L. H. Kalbus and R. H. Petrucci California State College San Bernardino, 92407
I
l
he Stoichiome+ry of Silver Chromate and Basic Copper Chromate Investigations for the freshman laboratory
Among the more significant recent developments in t,he teaching of freshman chemistry is the incorporation of quantitative experiments into the laboratory. This has varied from a few isolated experiments to rather complete courses in traditional quantitative analysis. Other ideas expressed in recent articles are that if student interest and achievement are to be heightened there should exist some reason for performing analyses (I); where possible, something unusual or unexpected should happen (3-4); and experiments should be open-ended (5). These several objectives may be met through the investigations of silver chromate and basic copper chromate described in this paper. The project begins with a continuous variation study of the formation of silver chromate. The results obtained are consistent with the expected formula, hut definite proof of this fact requires a quantitative analysis of the precipitate. I n dealing with silver chromate we are only confirming what the well-prepared student already k n o w s i t ' s Ag,CrOn. The project turns next to copper chromate and with this substance the results are unexpected, from beginning to end. A number of compounds have been reported in the system, Cu0-GOa-H20 (6). That of "normal" composition, CuCrOn, can only be obtained under carefully controlled conditions (7, 8). By contrast the basic salt, 2CuCr04.~ C U ( O HH,O, ) ~ . can be precipitated from solutions over a considerable range of concentrations of copper and chromate salts (9). The water of hydration in this compound is quite labile (Q), and we have worked with both the hydrate and the anhydrous compound. The experiments which are described in the following Based on a paper presented s t the 4th Western Regional Meet,ing of the American Chemical Society, San Francisco, November 1968. ' For the most part standard methods can be used, although modifications are required in certain instances. Furt,her details concerning the vari&s methods mentioned in this paper, and others not included, are available from the authors. Table 1.
Based on 6 to 12 student results. the chloride. Mohr chloride back-titration. 6 Ag and glass electrodes; NaCl titrant.
a
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Journal of Chemical Education
Experiments1and Results Silver Chromate
In the continuous variation study the methods followed are similar to those employed in the CBA chemistry course (10). All continuous variation plots correspond to a 2: 1 molar combining ratio of silver nitrate to potassium chromate, but definite proof of this fact is established by quantitative analyses for silver and chromate. Typical results are shown in Table 1. Basic Copper Chromate
I n a continuous variation study of the copper chromate system, precipitates are filtered by suction on a sintered glass crucible (without washing) and dried at 110°C for 2 hr. Since the solubility of basic copper chromate is strongly influenced by the presence of common and foreign ions, the results obtained are noticeably dependent on the concentrations employed. A typical set of results for conditions under which the compound, 2CuCrOc3Cu(OH)z.HzO, precipitates is shown in Figure 1. From this plot it is seen that the maximum yield of precipitate occurs for some molar ratio of the reactants other than 1 : 1, indicating that the compound may not be "normal" copper chromate a t all. As a matter of interest we have added to the usual experimental continuous variation plot (a) the experimentally determined Cu/Cr04 molar ratio for the series of precipitates, (b) a theoretical plot based on a 100yoyield of the compound having a Cu/CrOa ratio of 2.5:1, 2CuCr04. 3Cu(OH),, and (c) designations of certain Cu/Cr04 molar ratios. Pmpamtion of the Precipitate. The composition of the precipitate depends on the concentrations of the reactants (9), how the reactants are mixed ( I I ) , the extent to which the precipitate is
Analysis of Silver Chromate.
% Silver (Theoretical: 65.03) Gravimetricb Volumetric" Potentiometricd
section employ a wide variety of standard gravimetric, volumetric, and instrumental methods. Among the
% Chromate (Theoretical: 34.97)
65.01 f 0 . 2 64.61 10.5 64.99 + 0 . 8
Spectrophotometric' Iadometrid Potentiometric'
" I n 1 M NH. a t 440 mp. added; 4 titrated with Na&08.
1 XI 0
P t and calomel electrodes; excess ferrous ion titrated with dichromate.
