ELECTROMETRIC AND ANALYTICAL EVIDENCE FOR THE COMPOSITION O F PRECIPITATED BASIC COPPER PERCHLORATE RALPH A. BEEBE
AND
SEYMORE GOLDWASSER
Moore Laboratory of Chemistry, Amherst College, Amherst, Massachusetts Received April 4, 1955
The composition of the precipitate formed when carbonate-free alkali is added to solutions of copper salts can be estimated, without analysis, by electrometric titration, using the quinhydrone electrode as p H indicator. The application of the method to several copper salts has been previously described (1, 6 ) . In all cases where evidence was obtained for a definite basic salt, the composition was that of the 1:3 salt, for example, CuClz. 3CuO. (?)HzO. Such 1:3 precipitates have been formed under favorable conditions from solutions of the following copper salts: sulfate, chloride, nitrate, and trichloroacetate. To extend the list further, the precipitation of basic copper perchlorate has been studied by the electrometric method. Unlike the cases previ) ~ . (?)HzO. ously investigated a 1:6 salt is formed, Le., c ~ ( C 1 0 ~6CuO. Because of this anomalous behavior, the composition of the precipitated basic salt has been tested further by direct analysis for the copper to perchlorate ratio. The results confirm the formula derived by the electrometric method. EXPERIMENTAL
Preparation of solutions A solution of carbonate-free sodium hydroxide was prepared by the method of Cornog ( 5 ) . Copper perchlorate solutions were made by two methods. For solut'ion A, barium chloride was added in slight excess to a solution of copper sulfate, and the precipitated barium sulfate was removed by centrifuging. A small weighed excess of perchloric acid was added to prevent hydrolysis. In making solution B, an excess of vacuum-distilled perchloric acid was allowed to react with basic copper carbonate. Aside from the excess acid, this solution contained only copper and perchlorate ions. The acid present was estimated by the position of the initial breaks in the curves, as shown in figure 1 (7). Both solutions were standardized for copper content by the 1075 THE JOURNAI. OF PEYSICAL CHEMISTRY, V O L . X X X I X , N O . 8
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RALPH A. BEEBE AND SEYMORE GOLDWASSER
iodometric method. A blank test showing no liberation of iodine from potassium iodide by perchlorate ion in weak acid solution demonstrated that the method was applicable.
Electrometric titrations Both direct and delayed titrations were used as previously described in the case of copper trichloroacetate (1). Using a saturated calomel reference electrode, an E.M.F. value of + 0.007 volt was chosen as the end-point,
FIG. 1 TABLE 1 Use of direct titrations i n determination of alkali required per mole of copper perchlorate (1.7143 required for 1:6 basic salt)
II
SOLUTION A
Normality of copper perchlorate solution
Moles of NaOH
1.000
1.710 1.714 1.714 1.711
0.400 0.266 0.100
BOLUTION B
Normality of copper perchlorate solution
Molea of NaOH
1.000
1.713 1.714 1.715
0.400
0.100
this being the point of maximum slope of the titration curves. In all of the titrations of solutions of initial concentration greater than 0.1 N , very nearly 1.714 moles of alkali were required per mole of copper salt. This indicates a 1:6 basic salt.' The results for the direct titrations are shown 1
According to the equation, 7Cu(CIOJ2
+ 12NaOH = Cu(C104)2.6CuO. (1)HzO
+ 6NazSO~,the ratio of moles of sodium hydroxide to moles of copper salt is 1.7143:1.
COMPOSITION OF PRECIPITATED BASIC COPPER PERCHLORATE
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in figure 1 and table 1;2those for the delayed titrations, in table 2. In the delayed titrations it is certain that sufficient time was allowed to let the system come to equilibrium. Because the results were identical for the direct and the delayed titrations, it may be concluded that the precipitates were in equilibrium with the solutions even in the direct titrations. For solutions less concentrated than 0.1 N, more than 1.714 moles of alkali were required. This indicates that under these conditions the precipitate was probably a mixture of the 1:6 salt and hydrated copper oxide. TABLE 2 Use of delayed titrations i n determination of alkali required per mole of copper ~
TIME O F STANDING
SOLUTION OF cU(cIo4)zUSED
A A A A A A B B B B B B B B B
0.10 0.10 0.10 0.27 0.27 0.27 0.40 0.40 0.40 0.10 0.10 0.10 0.93 0.93 0.93
20 hours 20 hours 8 days 20 hours 20 hours 8 days 20 hours 20 hours 8 days 20 hours 20 hours 8 days 20 hours 20 hours 8 days
TREATMENT
a* bt b a b b a b b a b b a b b
MOLES NaOH ADDED INITIALLY
1.633 1.633 1.626 1.636 1.633 , 1.633 1.557 1.557 1.557 1.557 1.557 1.557 1.557 1.557 1.557
TOTAL MOLE6
NaOH
1.712 1.712 1.711 1.711 1.711 1.712 1.714 1.714 1.714 1.715 1.716 1.715 1.714 1.713 1.714
* a denotes constant stirring.
t b denotes occasional shaking. A n a l y s i s of the precipitate To 0.4 N copper perchlorate, alkali was added in amounts somewhat less than 1.714 moles per mole of copper salt.3 After three washings by centrifuging and decantation, the moist precipitate was dissolved in dilute sulfuric acid to make a stock solution. Separate aliquot portions of this solution were analyzed for copper iodometrically, and for perchlorate by
* Points on the curves above pH 7, approximately, do not represent equilibrium conditions. On standing a rather rapid decrease in pH is observed, owing t o the interaction of the excess hydroxyl ions with the precipitated basic salt. More than 1.7143 moles of alkali would, of course, convert a part of the basic salt into hydrated copper oxide, so that the precipitate would not have definite composition.
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RALPH A . BEEBE AND SEYMORE GOLDWASSER
the nitron method (4). In the basic salt Cu(C10&.3CuO. (?)H20, the ratio of moles of copper to moles of the perchlorate radical should be 7:2. Analysis of one precipitate from copper perchlorate solution A yielded a ratio of 6.96: 2. Two precipitates from solution B each yielded a ratio of 6.99:2. DISCUSSION
Previously both electrometric (1, 6) and phase rule studies (2) have established the individualities of several 1 :3 basic salts of copper of which CuS04.3CuO. 4Hz0 is an example. Britton (3) has given a review of the various theories, based on constitution, which have been proposed to account for this regularity of behavior. Now, with definite evidence for a 1 :6 basic copper perchlorate, any future theories must explain the structure of this salt as well. The authors believe that no satisfactory theory has yet been advanced. SUMMARY
1. When alkali is added to an excess of copper perchlorate in solutions above 0.1 N , the precipitate has the constitution: Cu(ClOJ2.6CuO. (?)H2O. This compound has not been reported in the literature. 2. This 1:6 salt is unlike any other basic salts of copper previously known. REFERENCES (1) BEEBE:J. Phys. Chem. 36, 3677 (1931). (2) BRITTON: J. Chem. SOC. 77, 2796 (1925). (3) BRITTON: J. Chem. SOC. 77, 2806 (1925). (4) COPEAND BARAB:J. Am. Chem. SOC.39, 504 (1917). (5) CORNOG, J.: J. Am. Chem. SOC.43, 2573 (1921). (6) HOPKINSAND BEEBE:J. Phys. Chem. 34, 570 (1930). (7) MULLERAND MULLER:Z. anal. Chem. 73, 47 (1928).
