structed a table of factors by which the weight of copper should be multiplied in order to yield invert sugar correctly in the presence of all ratios of sucrose to invert sugar up to 99 percent. Hiller's table of factors (p. 589) illustrates well the effect of sucrose. Each column. gives the factors (expressed in percent) for a constant weight of invert sugar in the presence of varying quantities of sucrose. In extreme cases an error of nearly 30 percent could be introduced by disregarding the effect of sucrose.

In order to select the proper factor, it is necessary to determine polariscopically the approximate sucrose content of the sample. The direct polarization in the presence of invert sugar is not a correct measure of sucrose but is sufficiently exact for the purpose, except in very low-grade products. If the sucrose is determined by the Clerget method, its percentage should be substituted for P, the direct polarization, in the formulas given below.

The Hiller factors were carefully determined, but not with the rigid specifications which are imposed at the present time. He used Fehling solution, boiled "2 to 3 minutes," and collected the precipitate on a paper filter, ignited, and reduced the oxide by hydrogen to copper. It is not essential that enough solution be taken to precipitate nearly all of the copper, but, in general, the results are more accurate at higher than at lower sugar concentrations.

Prepare a solution of suitable concentration of the material to be examined, clarify with neutral lead acetate, and remove the excess of lead with an alkali oxalate or sodium phosphate. Prepare a series of solutions in large test tubes by adding 1, 2, 3, 4, and 5 ml of this solution to each tube successively. Add 5 ml of mixed Soxhlet reagent to each, heat to boiling, boil 2 minutes, and filter. Note the volume of sugar solution that gives the filtrate lightest in tint but still distinctly blue. Place 20 times this volume of the sugar solution in a 100-ml flask, dilute to the mark, and mix well.

Transfer 50 ml of mixed Soxhlet reagent and 50 ml of the solution to a 250-ml beaker. Heat this mixture at such a rate that a period of approximately 4 minutes is required to bring it to the boiling point, and then boil for exactly 2 minutes. Add 100 ml of cold recently boiled water. Filter immediately through asbestos, and determine the copper by one of the methods described on page 178.

Let

Then

Cu the weight of copper obtained,

P=polarization of the sample (or sucrose by Clerget analysis), W the weight of the sample in the 50 ml of solution used for the determination,

F= the factor obtained from Hiller's table.

Cu

2

100

I'

=Z, approximate weight of invert sugar,

Y, approximate percentage of invert sugar,

100P R, approximate percentage of sucrose in mixture of

P+Y

sugars,

100-R I, approximate percentage of invert sugar,

Cu F

W

= percentage of invert sugar.

The factor, F, for calculating copper to invert sugar, is found in table 84.

Example: The polarization of a sugar is 86.4, and 50 ml of solution containing 3.256 g of sample gave 0.290 g of copper.

By consulting the table, it will be seen that the vertical column headed 150 is nearest to Z, 145, and the horizontal column headed 95:5 is nearest to the ratio of R to I, 95.1:4.9. Where these columns meet there is found the factor 51.2, which enters into the final calculation:

(h) BROWN, MORRIS, AND MILLAR METHOD FOR REDUCING SUGAR IN MOLASSES [21]

The general method of these authors has been adapted by W. A. Davis to the determination of reducing sugar in cane and beet molasses. The method is extensively used in Great Britain. It is applied to solutions containing 1 g of molasses in 100 ml (not defecated). The Fehling solutions contain 34.639 g of copper sulfate crystals in 500 ml, and 173 g of Rochelle salt and 65 g of sodium hydroxide in 500 ml, respectively. Mix 25 ml of each of the two solutions in a 250-ml beaker of tall lipped form and heat in a gently boiling water bath for 6 minutes. Add 50 ml of the molasses solution (1 g in 100 ml), cover the beaker, and continue the heating for 12 additional minutes. Collect the cuprous oxide in a Gooch crucible, wash with 200 ml of boiling water, and finally with alcohol. Dry and oxidize to copper oxide by placing the Gooch crucible in a larger ordinary crucible in a tilted position, heat gently at first, and finally strongly over a Teclu burner (but not with a blast lamp). Correct the weight of copper oxide for the quantity (usually less than 1 mg) obtained in a blank determination.