35.2 1 1 34.60 10.14 34.65 i 0.12
,, ,,
12.m
O . l O o v y
,
,
,*.,a,
WI
0.300
0.600
,
,\,
ilil.8.l
0.900
1.m
1500
4
Weight of K2CrOl (grams1 Figure 1. Eiperimentol resulb (solid line1 and theoretid plot (broken line), based on 100% yield of 2CuCrOn. 3Cu(OHI2. Cu/Cr04 molar ratio in Certain Cu/Cr04 molar precipitate (unwashed1 is given in parenthese,. ratios of interest ore given in porentheser with orrows to the abrcirm. Total weight of reactants, 1.500 g and tot01 reoction volume, 1 5 0 ml.
washed, and the manner in which i t is dried (9). A precipitate of satisfactory quality for analytical experiments can be obtained by adding, slowly and with stirring, 50 ml of a solution containing 0.50 g of K G K h to 50 ml of a solution containing 0.50 g of CuSO,. 5Hz0. Larger amounts of the reactants a t corresponding concentrations may he used if more precipitate is desired. The precipitate is washed with a minimum of water. Drying a t 110' C produces the anhydrous basic salt. To obtain the hydrated compound, air-drying works reasonably well. Speetropholmet~icDetermination of Copper lo Chromate Ratio. A small sample of freshly-prepared precipitate is dissolved completely in 1 M NHs and the resulting solution analyzed spectrophotometrically for Cu(NHa)2+ a t 610 mp and CrO? a t 440 mp. Neither species absorbs appreciably a t the wsvelength chosen for the analysis of the other. A series of standards is prepared by diluting solutions of K2Cr01 and CuSO, in 1 M NHI having concentrabions of about 0.3-0.5 g Cr/l end 1.0 g Cu/l, respectively. To determine the capper to chromate ratio it is not necessary to dry or weigh the sample of copper chromate. Thus, this analvais ., can he mrformed in a matter of minutes once the standsrdimtion data have been established. (This particular determination can be used as a separate short laboratory exercise. The authors have used i t successfully in a general studies chemistry course for non-science students.) Typical results for the Cu/CrO, molar ratio range from 2.6 to 2.7:l. The discrepancy from the theoretical value (2.5:l) for 2CuCrO4.3Cu(OH)%.H1Ocan be attributed almost entirely to the slight decomposition which occurs on washing the precipitate. EJed of Washing. Basic copper chromate is unlike the usual precipitate encountered in analytical chemistry in that it is somewhat more soluble than most and it dissolves incongruently. As the compound in question dissolves it decomposes to insoluble copper hydroxide. This means that the Cu/Cr04 molar r h o i n , the undissolved precipitate increases. This point may be demonstrated by washing freshly-prepared precipitates different,ly. Figure 2 shows the. result,s obtained wit,h no washing, washing with a 25-ml portion of water, two 25-ml portions, etc. Since precipitat,ion is carried out from a solution in which the molar concentration of C r O F exoeeds that of CUP+, t,he unwashed precipit,ate has s. v d u e of Cu/CrO, which is a bit low. Samples washed with increasing amounts of water give higher values. Results may be extrapolated to the idealized rat,io of 2.5: 1. For the following analyses washing should be held to a minimum. ~
~~~
~
~~~
~~~~~
~
~
.~ ~
12
8
16
20
Number of Washings ( 2 5 m l each1 Figure 2.
Effect of washing on Cu/CrOl ratio.
Methods for Copper and Chromate. I n ddition to its use for the Cu/Cr04 ratio determination the spectrophobometric method can be applied to the quantitative determination of copper and chromate using a weighed, dried sample of the precipitate. Copper may also be analyned by electrodeposition or iodometrically. Other methods for chromate are iodometric and potentiomet,ric. Results for copper and chromate are given in Table 2. Detemination of Hydrozide. That there must be an additional eonstituent(s) is apparent from the fact that the percentages of Cu and CrO, total only about 83%. Furthermore, it can be inferred from the 2.5: 1 ratio of Cu2+to Cr042- that the copper chromat,e precipitate must contain an anionic species in addition t o CrO.2-. That this species may be hydroxide can be demonstrated by a simple ~alculation.~That the species has basic properties, and its percentage composition in the compound can be established by an acid-base t,itration. The copper chromate precipitate contains two species that react with hydrogen ions, C r O F and OH-. A 0.2000 g sample of. the dried precipitate is dissolved in 50 ml of standard acid. The excess acid is titrated with standard NaOH to a pH of exactly 4.00. A sample of K2Cr0, containing the same amount of chromate as the sample of unknown is similarly titrated. (This necessitates a previous chromate determinat,ion on the copper chromate.) The difference in the volume of standard NaOH required for these two titrations is equivalent to the hydroxide present in the copper chromate precipitate. Some results obobtained on air-dried samples yield 16.16 & 0.06% OH. Sloichimelrg qf Basic Copper Chromate. Table 3 lists the complete analysis of three samples of copper chromate, one airdried and two dried a t 110°C. . Rather convincing evidence is ' T h e total percentage? of the copper and chromate on an oven-dried sample (essentially anhydrous) amount to about 837& The remaining constituent(s) secaunts for approximately 17y0. Designating this constituent(s) by X we may write the formula, Cuz.&rO,(X).. X, X 100 2.5 X 63.54 116
+
+ (X).
