High-energy modification of lower hydrates of copper(II) sulphate have been prepared and their

heats of solution measured. The values obtained are higher than previously found for the salt. A new

mechanism of dehydration has been proposed and supported by ir study of deuterated sample

Badri, M.

Crouse, Karen Anne

Universiti Putra Malaysia Institutional Repository

Pertanika 5(2),229-233 (1982)High-energy Modification Hydrates of Copper(II) Sulphate: heats ofsolution and infrared spectrumM. BADRl and K.A. CROUSEDepartment of Chemistry, Universiti Pertanian Malaysia,Serdang, Selangor, Malaysia.Key words: Copper(II) sulphate - high-energy hydrates; heat of solution of hydrates; ir of high-energyhydrates; vacuum dehydration; mechanism of dehydration.RINGKASANBeberapa hidratan kuprum(II) sulfat bertenaga tinggi telah disediakan dan haba larutan bahan-bahan itu disukat. Haba ini didapati lebih besar daripada yang diperolehi sebelumnya. Satu mekanismabaru pengontangan dikemukakan dan disokongkan dengan kajian inframerah sampel yang mengandungideutrium.SUMMARYHigh-energy modification of lower hydrates of copper(II) sulphate have been prepared and theirheats of solution measured. The values obtained are higher than previously found for the salt. A newmechanism of dehydration has been proposed and supported by ir study of deuterated sample.INTRODUCTIONThe plot of heat of solution data as a functionof percent water content in a salt would generallygive a relation similar to that shown in Fig. 1 forthe hydrates of copper(II) sulphate. In the figure,the lines AB and BC represent the heats of solutionfor the crystalline hydrates which were preparedby heating the pentahydrate in an oven (Badri andJamieson, 1979). The value increases linearly fromI, for the pentahydrate, to B, for the monohydrate.From the monohydrate to the anhydrous salt, A,there is an increase in the slope of the curve togive the line BA.The line 11K, which is to be reported here,represents the values for the noncrystalline high-energy modification hydrates which were preparedby dehydrating the pentahydrate under vacuum.This line is parallel to the line AB and passesthrough I, the heat of solution of the startingmaterial. To facilitate discussion, we propose thisline be called the "maximum line" because it maynot always be obtained exerpimentally (Badri andJamieson, 1979; Jamieson and Frost, 1956) andbecause there are cases where intermediate lineshave also been found together with this "maximumline" (J amieson et al. 1965). In the case of CuS04 ·5H2 0, only the intermediate line FGH was obtained229previously. This line intercepts the heat of solutionline BC at F, the trihydrate composition (Badriand Jamieson, 1979).It was suggested that under vacuumCuS04 · 5H2 0 would first form the crystallinetrihydrate before proceeding to form the corres-ponding high-energy modification monohydrate.Even at sufficiently low pressures, and havingdesiccants such as barium oxide and carbontogether with a liquid nitrogen cold trap, a maxi-mum high-energy modification product with aheat of solution falling on the "maximum line"was not obtainable. Invariably all the vacuumdehydration products of CUS04 .5H2 0 under suchconditions have heats of solution falling on a linewith a lower slope. The reason for this seems tobe that these products are mixtures of the startingmaterial, the pentahydrate, and the high-energymodification monohydrate which originates fromthe crystalline trihydrate, CuS04 .3H2 0, with itsheat of solution represented by the point G inFig. 