Indian Journal of ChemistryVol. 21A, May 1982, pp. 456-459

Reaction of Cobalt(II), Nickel(II) & Copper(II) Sulphates In SodiumNitrate-Potassium Nitrate Eutectic Melt

R. P. RASTOGI·, N. B. SINGH & S. P. PANDEYDepartment of Chemistry, University of Gorakhpur, Gorakhpur 273 001

Received 2 November 1981; revised and accepted 19 December 1981

TG, DTG and DTA studies have revealed that anhydrous Co(Il), Ni(Il) and Cu(Il) sulphates decompose inNaNOa-KN03 eutectic melt at a temperature much lower than the decomposition temperatures of the correspondingsulphates in air. Powder X-ray dHJraction studies confirm the presence of mixture of Co"O., Na.SO., K2SO.; NiO,Na2SO., K2SO.; and CuO, Na!SO., K.SO. in the reaction systems. The presence of NO. and O. has been detected.It has been suggested that the reactions proceed according to the equations.2MSO.(s) + 2NaN03(l) + 2KNOil) -+ 2M(N03)a(l) + NatSO.(s) + K.SO.(s)2M(N03)'(/) -+ 2MO(s) + 4NO.(g) + O.(g) (M = Cu, Ni)2COSOls) + 2NaNO.(l) + 2KN03(1) ....•.2Co(N03)2(l) + Na.SO.(s) + K,SO.(s)3Co(N03M/)-+ C030.(s) + 6NOb) + O.(g)The reactions in eutectic melts are essentially solid-liquid reactions. Kinetics of decomposition of metal sulphateshas been investigated by isothermal TG at ditIerent temperatures. It is found that Jander's equation,{1- (1- ex)l/'}" = kt (1)fits the data where ex is the fraction decomposed, t is the time and k is constant.

RECENTL Y Singh and Rastogi- studied theformation of maleamic acids in a eutecticmelt and found that the acids were obtainedat a lower temperature and in a better yield. A seriesof reactions=P in nitrate and nitrite eutectic meltsindicate that reactions generally occur at lowertemperatures and interesting reaction products areobtained. However such studies are limited in thelitera ture and hence more research work is neededin order to assess the technical and fundamentalpotentiality of reactions in eutectic melts.

The present paper reports the results of studies onthe reactions of cobalt(II), nickel(II) and copper(II)sulphates separately in NaNOa-KNOa eutectic melt.

Materials and MethodsAnalar (BDH) grade NaNOs, KNOa, CuS04.5H20,

NiS04.7H20 and CoS04.7HzO were used as such.

Preparation of eutectic melt and of anhydrousmetal sulphates-Dried samples of NaNOs andKN03 were mixed in 45 : 55 ratio (wt %) in acleaned test tube and the eutectic mixture obtainedby the method as described earlier-". The meltingpoint of the eutectic was found to be 226°C.

The preparation of anhydrous sulphates involvedheating CuS04.5H20, NiS04.7H20 and CoS04.7H20respectively at 250° >300° and >250°C, to a cons-tant weight. The samples were stored in a desi-ccator.

The thermal behaviour of eutectic melt, metalsulphates, mixtures of metal sulphates and eutecticand mixtures of metal sulphates with sodium andpotassium nitrates was investigated employing ther-mogravimetry (TG), differential thermal analysis

456

(DTA) and derivatographic thermogravimetry(DTG). TG studies were carried out with a manualthermogravimetric analyzer in air in a gold cruciblewith a heating rate of 4°C/min. The minimum massloss could be recorded upto 0.0001 g and the uncer-tainty in temperature was ± lOoC. Each experimentwas repeated three times and the reproducibility ofthe results was within ± 1%.

DTA, DTG and TG studies of the sulphates ineutectic melt were simultaneously carried out with arecording thermal analyzer (Panlik-Paulik-ErdeyMOM derivatograph, Hungary). The results ofmanual and recording TG studies were in goodagreement.

Powder X-ray diffraction studies - The powderX-ray diffraction patterns of the residues left in TGexperiments were obtained with a X-ray diffracto-graph (XRD-5 General Electric, USA) using CuK a.radiation.

Gravimetric estimations - Copper, nickel and co-balt in the reaction products were estimated gravi-metrically by standard methods-s.

Qualitative analysis of the evolved gases-N02 andO2 evolved during the reaction were tested qualita-tively in the usual way.

Isothermal decomposition studies - Isothermal TGexperiment was performed with a manual thermo-gravimetric analyser in air. Corning glass cruciblewas used during the experiment. It was hanged intothe electric furnace with the help of platinum wire.The temperature was recorded with a pyrometer(Toshniwal). The sample could be maintained atconstant temperature with the help of a dimmerstat(A.E.D. Co., Bombay). The mass loss was notedwith an electrical balance ('VTU' Balance model No.