=
17
(1). = 56 The fallowing combinations are possible with respect to the formula weight, of X: X' = 56, n = 1; X = 28, n = 2 ; X = 19, n = 3; X = 14, n = 4. Three hydroxide ions per formula unit of compound corresponds very well to one of these possibilities (and also leads to an electrically neutral formula unit).
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Table 2.
Determination of Copper and Chromate in Basic Copper Chromatea
% copper
% Chromate
Spectm hotometric" ~leetro$eposition Iodometric'
In 1 M NH8 at 440 mw. P t and calomel electrodes; excess ferrous ion titrated with dichromete.
Based on 6-12 student results. V n 1 M NH8 at 610 mw. XI added; 4 titrated with Na&03. Table 3.
Cu/CrO, molar ratio %cu % CrO. % OH
% so, % H*O % Total
Analysis of Basic Copper Chromate
Sample 115 (dried at 110°C).
Sample 117 (dried at llO°C) .
2.71 49.38 33.28 16.69 0.12
2.68 49.19 33.57 16.43 0.12
99.G
-
Summary
This paper outlines a laboratory investigation in which quantitative analyses are applied to determining the stoichiometry of two compouuds-one rather conventional and the other unusual. From the variety of methods presented an instructor or student may select those most appropriate to his objectives. In addition further investigations are possible, especially in the copper chromate system. The effect of reactant concentrations, the rate of mixing, temperature of precipitation, and washing on the composition of the precipitate may be studied. Precipitation from homo-
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Journal of Chemical Education
Theoretical 2CuCr0,. 3CdOHh
Sample 117 (air-dried)
Theoretical ZCUCI-0,. 3Cu(OH)%. HzO
99'. Bi
obtained in this project that the Cu/CrO, ratio is 2.5:l or 5:2 and that hydroxide is present. This provisional formula can he written (for the anhydrous compound), 5Cu.2CrOd.nOH. Ordinary valence rules would suggest that n = 6, leading to the formulation, ~ C U C ~ O . . ~ C ~ ( OMost H ) ~ . of the discrepancy between experimental and theoretical values can be attributed to the decomposition of the precipitate which occurs on washing. That this occurs is suggested by the slightly high experimental values of the Cu/CrO, molar ratio, hut that the experimental data are internally consistent is suggested by charge balance. For example, for sample 117, dried at llOSC, the number of moles of charge per 100 g of compound would he: +1.5483 (Cua+), -0.5788 ( C r O F ) , -0.9659 (OH-), -0.0025 ( S O P ) ; excess charge f0.0011.
778
33.6 + 1 33.06 + 0.1 32.76 + 0 . 3
Speetrophotometricd lodometric" Potentiometric"
48.6 f 1 48.18 1 0 . 1 47.94 + 0.07
geneous solution is also possible, using hydrolysis of urea to regulate the OH- and C r O F ion concentrations. Acknowledgments
The authors are pleased to acknowledge the assistance of their colleague, Arlo Harris, and of a large number of students, in particular William Woerz, Michael Tallman, David Randolph, and James Bishop. Literature Cited (1) NORDMANN, J., J . CHEM.EDUC.,44,691 (1967). (2) KING,L. K., A N D COOPER,M., J. CHEM.EDUC.,42, 464 (1965). D., AND BARNARD, W., J. CHEM.EDUC.,44, 693 (3) DINGLEDY, (1967). (4) HAIGHT, G. P., JR.,J. CHEM.EDUC.,44, 766 (1967). (5) Anon., Chemical and Engineering News, 53, (Nov. 20, 1967). (6) SNEED,M., MAYNARD, J. L., A N D BRASTED, R. C., "Comprehensive Inorganic Chemistry," D. Van Nostrand Ca., Princeton, 1954, Vol. 11, p. 96. (7) BRIGGS,S. H. C., J.Chem. Soc., 242 (1929). J., Z. anorg. C h a . , 10,148 (1895). (8) SCHULZ, (9) HAYEK,E., Z. anorg. u. allgm. Chem., 216,315 (1934). (10) NEIDIG,H. A. (Edilo?), "Investigating Chemical Systems," McGraw Hill Book Co., Webster Division, St. Louis, 1963, p. 20. P. E., et al., Can. J . Research, 16B, No. 2, 37 (11) PELLBTIER, (1938). ~
