Thermal decomposition of some simple nitro complexes of cobalt(III), nickel(II) and mercury(II)

J. inorg,nucI.Chem.,1972,Vol. 34, pp. 2171-2178. PergamonPress. PrintcdinGreatBritain

THERMAL DECOMPOSITION OF SOME SIMPLE NITRO COMPLEXES OF COBALT(III), NICKEL(II) A N D MERCURY(II) M. B. D A V I E S and J. W. L E T H B R I D G E Science Department, Stockport College of Technology, Stockport, SKI 3UQ (Received 9 August 1971) Abstract-I.R. spectroscopy, X-ray powder diffraction and atomic absorption spectrophotometry were used to examine the products of the thermal decomposition of sodium and potassium hexanitrocobaltate(III), potassium and potassium lead hexanitronickelate(II) and potassium tetranitromercurate (II) nitrate. A n attempt is made to reconcile the differences between various published reports for the thermal decomposition of hexanitrocobaltate(III) by examining thermogravimetric curves and product analyses for various weights and particle sizes of materials. INTRODUCTION

ThE HEXANITROCOBALTATE(III) compounds are extremely useful in analytical chemistry. It would seem important, therefore, that the products of the thermal decomposition of these materials should be known unambiguously. Duval et al. [ l a, b] and also Wendlandt and Southern [2] have examined compounds containing the hexanitrocobaltate(III) ion using thermogravimetric techniques and there is some disagreement between these groups about the exact nature of the products. This paper is an attempt to reconcile the difference between the two sets of results and also presents, for comparison purposes, thermal decomposition studies of some other complex nitro compounds of nickel(II) and mercury(II). EXPERIMENTAL Thermogravimetric experiments were carded out either under static conditions in air or under dynamic conditions in nitrogen on a Stanton TR-01 thermobalance. Some samples of sodium hexanitrocobaltate(III) and potassium hexanitronickelate(II) were examined on a Stanton TR-1 thermobalance. Heating rates were 5°C per min., 4°C per min. or 1.4°C per min. Sample temperatures were assumed to be those measured by a P t - P t 13% Rh thermocouple having its hot junction placed in the crucible support so that is just touched the bottom of the crucible or a chromel-alumel thermocouple in the thermobalance situated just above the crucible. Porcelain crucibles were used throughout except for one sample of KaCo(NO,)n where a platinum crucible was used. These were always the same shape and blank runs established that there was no significant change in weight of the crucible over the working temperature range. Sodium hexanitrocobaltate(III) was used both as B.D.H. "AnalaR" and also prepared by the method described by Brauer [3]. Cobalt in this compound was determined by precipitation with sodium hydroxide and titration of the iodine liberated with added potassium iodide [4]. Nitro group was 1. (a). C. Duval, Inorganic Thermogravimetric Analysis, 2nd Edn., p. 343. Elsevier, Amsterdam (1963). (b). Ibid. 1st Edn., p. 211 (1953). 2. W.W. Wendlandt and T. M. Southern, J. therm. Anal. 2, 87 (1970). 3. G. Brauer, Handbook o f Preparative Inorganic Chemistry, Vol. II, 2nd Edn., p. 1541. Academic Press, New York (1965). 4. W . G . Palmer, Experimental Inorganic Chemistry, p. 541. Cambridge University Press (1954). 2171

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determined with ceric sulphate[5]. Calculated: Co, 14.61%; NO2, 68.3%; experimental: Co, 15.05%;

NO2, 68.0%. Potassium hexanitrocobaltate(III) was used as B.D.H. reagent grade material. Potassium and potassium lead hexanitronickelate(II) were prepared by the method of Elliott, Hathaway and Slade [6]. Nickel and lead were determined by atomic absorption spectrophotometry. Theory for K4Ni(NOz)eH20, Ni, 11-6%; experimental: Ni, 11.1%. Theory for KzPbNi(NO~)o, Pb,