RASTOGI et a/. : DECOMPOSITION OF SULPHATES IN A EUTECTIC MELT

V-3). Known weights of the eutectic and the anhy-drous metal sulphates were taken in the crucible andmixed thoroughly. The crucible was then hanged inthe furnace. The kinetics of decom position of thereaction mixture was followed by noting the massloss at different intervals of time at constanttemperature (± 5°C). Each experiment wasrepeated three times at different temperatures.The mass loss could be recorded correct to 0.0001 gwhereas the temperature variation was less than± 5°C.Results and Discussion

TG studies (Fig. 1) indicate that anhydrouscobalt(Il), nickel(II) and copper(I1) sulphates decom-pose at a much lower temperature when heated withNaN03, KNOs or NaN03-KNOs eutectic. The de-composition temperatures are recorded in Table 1.Sodium and potassium nitrates and the eutectic donot decompose upto 600°C. DTA curves for eutecticalone and the mixtures of metal sulphates in eutectic,given in Fig. 2, indicate that upto 600°C the eutecticis quite stable and after that there is a continuousmass loss. This is due to decomposition of eutectic

0

20

40

60

0 ..

~20

~400....•c/.) 60I/)c(

~0

20

40

C60

80

1000 • 100 200 300 400 500 600TEMPERATURE (Oe)

Fig. 1 - TG curves for the decomposition of different systems.(A(l) CoSO, (0.0315 g) + KNO. (0.0610 g); (2) CoSO,(0.0295 g) + NaNOs (0.0675 g); and (3) CoSO. (0.0218 g) +NaNOs/KNO. (0.0650 g). B (1) NiSO. (0.0204 g) + KNO.(0.0504 g); (2) NiSO. (0.0268 g) + NaNOs (0.0554 g)' and(3). NiSO. (0.0235 g) + NaNO./KNO. (0.0656 g). C (I)CUS04 (0.0337g) + KN03 (0.0858 g); (2) CUSO, (0.0490 g)+ NaNO. (0.1350 g) ; and (3) CUSO. (0,0313 g) + NaNO.l

KNO. (0.0615 g)].

TABLE 1 - DECOMPOSITION TEMPERATURES OF Ptraa METALSULPHATF3, EUTECTIC MELT AND MIXTURES IN EUTECTIC MELl

System Decompositiontemp.CC)

System Decompositiontemp.CC)

CUSO, 750 CoSO,-NaNO. 320CuSO,-KNO. 340 CoSO,-NaNOs/KN03CuSO,-NaN03 320 eutectic 280CuSO,-NaNO.lKNO. NiSO, 764

eutectic 300 NiSO,-K..NOa 420CoSO, 735 NiSO,-NaN03 400CoSO.-KNO. 340 NiSO,-NaN03/KNO.

eutectic 340

and sublimation of the decomposition products. Thefirst endotherm in DTA around lOO°C is due tophase transformation of KNOs and the other endo-therm around 220°C is due to the melting of theeutectic. Two more endotherms in DTA (Fig. 2)below 600° C are in the region where the decompo-sition reactions take place. This indicates that thereare two stages of decomposition,although this is notindicated by TG. It may be inferred that the two de-composition stages are taking place simultaneouslyand hence could not be detected by TG.

Powder X-ray diffraction studies indicate that in°CuSO«-eutectic system, CuO (2.52, 2.32, 2.53 A),

Na2SO« (2.79 A), K2SOl2.90, 3.00, 2.89 A); inCoSO«-eutectic system, COa04 (2.44,2.84 A), Na2SO«(2.80,4.67 A), K2S04 (2.92,2.88 A); and in NiS04-eutectic systems, NiO (2.08, 2.41 A), Na2SO«(2.78,2.63 A), K2SO« (2.91, 3.0l, 2.87 A) are formed as

1

200 400 600 eoo 1000TEMPERATURE to,)

Flg. 2 - DTA curves for (A) NaNOs/KN03 eutectic (wt ofeutectic 600 mg); (B) NaN03/KN03-CoSO. system (eutectic420 mg; CoSO, 200 mg); (C) NaN03/KN03-NiS04 system(eutectic 450 mg; NiSO. 230. mg); and (0) NaN02fKN03-CuSO, system (eutectic 440 mg; CuSO, 180 mg); [Heating

, rate 5°C/nUn].

457

INDIAN J. CHEM •• VOL. 21A, MAY 1982

a result of reaction. The observed d-values for theabove compounds are in good agreement with thereported values. In addition to these the presence ofunreacted NaNOa and KNOa was also detected inXsray.

In order to verify whether oxides are produced,the residues left after TG studies were washed severaltimes with water in order to remove the metal sul-phates and unreacted nitrates. Percentage of metalin oxides produced in CoS04·NaNOa/KNOa• NiS04-NaNOa/KNOa and CuS04-NaNOa/KNOa werefound to be 71.7 (calc. for Co : 73.4), 75.7 (calc. forNi: 78.6),82.0 (calc. for Cu : 79.9). The agreementbetween observed and the calculated values is satis-factory confirming the formation of oxides. In orderto provide further confirmation,per cent mass loss inthe reaction system was determined by TG (bothrecording and manual) which was found to be65.0 (manual 65.6 ± 0.6), 69.6 (manual 70.8 ± 0.4)and 66.7 (manual 68.0 ± 0.5) for the above threesystems. The calculated values for per cent massloss, assuming that oxides are formed are 64.5, 69.1and 67.2 respectively. These results confirm the stoi-chiometry of the reactions. The gases evolvedduring the reactions were detected as N02 and O2,

From the analytical results it follows that the stoi-chiometry of the reactions are given by Eq. (1) (forCu and Ni) and Eq. (2) for Co.