1.Several investigations have been reported onthe kinetics of dehydration of CuS04 .5H2 0 andother salts. It is well known that the ambientpressure can influence the rate of dehydrationand the nature of the products. Perhaps the mostM. BADRI AND K.A. CROUSEK EXPERIMENTALInfrared StudyAbout 0.1 g of CuS04 .3H2 O. 202 0 previouslyprepared (M. Badri and Jamieson, 1979) wasintroduced into a glass tube. The tube wasstoppered and then cooled in liquid nitrogen forabout 15 minutes. It was then connected to avacuum line through a stopcock. The pressurewas kept low, at about lxlO-4 Torr, by coolingthe sample with ice. After about six hours thetemperature was raised to that of the room (30°C)and dehydration was continued further for another30 hrs at about lxlO-6Torr. The ir spectrum wasobtained from a Nujol mull of the product on aBeckman Acculab 7 ir spectrophotometer.The heat capacity of the calorimeter wasdetermined by the chauge in the thermally inducedemf of the thermocouple caused by a knownamount of the electrical energy supplied by theheater. This was registered on the voltmeter,amplified and recorded by the recorder.The calorimeter was made up of a 200 mlsilvered Dewer flask filled with distilled water.The flask was inserted into a cut plastic bottlewhich was placed at the centre of a plastic bucketcontaining decanoic acid (m.p. 31°C). The bucketwas thermostated at 31°C in a water-bath. Theflask was fitted with a cover accomodating astirrer, a heater, a small glass tube for the variablejunction of a copper-constantan thermocouple anda glass funnel for sample delivery. The referencejunction of the thermocouple was placed in a glassencasement and dipped into the decanoic acidequilibrated at its melting point. The thermocoupleleads were connected to a DC microvoltmeter,(TOA electronic Ltd., Model PM-16A), and arecorder.Heat of SolutionAbou t 0.2 g sample of CuS04 . 5H2°of BDHanalar grade was put in a pyrex glass boat. Theboat was then cooled with some liquid nitrogenin a watch glass covered with a beaker to preventtoo much condensation of water vapour on theboat. When the sample was sufficiently cooled,the boat was introduced into a large glass cham berand evacuated to a pressure of about lxlO-sTorrEvacuation was carried on for a few hours afterwhich dried air was introduced and the productwas weighed. The composition of the salt wasdetermined by the loss in weight. The heat ofsolu tion was measured immediately in a calori-meter.c40SALThydrates ofotiwIIt has also been suggested that the initialpressure of dehydration is particularly importantas a crystalline product may be formed prior tofurther dehydration to the corresponding high-energy modification lower hydrates (Jamiesonand Frost, 1956). This is certainly true as dehy-dration may start long before the apparatus iscompletely evacuated. In addition to this, thevapour pressure due to the decomposing sample isquite high and this alone may cause the formationof a crystalline lower hydrate as the product.In this paper, we report the method of pre-paring the maximum possible high-energy modifi-cation for the hydrates of copper(II) sulphate andto use the ir spectrum to support the mechanismsuggested.10 20 30PERCENT WATER INFig. 1. Heats of solution for thecopper(II) sulphate.pertinent are the experiments reported by Topleyand Smith (1935) and Volmer and Seydel (1937)where the dependence of rate of dehydration onthe pressure was shown to have a minimum.According to them, dehydration of a salt at apressure greater than the minimum for the saltwould yield a crystalline product.230HIGH-ENERGY MODIFICATION HYDRATES OF COPPER(II) SULPHATERESULTSmadi-whereI2000I3000WAVENUMBERS eM-Ispectrum far high-energyCuS04·xH2 0 .yD 2 0Fig. 2. Infraredficatianx + y <1f-ZWUCl:::wa...zo(f)(f)~(f)Z<!Cl:::f-The heats of solution for the high-energymodification hydrates of copper(II) sulphateare listed in Table I and plotted as a function ofpercent water content in Fig. 1. The v~ues werecorrected for the dilution effect (Lange et ai.1933). They are shown by the circles which seemto fall on a straight line 11K in Fig. 1. By usingthe method of least squares, the equation for theline UK was found to beYIK = - 21.25x + 740.6 73where y is the heat of solu tion and x is percentwater in salt.The heat capacity of the calorimeter wastested for its consistency by measuring the recorderdeflections due to various amounts of energyinpu1. The average value obtained was 39.6 JJinfor ten different experiments with a deviation ofabout ± 0.6 1.TABLE 1Heat of solution for high-energy modification hydratesof copper(II) sulphate.In Fig. 2, the ir spectrum of a vacuum dehy-drated product is shown. The sample containsabout 9.8% H2 0 and D 2 0, which is less than thatof the monohydrate composition. The spectrumshows both the absorptions at 3300 cm -1 and2400 cm- 1 due to 0 - 11 and 0 - D stretchingfrequencies respectively. For clarity, the absorp-tion band due to Nujol at about 3000 cm-1 hasbeen ommitted.