For weights of copper oxide less than 0.245 g multiply the weight of copper oxide by the factor 81.5 to obtain directly the percentage of invert sugar in the sample. For weights greater than 0.245 g, use the factors given in table 20.

TABLE 20.-Factors for calculating invert sugar from copper oxide

3. DETERMINATION OF COPPER

(a) GENERAL

The estimation of copper in the cuprous oxide precipitate may be accomplished gravimetrically or volumetrically. The gravimetric methods, which consist in the weighing of the cuprous-oxide precipitate either directly or after conversion to copper or cupric oxide, may be employed only in case the precipitate is uncontaminated. Contamination may be caused by the precipitation or inclusion of inorganic or organic impurities in the sample. Such precipitation is more likely to occur when cruder samples are analyzed.

Sherwood and Wiley [22] made an extended series of analyses of pure and crude products, which, in table 21, is condensed by computing the averages. Included in the table is the mean of a number of analyses made by Hammond.

TABLE 21.—Comparison of methods for determining reduced copper

The data presented indicate that the iodometric and electrolytic methods yield the true weights of reduced copper. It is evident at once that other methods yield correct results only with relatively pure substances. Unfortunately, the materials selected for illustration are extremely crude products and do not permit judgment as to what classes of materials can safely be analyzed gravimetrically. Unquestionably there are many classes of commercial products sufficiently free from impurities which may contaminate the copper precipitate, but the analyst must exercise discretion in the selection. of a method of copper analysis. Inasmuch as some of the volumetric methods are less time-consuming than the gravimetric methods, the recommendation appears justified that each analyst select one which best meets his requirements, leaving the gravimetric methods for materials of unquestionable purity. The data in table 21 show that the error of analysis due to contamination is greatly diminished by ignition to cupric oxide, which is weighed directly or reduced to copper. Inorganic contamination is not removed by this procedure.

(b) GRAVIMETRIC METHODS

Preparation of asbestos [11].-Digest the asbestos, which should be of the amphibole variety, with hydrochloric acid (1+3) for 2 to 3 days. Wash free of acid, digest for a similar period with 10-percent sodium hydroxide solution, and then treat for a few hours with hot

alkaline tartrate solution (old alkaline tartrate solutions are suitable) of the strength used in sugar determinations. Wash the asbestos free from alkali, digest for several hours with nitric acid (1+3), and after washing free from acid, shake with water into a fine pulp. In preparing the Gooch crucible, make a film of asbestos 4-inch thick and wash thoroughly with water to remove fine particles of asbestos. If the precipitated cuprous oxide is to be weighed as such, wash the crucible with 10 ml of alcohol, then with 10 ml of ether, dry for 30 minutes at 100° C, cool in a desiccator, and weigh. For the most careful work, the analyst should assure himself that the weight of the crucible remains constant, by pouring 100 ml of clear hot alkaline. solution through it, washing, drying, and reweighing.

A more rapid and perhaps equally effective method developed by Brewster and Phelps for the preparation of asbestos is described on page 324.

Determination.-The gravimetric estimation of copper by direct weighing of cuprous oxide has been described in detail on page 170. To eliminate organic contamination of the copper precipitate, it may be converted to cupric oxide. Ignite at red heat for 15 to 20 minutes, preferably in a muffle or in such a manner as to avoid exposure of the precipitate to hot reducing gases. Too intense heating must be avoided. Cool in a desiccator and weigh rapidly, since cupric oxide is hygroscopic. Multiply the weight of cupric oxide by 0.7989 to obtain the weight of copper. For the most careful work the crucible containing the asbestos should be heated previous to the filtration in order to insure its constancy of weight during the analysis.

Because of the hygroscopic nature of cupric oxide, and to eliminate the possible error arising from its incomplete oxidation, it is sometimes advisable to reduce the precipitate to metallic copper. This can be effected readily by exposing the precipitate to a continuous stream of hydrogen and at the same time heating gently with a Bunsen flame until reduction is complete. Cool in a stream of hydrogen. This method is facilitated by use of a filtering tube constructed of hard glass, the asbestos being supported by a perforated disk or platinum cone. This tube permits the direct application of the flame during reduction, whereas the Gooch crucibles must be supported in a suitable glass chamber.