2MS04(s) + 2NaNOs(l)+2KN03(l) = 2MO(s)+Na2SOls) + K2SOls) + 4N02(g) + 02(g) .. (1)6CoSOls)+6NaN03(l) +6KNOs(l) = 2CosOls)+3Na2S04(s) + 3K2SO.(s) + l2N02(g)+202(g) .. (2)

The manner in which these reactions occur needsexplanation. Metal sulphates and alkali nitrates donot yield oxides and N02 respectively at lower tern-peratures (..... 300"C). Hence the reaction must betaking place in steps. It appears that exchangereaction between SO;- and NO; ions takes place atthe surface of metal sulphates. After the exchange,metal nitrate can easily decompose into oxide,N02 and O2, Accordingly, the mechanism of thereaction can be postulated as shown in Scheme 1where M2+behaves as a Lux-Flood acid and NO;acts as a lux-flood base.

2MS04 + 4NO; -+ 2M2+ + 4NO; + 2S0:-2M2+ + 4NO; -+ 2MO + 4N02 + O22CoS04 + 4NOJ: ~ 2C02+ + 4NO; + 2S0!-3C02++ 6NO; -+ C0304 + 6N02 + O2

Scheme 1

There are two possibilities for the above reactionsto occur. The metal sulphates may dissolve in theeutectic melt and a homogeneous reaction may takeplace or there may be a heterogeneous solid-liquidreaction. However visual observation indicates thatmetal sulphates are practically insoluble in the eutec-tic melt at the temperatures under investigation. Sothe first possibility is ruled out. It should be notedthat the above processes are solid-liquid reactionsand in the reaction zone, influx of NO;, outflux ofN02 and O2 and formation of porous oxide crust

458

0.6

c

360 "C

40 80 120 160TIME lMUIUTESI

Fig. 3 - Test of Jander's equation for the decomposition of(A) COSO,; (B) NiSO,; and (C) CuSO, in NaNOJKNOa

eutectic melt.

Fig. 4- Arrhenius plots for (8) NaNO./K.N03-CuSO,;«!» NaNO./KN03-NiSO,; and (e) NaN03/KNO.-CoSO,.

would simultaneously be occurring since the reactiongoes to completion.

In order to study the kinetics of the reaction, theratio of mass loss at time ( t ) and total mass loss wasdetermined as a function of time from isothermalTGA. The mass loss of the reaction mixture wouldbe due to liberation of N02 and O2, On the basis ofthe stoichiometric equations it is easy to show that IXwould be the ratio of mass of metal sulphatesdecom-posed at any time ( t) and the total mass of metalsulphates.

The kinetic data for isothermal decompositionfollow Jander's equation"

{I- (I - 1l)1/3)3 = kt.where k is the apparent rate constant which dependson the radii of the particles and diffusion coefficient

RASTOGI et al. : DECOMPOsmON OF SULPHATES INA EUTECTIC MELT

where the subscripts refer to the reaction involvingcorresponding sulphate.TABLE 2 - APPARENT RATE CoNSTANTS AND ENERGY OF

ACTIVATION FOR DECOMPOSmON OF METAL SULPHATES INEUTECTIC MELT

Temp. Wt of Wt of k x loa(0C) eutectic metal sulphates (mirr? )

(g) (g)

CoSO.-NaN03/KNOJ320 0.1923 0.1030 0.62360 0.1652 0.0995 2.5400 0.1035 0.0520 38.0

NiSO.-NaN03/KNO·.400 0.2054 0.102& 1.7420 0.6072 0.3145 4.&440 0.1920 0.0525 29.7

CuSO.-NaN03/KNO·,360 0.2050 0.0&55 1.2400 0.0640 0.0295 11.1440 0.1635 0.0660 35.2

Activation energies : 40.0, 71.0 and 40.0 kcal/mol forCoSO,. NiSO. and CUSO. for decomposition respectively.

of species involved in migration. When {l-(l-cx)-ct1/3}2is plotted against t linear plots are obtained (Fig. 3).The apparent rate constant was calculated from theslope of the linear plots by least square method.The values are given in Table 2. Logarithm of k hasbeen plotted against reciprocal of absolute tempera-ture in Fig. 4 from which the values of energy ofactivation were estimated. The values are given inTable 2. Higher values of energy of activationshow that bulk diffusion of the ion is involved inthe process. The energy of activation for the re-actions are found to be in the following order :EeoBo. = EeoBo4 < ENt804

AcknowledgementOne of us (S.P.P.) is thankful to the CSIR, New

Delhi for financial support. The authors are thankfulto the lIT, Kanpur for providing necessary facilitiesfor X-ray diffraction and simultaneous TG, DTGand DTA studies.

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