%waterin salt36.0835.1030.7529.3129.2421.3718.0716.0215.2914.9713.1811.558.968.46heats of solutionJOUles/gram- 34.774.477101.1114.6100.0272.9342.7443.5447.7418.0457.3464.4556.5548.5DISCUSSIONThe result clearly shows that it is possibleto obtain the maximum heat of solution line forthe high-energy modification hydrates if the initialdehydration pressure were kept very low. Thiscould be achieved by reducing the temperature ofthe sample which in trun would reduce the rate ofevolu tion of water vapour. It is in agreement withthe suggestion of earlier workers that high watervapour pressures would facilitate crystallization ofthe products (Topley and Smith, 1935 and Volmerand Seydel, 1937). Such crystallization wouldundou btedly result in the lowering of the heat ofsolution of a sample.The occurrence of the intermediate heat ofsolu tionlines in some systems can now be explained.For example, in the case of cobalt(II) sulphate,this intermediate line seems to originate from thetrihydrate (Jamieson et ai. 1965) whereas in thecase of zinc(II) sulphate there are two intermediatelines: one is from the trihydrate and the other isfrom the hexahydrate (1 amieson and Frost, 1965).However, contrary to the case of copper(II)sulphate, in both cases the maximum heat ofsolution line for the high-energy modificationhydrates were obtained when various powerfuldesiccants were used.Thus it is possible to assume that in thoseexperiments the initial dehydrating pressures ~erehigh due to the rapid evolution of water vapourfrom the salts. The first stage of dehydrationunder such conditions would lead to the formation231M. BADRI AND K.A. CROUSEof crystalline lower hydrates with their heats ofsolution falling on a line such as BC in Fig. 1. Themanner in which the second dehydrating stagewould proceed would depend on the salt con-cerned. In the case of copper(II) sulphate, theambient pressure was low enough to cause thecrystalline trihydrate formed to be further dehy-drated instantly to the corresponding high-energymodification lower hydrate even though someparts of the sample were still of pentahydratecomposition. Thus the final product would be amixture of this high-energy form and the originalpentahydrate. The heat of solution for such amixture would then fallon a line joining the twoindividual values (Donnan and Hope, 1909). Atruely in termediate line such as FGH in Fig. 1could only be obtained by using the trihydrate asthe starting material (Badri and Jamieson, 1979).In the case of zinc(II) sulphate it seems thata slightly different mechanism is operating. Thefirst stage of dehydration resulted in the completeformation of the crystalline hexahydrate (or thetrihydrate). Only when all the heptahydrate wasthus decomposed would the second stage proceedto the corresponding high-energy modificationlower hydrate. The product is then a mix ture ofthis high-energy lower hydrate and the crystallinehexahydrate and not' the starting material, theheptahydrate. Thus the heat of solution of thesample would fallon an intermediate line originat-ing from the hexahydrate. The purpose of havingthe desiccants in the system is to speed up theremoval of water vapour. Consequently the initialdehydrating pressure would be low and this wouldfacilitate thf' formation of the non-crystalline high-energy produ cts directly from the starting material.In other c<ses, for example, the chlorides (J amiesonand Frost, 1956), desiccant alone seems to beinsufficient to maintain a low initial pressure.Therefore in this case perhaps the maximum linecould be obtained by applying the present tech-nique.It may not always be necessary that vacuumdehydration be carried out only after the sampleswere first cooled, particularly when small samplesare used such as in the thermogravimetrictechnique. Here a sample weight of about 20 mgis typical. A good vacuum pump may be able