33.4%: Ni, 9.38%; experimental: Pb, 33.3%; Ni, 9.50%. Potassium tetranitromercurate(II) nitrate was prepared by the method of Hall and Holland [7]. Mercury was determined using e.d.t.a, and nitro groups using a colorimetric method[8]. Theory

Hg, 35.47%; NO2, 32.63%; experimental: Hg, 35.60%; NOz, 32.30%. Solid products were examined using a Perkin-Elmer 137 Infracord spectrophotometer as potassium bromide discs. Nickel, lead and cobalt were determined in the product mixtures using a UNICAM SP 90 atomic absorption spectrophotometer after dissolution of the solids in acid. Calibration of the instrument was accomplished using standard solutions, suitably diluted, supplied by B.D.H. Heavy metal residues were examined by X-ray powder ditfraction, after extraction of water soluble products and drying at 110°C. Copper K~ radiation (Raymax 60) was used except for the cobalt containing samples for which nickel K~ radiation was used. RESULTS AND DISCUSSION

Sodium hexanitrocobaltat e( I l l ) This compound has been studied by Wendlandt and Southern [2] using thermogravimetric analysis, differential thermal analysis and analysis of the gaseous products using mass spectrometric analysis. They proposed that the first stage of the decomposition was:

3NaaCo(NO~)0(s) ~ 9NaNO2(s) + Co~O4 + 5NO2(g) + 4NO(g).

The same compound had previously been studied thermogravimetrically by Duval et al. [la, b]. They concluded that the solid products of the decomposition of compounds of this type are cobalt(II) oxide and sodium nitrate or sodium nitrite. The thermal decomposition weight-losses as percentages and the temperature ranges of the reactions are shown in Table 1. The i.r. spectra of the products are shown in Fig. t. Comparison of these spectra with published spectra[9] and prepared nitrite-nitrate mixtures shows that the products commonly contained a mixture of both nitrate and nitrite. Examination of the results shown in Table 1 and of the spectra in Fig. 1 for the first stage of decomposition clearly shows that as the percentage weight-loss decreased the proportion of nitrate in the product increased. For the highest weight used the i.r. spectrum indicates nitrate to be the only solid nitrogen containing product of the first stage of the decomposition. Table 1 also shows that a similar decrease in weight-loss and increase in the proportion of nitrate occurred with increasing particle size for a fixed sample weight. 5. 6. 7. 8. 9.

W.G. Palmer, Ibid., p. 540. H. Elliott, B. J. Hathaway and R. C. Slade, lnorg. Chem. 5, 669 (1966). D. Hall and R. V. Holland, Proc. Chem. Soc. 204 (1963). N. M. Hughes, Ph.D. Thesis, University of Wales, 1964. K. Nakamoto, Infrared Spectra of Inorganic and Co-ordination Compounds, 2nd Edn., pp. 88, 91. Wiley Interscience, New York (I 970).

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Table 1. Thermal decomposition data for sodium and potassium hexanitrocobaltate(III)

Substance

Sodium hexanitrocobaltate(III)

Potassium hexanitrocobaltate(III)

Initial wt. (g)

Particle size (mm)

0.0984T

Random

0.0979 0.1948 0.3982 0.4277 0.0981 0.0981 0.0981 0.4290* 0.1055,

Random Random Random Random 0.104 0.178-0.152 1.00-0.84 Random 0.355-0.178

0.4761 0.0855

0.355-0.178 0.355-0.178

Reaction temp. range (°C)

Weight-loss Postulated reaction type

Observed (%)

A C A A B B A B B B A C B A

23.0 21.4 20.5 19.1 18"7 22.5 20.1 19.3 19.3' 20.2 17.8 19.6

165-210 560-950 169-214 169-214 165-219 160-219 177-224 177-205 173-210 170-224 210-305 585-980 204.-292 216-276

Calculated (%)

23.0 23.0 23.0 17.05 17.05 23"0 17.05 17.05 17.07 20.7 15.1 20.7

Key: *Under nitrogen. tHeating rate 1-4° per min. SHeating rate 4 ° per min. A = 3M3Co(NO2)e --* CoaO4 + 4MNOa + 5MNO2 + NO2 + 8NO. B = 3MaCo(NO~)6 --~ CoaO4 + 9MNOa + 5NO + 2N2. C = 2MNO3 -~ 2MNO2 + O~ and 2MNO2 --* M20 + NO + NO2.

