Arlo D. Harris and Lee H. Kalbus I ~echn~osit ion of Copper(l) Sulfate Pentahydrate

California State College San Bernardino, California 92407 I A sequential gravimetric analysis

Thermal dehydration of copper(I1) sulfate pentahydrate is used commonly to demonstrate gravimetric analysis (1,2, 3). Ease of water loss and a marked color change during the process are reasons for this. However, experimental difficulties arise since little or no control of the heat source (usually an open flame) is maintained. Results obtained are often poor due to some decomposition of the anhydrous salt to copper(I1) oxide. Improvements described here are use of a controlled temperature environment and a quantitative study of the decomposition reaction to a thermally stable oxide.

The number of molecules of water per formula unit of a hydrate is variable and in some cases may not even he an in- teger. However, the formula in no way distinguishes how these are attached to the parent compound. They may he held by (a) coordinate covalent bonds, (h) hydrogen bonds, or (c) uniform or random arrangement in a crystal lattice. In any given hydrate they need not he held in the same way.

Copper(I1) sulfate pentahydrate has four water molecules hound covalently through oxygen to copper. These form a square planar configuration with CuO distances of 2.0 A. The fifth water molecule is attached via hydrogen honding to sulfate ions and to the water molecules attached to copper (see the figure). As such it is held in the crystallattice in auniform manner (4.5).

Gentle heating of CuS01.5 Hz0 causes loss of the lattice water and one water attached to copper yielding CuS043 H20 (6). One sulfate and three water molecules f o m a square plane with CuO distances of 1.96 A. The irregular octahedron is completed by two oxygen atoms from two other sulfate ions a t 2.34 .&and 2.45 A. These coordination polyhedra are held together hy hydrogen bonding (7,s).

On controlled heating, CuS045 Hz0 effloresces to an iso- lable monohydrate containing one water attached. to the copper while the other five positions of a distorted octahedron are taken u p by oxygens from sulfate ions(9.10). Intermedi- ates containing 2 and 4 waters are also reported (11); however, these do not interfere with the experiment described here.

Stronger heating effects total dehydration (12, 13) to an- hydrous copper(11) sulfate. This has a much more distorted structure with two oxygeus a t 1.89 A, two a t 2.00 A and two a t 2.37 A (14, 15).

Ignition degrades the product to copper(11) oxide which has the crystal lattice of a giant molecule. Each copper atom is surrounded by four oxygens in a square plane with CuO dis- tances of 1.95 .& (16).

The Experiment The purpose of the experiment is fourfold: (a) to determine the

value for X in the starting material, (b) to determine the formula of the intermediate hydrate, (c) to determine the formula ofthedegra- dation product, and (dl to write all balanced equations.

Far the dehydration steps, the only information given is the formula CUSOIX H20 and that at 4W°C all the water of hydration will be lost. For the degradation step, the student is told that both CuS and CuO are black.

Materials

CuS04.5 H20: Baker AR 5-1843 Fine Crystal Crucible, procelain: Cwrs 25006-0 Spot plate, porcelain: Coors 550-00 Balance: Torhal Model ET-1 Oven: National Appliance Model 5510-6

The environment of the waters of hydmTion in copper(l1) sulfate pentahy- drab

Furnace: Lindherg Heavy Duty, Control Model 59344, Ignition Chamber Model 51442

Procedure (3 Parts)

I. A 2.5 g sample of CuSOc5 HzO, weighed to the nearest milligram, is placed into a previously cleaned, oven dried, and weighed crucible. The crucible is placed in an oven at 140°C. Aporcelain spot plate may be used to support up to four crucibles in the oven. After 1 hr the crucible is removed and placed on a second porcelain spot plate at room temperature. The porcelain plate method dissipates heat very quickly. After 5 min cooling, the crucible is reweighed. A second 1-hr heating period may be used to verify completion of this step. Weight loss at this point corresponds to 4 waters of hydration. The residue is pale blue compared to the initial deep blue color.

11. Next, the crueihle is placed on a spot plate in afurnace at 4W°C for 1 hr. After cooline as described ahove. the erueihle is re- weiehed. A second 1-ir heatine oeriod ma; heused forverifi

anhydrous copper(I1) sulfate, with no evidence of copper(I1) oxide.

111. Finally, the crucible is placed on a spot plate in a furnace at 1000'C for 1 hr. After cooling as described above, it is re- weighed. A second 1-hr ignition period may be used to verify complete degradation. Weight loss in this step corresponds to sulfur trioxide. The black residue is copper(I1) oxide.

Results and Calculations

Data including standard deviation and range for 12 samples run simultaneously are as follows: (a) average sample weight 2.574g; (b) weight loss at 140°C, 0.741 f 0.009 g, range 0.721-0.752 g; (c) weight loss at 400°C,0.186 &,0.003 g, range 0.179-0.192 g; (d):residue at 1000°C, 0.812 & 0.013 g, range 0.794-0.834 g. Using these data the required calculations and equations determined by students are as follows.

