopcock, F, and the whole placed in a thermostat and allowed to remain until the temperature has reached equilibrium and all of the bubbles have collected in the expansion chamber. The expansion chamber is removed and the volume fixed by carefully sliding plate A over surface B. The picnometer is then removed from the thermostat, wiped clean, placed in the balance case, and weighed. The weight of the contained sample is corrected to vacuo and compared with the weight of an equal volume of water at 4° C in vacuo.

C

FIGURE 42Newkirk picnometer.

(3) WEIGHT PER GALLON OF MOLASSES.-Since, in commercial transactions, molasses is sold both by volume and by weight, the determination of the weight per gallon is of considerable importance. The method employed at the National Bureau of Standards and adopted by the United States Customs Service is as follows:

A special 100-ml calibrated volumetric flask with a neck of approximately 8 mm inside diameter shall be used. Weigh the flask empty and then fill it with molasses, using a long-stem funnel reaching below the graduation mark, until the level of the molasses reaches the lower end of the neck of the flask. The flow of molasses may be stopped by inserting a glass rod of suitable size into the funnel so as to close the stem opening. Remove the funnel carefully to prevent the molasses coming in contact with the neck, and weigh flask and molasses. Add water almost up to the graduation mark, running it down the side of the neck to prevent mixing with the molasses. Allow to stand several hours or overnight to permit the escape of bubbles. Place the flask in a constant temperature water bath at 20° C for a sufficient time for it to reach the temperature of the bath, then make to volume at that temperature, with water. Weigh. Reduce the weight of the molasses to vacuo and calculate the density.

Example.

Weight per gallon determined at 20° C.

Weight of flask, 37.907 g.

Weight of flask and molasses, 167.148 g.

Weight of flask, molasses, and water, 174.711 g.

167.148 g-37.907 g=129.241 g=weight of molasses (in air with brass weights). 174.711 g-167.148 g=7.563 g=weight of water (in air with brass weights). Calculating volume of water from weights in air at 20° C.

Divide weight of water in air by weight of 1 ml of water in air at 20° C (table 106, p. 612), 7.563/0.99718=7.584 ml.

Volume of flask at 20° C, 100. 060 ml

7. 584 ml

92. 476 ml=volume of molasses.

To reduce weight of molasses to vacuo:

129.241/8. 4=15.4 ml=volume of brass weights.

77.1X0.0012

0.093 g, buoyancy correction to be added to weight of molasses.

(0.0012 g = weight of 1 ml of air at 760 mm at 20° C).

(c) BY MEANS OF DIRECT-READING BALANCE

The weight per gallon of molasses may be determined directly by means of a torsion balance designed by H. J. Bastone, of the American Sugar Refining Co. It is fitted with two beams, one a double beam for taring the sample bottle, and the other a recording beam graduated in pounds per gallon from 10.80 to 12.05 in 1/100 pound per