A convenient method of reduction to copper is in use by the United States Customs Service (Treasury Decision 39350; 1922). The method was devised in principle by Wedderburn [23].

Wash the cuprous oxide thoroughly with water at a temperature of about 60° C, then with 10 ml of alcohol, and dry for 30 minutes in a water oven at 100° C. Heat the crucible for 30 minutes over a Bunsen burner. The precipitate is reduced to metallic copper in methyl-alcohol vapors. This is done by placing about 100 ml of methyl alcohol in a 400-ml beaker, and placing a triangular support in the beaker so that the crucible is above the level of the alcohol. Heat the covered beaker on a hot plate to boiling, remove the cover, and place the hot Gooch crucible on the triangle. This ignites the alcohol vapors. Immediately cover the beaker with a watch glass and allow the Gooch to remain for about 3 minutes, remove, cool in a desiccator, and weigh.

(c) ELECTROLYTIC DEPOSITION FROM NITRIC ACID SOLUTION [24]

Upon completion of the copper reduction reaction, decant the hot solution through an asbestos mat in a Gooch crucible and wash the beaker and precipitate thoroughly with hot water. Transfer the asbestos mat from the crucible to the beaker with a glass rod and rinse the crucible with 14 ml of nitric acid (1+1), allowing the rinsings to flow into the beaker. After the cuprous oxide is dissolved, dilute to 100 ml, heat to boiling, and continue the boiling for about 5 minutes to remove the oxides of nitrogen. Cool, filter, and dilute to 200 ml. Add 1 drop of 0.1 N hydrochloric acid and mix thoroughly.

If extreme care is exercised to avoid spattering, the cuprous oxide can be dissolved by allowing the nitric acid to flow down the walls of the crucible. Keep the crucible covered as well as possible with a small watch glass. Collect the filtrate in a 250-ml beaker and wash the watch glass and the tip of the pipette with a jet of water. Continue as described above, beginning with "heat to boiling.'

For electrolysis use cylindrical electrodes of platinum gauze 1.5 and 2 inches, respectively, in diameter, and 1.75 inches in height, thoroughly cleaned, ignited, cooled in a desiccator, and weighed. Insert the electrodes in the copper solution so that the surface of the cathode clears the anode by at least 5 mm and both electrodes almost touch the bottom of the beaker. Electrolyze with a current of 0.2 to 0.4 ampere until deposition is complete, usually overnight. Without interrupting the current, slowly lower the beaker and at the same time wash the electrodes with a stream of distilled water. Immediately immerse the electrodes in another beaker of water, lower the beaker, and break the current. Rinse the cathode with ethyl alcohol and dry for a few minutes in an oven at 110° C. Cool in a desiccator and weigh.

(d) THIOSULFATE METHOD

When potassium iodide is added to a cupric copper solution, cuprous iodide is precipitated and iodine liberated according to the equation Cu+++21 Cu++I+1/2 I2.

The reaction is reversible and has been shown by Bray and MacKay [25] to obey the mass law within certain limits in dilute solutions. For the purposes of titration the reversible reaction can be made to run to completion in either direction by adjustment of conditions. Thus by the removal of cupric ions in the form of a complex ion or by great dilution the reaction can be made to run quantitatively from right to left, while the presence of a large excess of iodide ions, together with the removal of cuprous ions in the form of insoluble cuprous iodide, causes the reaction to run quantitatively from left to right.

Shaffer and Hartmann [26] determined the positions of the equilibria and concluded that for the determination of cupric salts potassium iodide must be added to give a final concentration of about 0.25 M (4 to 5 g per 100 ml of solution). For the determination of cuprous salts, the solution must be so diluted that the final concentration of copper and of iodide does not exceed about 0.005 M each. Equivalent to this dilution, which curtails the general usefulness of the method, is the addition of potassium oxalate, which forms anions containing both copper and oxalate. The cuprous titration can thus be made conveniently without excessive dilution.