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.

The principles established by Shaffer and Hartmann are the basis of the cupric titration given in detail below. The cuprous titration is generally made directly in the reaction mixture, and examples are given in several of the special reducing-sugar methods described under "volumetric processes" page 185. (See also Methods of Analysis of

AOAC [27].)

Reagent. Standard thiosulfate solution.-Prepare a solution containing 39 g of pure Na2S2O3.5HO2 in 1 liter. Weigh accurately 0.2 to 0.4 g of pure Ĉu and transfer to a 250-ml Erlenmeyer flask roughly graduated by marks at 20-ml intervals. Dissolve the Cu in 5 ml of a mixture of equal volumes of HNO3 and H2O, dilute to 20 or 30 ml, boil to expel the red fumes, add a slight excess of strong Br water, and boil until the Br is completely driven off. Cool, and add NaOH solution with agitation until a faint turbidity of Cu(OH)2 appears (about 7 ml of a 25-percent solution is required). Discharge the turbidity with a few drops of acetic acid and add 2 drops in excess.

Prepare a solution of 42 g of KI in 100 ml of solution made very slightly alkaline to prevent formation of HI and its oxidation.

It is essential for the thiosulfate titration that the concentration of KI in the solution be carefully regulated. If the solution contains less than 320 mg of Cu, 4.2 to 5 g of potassium iodode should have been added at the completion of the titration for each 100 ml of total solution. If greater quantities of Cu are present, the amount of KI should be proportionately greater. The KI solution should be added slowly from a burette, with constant agitation.

Observe the volume of the Cu solution and add 1 ml of KI solution for each 10 ml of the solution undergoing titration. Titrate at once with the thiosulfate solution until the brown color becomes faint. Again observe the volume and add an additional volume of KI to make the required concentration, noting from the volume of the thiosulfate the approximate Cu content of the solution. Add sufficient starch indicator to produce a marked blue coloration. Continue the titration cautiously until the color changes toward the end to a faint lilac. As the end point is approached, add the thiosulfate in fractions of drops, allowing the precipitate to settle slightly after each addition. One ml of thiosulfate solution equals about 10 mg of Cu.

Determination.-Wash the precipitated Cu2O, cover the Gooch crucible with a watch glass, and dissolve the oxide by means of 5 ml of HNO3(1+1) directed under the watch glass with a pipette. Collect the filtrate in a 250-ml Erlenmeyer flask roughly graduated by marks at 20-ml intervals, and wash the watch glass and Gooch crucible free from Cu. Proceed as directed under "Reagent," beginning with "boil to expel the red fumes."

Foote and Vance [28] have studied the titration and have proposed that an addition of 2 g of ammonium thiocyanate be made when the tiration has proceeded nearly to the end point. In the usual procedure the cuprous iodide is not white but slightly discolored, apparently as a result of the adsorption of iodine. Because of the greater insolubility of cuprous thiocyanate, the surface particles of cuprous iodide are changed to thiocyanate, releasing the adsorbed iodine, which thus consumes a slight additional volume of thiosulfate. The end point is exceedingly sharp, the precipitate turning completely white. This modification can be introduced into the procedure described above,

provided both standardization and determination are conducted in the same manner.

Foote and Vance performed the titration in the presence of 5 ml of sulfuric acid and in the presence of the same reagent buffered by 3 g of ammonium acetate and obtained the same analytical results. Even a small volume of nitric acid (1 ml) produced no measurable deviation. In view of these results, it is probable that the procedure given on page 181 could be materially simplified.

(e) PERMANGANATE METHOD

When cuprous oxide is dissolved in a ferric sulfate solution the latter is reduced to ferrous sulfate and can be titrated with standard permanganate. The reduced copper can then be computed on the basis of the stoichiometric relations. This method of determination of reduced copper was originally proposed by Mohr [29], but has erroneously been attributed to Bertrand [16]. Mohr prescribed that the ferric sulfate be dissolved in sulfuric acid, and this procedure was adopted by Bertrand, who based his tables (see page 587) upon the volume of permanganate consumed, instead of referring his sugar to copper. Subsequently, it was discovered that the permanganate volumes, when converted to copper, gave results which were invariably about 1.4 percent too low, and many authors advocated a blanket correction by this amount. Schoorl and Regenbogen [30] ascribed the low results to the rapid oxidation of ferrous sulfate by air. The oxidation is more rapid than is ordinarily the case with ferrous sulfate, but in the presence of copper, which increases the rate of oxidation, and in the presence of the asbestos, which carries finely subdivided air, the amount of oxidation appears to be accounted for.

Schoorl and Regenbogen found that if the cuprous oxide was dissolved in neutral ferric sulfate, or better, ferric alum, before addition of sulfuric acid, correct results were obtained. A brownish-red clear solution is obtained in which the Fe2O3 apparently remains dissolved in the form of a basic salt. The authors suggest that the following reaction occurs:

Cu2O+2Fe2(SO4)3 →→2CuSO4+2FeSO4 + Fe2O3+SO3.

Reagents. Prepare a potassium permanganate solution, about 0.1573 N, containing 4.98 g per liter. After several days' aging, filter through asbestos or sintered glass. Standardize by either of the following methods.

(a) Transfer 0.35 g of sodium oxalate (dried at 103°C) [31] to a 600-ml beaker. Add 250 ml of sulfuric acid (5+95) previously boiled for 10 minutes and cooled to 27±3°C. Stir until the oxalate is dissolved. Add 29 to 30 ml of permanganate solution at a rate of 25 to 35 ml per minute while stirring slowly. Let stand until the pink color disappears (about 45 seconds). Heat to 55° to 60°C, and complete the titration by adding permanangate solution until a faint pink color persists for 30 seconds. Add the last 0.5 to 1 ml dropwise, allowing each drop to become decolorized before the next is added. Determine the excess of solution (usually 0.03 to 0.05 ml) required to impart a pink color to the same volume of acid boiled and cooled to 55° to 60°C.

In potentiometric titrations the correction is negligible if the end point is approached slowly.