2.574 g 1.647 g 0.927 g 1.647 g

= 0.01032 mole CuS04 159.6 g/mole

0'927 = 0,05144 mole Hz0 18.02 glmole

Volum 56. Number 6, June 1979 1 417

0.01032 - 1 mole CUSO* is followed visually by observing color changes during the decompo-

Mole ratio: - - 0.01032 sition.

Most importantly, these refinements demonstrate some of the more -- 0'05144 - 4.98 mole H 2 0 0.01032 subtle aspects of crystal chern~stry. For example, students discover

that all 5 waters of hydration in CuSOu5 H20 are neither thermally :. CuS0a:HzO = 1:5, X = 5 and the formula is CuSOl. 5 Hs0 (1) ,t,ucturally equivalent, llODC

cuS01.X Hz0 A cuS01.Y H 2 0 + Z H 2 0 Several adaptations of this experiment are possible. For a single A 3-hr laboratory, the two separate dehydration steps may be com-

2.574 e 1.833 e 0.741 e pleted. If asecond 3-hr laboratory period is available, the additional - - - - 1.833 a - 1.647 a = 0.186 a Hs0 far Y. . . . -

0'18' = 0.01032 mole Hz0 18.02 glmole

degradation step makes a concise experimental package. A combination experiment is also possible using a stoichiometric

study of copper sulfate pentahydrate previously published in this Journal (17). Our ex~eriment determines water eravimetricallv. The . . "~ ~~~~~~~~~ d~ - - ~ -

Mole ratio: 6.01032 = 1.0 mole HzO experiment by Silber determines copper and sulfate, thereby ob- 0.01032 taining the amount of water by difference. These may be combined

:. CUSOLH~O = 1:l. Y = 1 and Z = 4. (2) to produce an experimental package covering several quantitative

The balanced equations for (1) and (2) are: techniques utilizing this interesting pentahydrate salt. If combined, the total time would he from five to six 3-hr laboratory periods.

~ ~

CUSOCH~O ~ C U S O * + HzO (1) Acknowledgment T h e authors wish t o thank t h e members of t h e Chemistry

CuS01.5 Hz0 5 - CuSOcH20 + 4 HzO 4 Department for reviewing the manuscript, a n d i n particular

For the degradation step: Professor Ralph Petrucci whose critical comments were in-

1WO'C valuable to the success of th is project.

CuSOd - black residue + lost material Literature Cited 1.647 g 0.812 g 0.835 g (3)

i l l H e . G. G. and Kask. U.. "Exoerirnental General Chemistrv." Bamesand Noble.Ine..

the equation far degradation is: lWO0C

CuSO4 - CuO + SOa . 4

Dickinnon Publishing Cn.. Ine. 1967, p. 41. 141 Beeuers, C. A. and I.ipson, H., Pme. Roy. Soc.. l46A. 570 l1934l.

(3) 15) Bac0n.G. F.. and Curry, N. A.,Pror. Roy. Sac, 266A.95, 1962. 161 Galimherti.L.,Boil. Sci. Foc. Chim. ind.. Bologns.,272.1940. 171 Zahrohrky,R. and Baur, W. H.,Noturuias. 52,388,1965. is1 Zahrohrky, R. and Baur. W. H..Acto Cryst., 824,508 1968.

Discussion 191 Marin, Y.and Duva1.C.. Anal. Chim Aeto.. 6.47 ,952. i101 Duva1.C.. Anal. Chim. Arfo.. 16.223.1967.

This improved yields data accurate enough to dem. ("1 Taylor. T I. and Klug. H. P. ;J ~ h r m Phys ,4.601.1936. 1121 Kohler. K.and Zaske,P..Z. an or^. und A l l ~ e m . Chm.. 7,331.19M.

onstrate sequential gravimetric analysis. Advantaw gained by using 1131 pannetier, G., G U ~ ~ S ~ , J , and Manoli. J. M.. RUII soc. them pmnc., 2832. ISM. a controlled temperature environment are (a) isolating a stable 1141 Rae. B. R.,Aeta Cryst., 14.321,1961.

monohydrate intermediate and (h) preserving the purity of the an. 1151 Almdov~,I..Fra2er.B.C.,Hunt.J. J.,Fox.D.E,andBrown,P.H.Phys.Rsu., MA, 153, 1965. hydrous salt' The added aspect of degradation to oxide 1161 Tunnell, G..Pomjsk, E.. end Ksanda, C. J. Z Krisl.. W.120.1935.

makes i t more interesting and instructive. Progress of the reaction (171 Silber. H.B. , J.CHEM. EDUC., 43,586,1972.

418 1 Jwmal of Chemical Education