The increase in the proportion of nitrate at the expense of nitrite in the reaction products could occur in one or more of the following ways: (a) by aerial oxidation, Co) by reduction of the cobalt oxide species, or (c) by the reaction with the oxides of nitrogen which are evolved in the reaction. The results in Table 1 show that there was little significant change in weightloss between identical samples heated in air and in nitrogen. Examination of the i.r. spectra of the products similarly showed no decrease in the relative intensity of the nitrate band when the sample was heated in an atmosphere of nitrogen. Aerial oxidation therefore appears to be unlikely. Duval et al. [lb] assumed that the formation of nitrate in the decomposition was at least in part due to oxidation of nitrite by cobalt(III) leading to the postulation of cobaltous oxide in the reaction products. We have examined the water insoluble products of the decomposition by X-ray powder diffraction and found that the cobalt product is invariably CoaO4 regardless of the sample or particle size or of the atmosphere used. Co304 is the thermodynamically favoured oxide of cobalt in the temperature range 265°C-850°C [ 10]. It follows from the overall stoichiometry of the decomposition that the oxidation of nitrite to nitrate by Co(III) can only account for a maximum of 1 mole of nitrate from 2 moles of complex (and only 1 mole from 6 moles of complex 10. H. H. Willard and D. Hall, J. Am. chem. Soc. 44, 2219 (1922).

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M . B . D A V I E S and J. W. L E T H B R I D G E

2000

I 1500

I I000

I 900

I 800

I

700

cm'l

Fig. 1. I.R. spectra of products of thermal decomposition of sodium hexanitrocobaltate (III) for various weights of starting material. Key: Initial wt. A = 0.0979 g; B = 0-1948 g; C = 0.3982 g; D = 0.4277 g.

w h e r e C 0 3 0 4 is

the cobalt product):

2Na3CofNO2), --->NaNO3 + 5NaNO2 + 2CoO + 3NO + 3NOu. We therefore conclude that the high proportions of nitrate observed in our products must arise from oxidation of nitrite by the oxides of nitrogen produced in the primary decomposition. The dependence of the nitrite/nitrate ratio on the sample weight and particle size can be explained on this basis since increase in either or both of these factors would increase the time for which the escaping gases were in intimate contact with the solid thus allowing more extensive oxidation. On this basis, although we have been unable to reproduce conditions where nitrite is the only nitrogen containing product the formation of nitrate as reported by Duval et al. [ 1b] and of nitrite as reported by Wendlandt and Southern [2] can be accounted for by physical differences in the samples used. With regard to the final gaseous products, it again follows from the overall stoichiometry of the decomposition that the formation of nitrate must result in a corresponding decrease in the proportion of nitrogen(IV) oxide relative to nitrogen(II) oxide: NaNO2 + NO~ --, NaNOa + NO.