toattain a very low pressure right from the start.Thus very little of the intermediate high-energyform would be formed.However, in the experiments where samples ofas much as 0.5 g were used for the measurementof some thermodynamic properties (Frost et al.2321951), the amount of water vapour evolved mighthave been a little bit excessive for fast evacuation.Thus the product might have been crystalline, orat most, might have assumed one of the severalpossible intermediate high-energy froms. As aresult, its heat of solution would be lower than themaximum possible. This may be a reasonableexplanation for such a low excess-energy contentof vacuum dehydrated product of MgS04 7H2°and CoC12 .6H 2 0 over that of the crystallinematerial (Jamieson and Frost, 1956).The mechanism of dehydration under thepresent condition differs from the one describedearlier (Badri and Jamieson, 1979). This is clearlyshown by the spectrum of the deuterated salt,CuS04 .xH2 0.yD2 0 with (x + y) < 1, Fig. 2The presence of D2°clearly indicates. that vacuumdehydration of CUS04 .3H2 0. 2D2° does notcompletely remove what is supposed to be the lessstrongly bonded D2 0. Instead it shows that theremoval of D2 0, H2° and anionic H2° in thesalt occur simultaneously and nonselectively. Thisis not impossible since at such a low pressure thedriving force might have been large enough toovercome the slightly stronger bonds holding theanionic water in the lattice. This means that theremaining water molecules in the product areoccupying different possible positions around themetal ions as well as on the sulphate ions. In thismanner, the hydrated CUS04 entities in the latticeare dissimilar and show different molecular struc-tures. Thus there is no repetitive molecular or ionicpattern throughout the solid. Rearrangement ofthese ions to form a crystalline material is renderedimpossible because they do not have a commonfeature in their structure even though they mighthave a similar composition. High water vapourpressure and heat would increase their mobilityand the probability of rearrangement of the bondsso that the different entities are now able to havea more stable common structure to turn into andthus form a crystalline material.REFERENCESBADRI, M. and JAMIESON, J.W.S. (1979): Thermo-chemical measurement of the ligand field splittingenergies for hexaaquocopper(II) and hexaammine-copper(II) ions. Can. 1. Chern. 57,1926-1931.JAMIESON, J.W.S. and FROST, G.B. (1956): The heatsof crystallization of vacuum-dehydrated MgS04 .7H2 0 and Co02 .6H2 0. Can. J. Chern. 34,583.JAMIESON, J.W.S., LA MONTAGNE, R.A., PATTERN,B.S. and BROWN, G.R. (1965): High~nergymodifi-cation of the hydrates of zinc sulphate. Can. 1.Chern. 43,3129-3132.HIGH-ENERGY MODIFICATION HYDRATES OF COPPER(II) SULPHATETOPLEY, B. and SMITH, M.L. (1953): Kinetics of salt-hydrate dissociations: MnC20 4.2H20 = MnC204+ 2H2 0. J Chern. Soc. 321.VOLMER, M. and SEYDEL, G. (1937): The velocity ofdehydration of manganese oxalate dihydrate. Zphysik Chern. A179, 153-71.LANGE, E., M ONHEIM, J. and ROBINSON, A.L. (1933):Heats of dilution of aqueous solutions of zinc,cadmium and copper sulphate and sulfuric acid at25°C. J Arner. Chern. Soc. 55,4732.JAMIESON, J.W.S., BROWN, G.R., GRUENER, D.W.,PEl LUCK. R. and LA MONTAGNE, R.A. (1965):233Possible calorimetric method for measurement ofthe ligand field splitting energies for the hexaaquocations of some transition elements. Can. J. Chern.43,2148-2156.DONNAN, EG. and HOPE, G.D. (1909): Calorimetricf\nalysis of hydrated salts. Trans. Faraday Soc.5,244-50.FROST, G.B., MOON, K.A. and TOMPKINS, E.H. (1951):Amorphous intermediate products in the dehydra-tion of certain hydrated salts. Can. J Chern. 29,604-32.(Received 4 August 1982)

High-energy Modification Hydrates of Copper(II) Sulphate: heats of solution and infrared spectrum

http://psasir.upm.edu.my/2152/1/High-energy_Modification_Hydrates_of_Copper%28II%29_Sulphate_heats_of.pdf

High-energy Modification Hydrates of Copper(II) Sulphate: heats of solution and infrared spectrum

Abstract

Similar works

Full text

Available Versions

Universiti Putra Malaysia Institutional Repository