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Freeman [ 11] suggested a similar situation in the oxidation of sodium nitrite. If 1 mole of complex gives 3 moles of sodium nitrate the stoichiometry demands that some nitrogen (or less likely N20) would be formed. The gas analyses reported by Wendlandt and Southern are consistent with the required stoichiometry. Mass spectrometric analysis (M.S.A.) data revealed the presence of nitrogen and nitrogen(II) oxide as the major species. This would be consistent with the formation of the nitrate product found by Duval and by ourselves. Classical gas analyses by Wendlandt and Southern of other samples showed nitrogen(IV) oxide and nitrogen(II) oxide as main products in proportions agreeing with those expected for the decomposition of these particular samples to give only nitrite. The formation of NO2 as implied by Duval et al. is not however consistent with the formation of only nitrate. At temperatures above 280°C our thermograms are the same as that obtained by Wendlandt and Southern and agree with the decomposition of sodium nitrate and sodium nitrite, previously explained by Freeman [ 11]. Potassium hexanitrocobaltate( l l l ) The pattern for the decomposition of this compound is as expected very similar to that of the sodium salt. The decomposition begins at a somewhat higher temperature than that of the sodium salt. When the first decomposition is complete the products are molten. Examination of the i.r. spectra of the products showed once more that the ratio of nitrate:nitrite increases as the initial bulk of the starting material increases. However, the effect is not as marked. At temperatures above 210 ° the decomposition pattern is that of a mixture of potassium nitrite and nitrate. Potassium hexanitronickelate( l I ) This compound was prepared as the monohydrate [6]. The first stage of the decomposition was, as expected, simple loss of a molecule of water. This is shown by the excellent correspondence between the theoretical and experimental weight-losses shown in Table 2. The second weight-loss for this compound is also shown in Table 2. The i.r. spectrum of the products after this stage showed the presence of nitrite and a smaller proportion of nitrate, as in the case of the cobalt(II) compourml. Unlike cobalt, nickel is not able to use two oxidation states readily. Thus any mechanism for this decomposition involving oxidation of nickel oxide is unlikely. The X-ray diffraction pattern of the material remaining after the reaction products were extracted with water is unambiguously that of nickel(II) oxide. Evidently a similar mechanism is operating for this compound as for the cobalt(III) compound. The simplest stoichiometry for the reaction is:

K4Ni(NO2)6 -* 3KNO2 + KNO3 + NiO + 2NO. In this case, however, the initial quantity of material present did not affect the nature of the products. 11. E. S, Freeman, J. phys. Chem. 60, 1487 (1956).

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M.B. DAVIES and J. W. LETHBRIDGE Table 2. Thermaldecompositiondata for nickel(II)and mercury(II)nitro compounds Reaction s t a g e temp. (°C)

Substance Potassium hexanitronickelate(II)

Potassium lead hexanitronickelate(II)

Wt.-loss (%) O b s e r v e d Calculated

105-125 210-264 605-980 96-128" 216-268'

anhydrous salt KNO3,KNO~,NiO I(20, NiO anhydroussalt KNOs,KNO~,NiO

3.16 11.9 3.6 12.1

3.5 11.8 3.5 11.8

225-322

KNOs,nickel plumbite (NiO + PbO) I(20, nickel plumbite (NiO+ PbO) KNOa,nickel plumbite (NiO + PbO)

19.3

19-3

58,4--900

212-282" Potassium tetranitromercurate(II) nitrate

Postulated solid products

190-255 330-470 560-685 180-250"

KNOa, KNO~ (some HgO) 2HgO ---, 2Hg + O, 1(20 KNO3,KNO~

19-0 44-0 4.4 47-7

19-3 51"9 (no HgO) 51-9

*Under nitrogen. Under an atmosphere of nitrogen the reaction was similar and again the product contained nitrite and nitrate in a similar ratio. The thermogravimetric curves above 225°C are consistent with the decomposition of potassium nitrite and nitrate.

Potassium lead hexanitronickelate( I I ) This compound decomposed at 225°C to give a weight-loss as shown in Table 2. The i.r. spectrum of the products after this loss in weight in air or in nitrogen has a very strong broad peak at 1380 cm -1 and a weaker, sharper peak at 822 cm -1. This is clearly the spectrum of nitrate ion [9]. The percentages of lead and nickel in the products were determined using atomic absorption spectrophotometry. The formation of nitrate as the only nitrogen containing solid product and the elemental analyses suggest the following simple stoichiometric equation KzPbNi(NOz)6 ~ 2KNO3 + PbO + NiO + 4NO. The products of which require Pb, 41.2%; Ni, 11-8%; found: Pb, 42.0%; Ni, 10.9%. The lines observed in the X-ray powder diffraction of the product obtained after removal of water soluble material are listed in Table 3. This pattern does not correspond to that obtained for authentic samples of NiO, PbO, Pb304 (B.D.H. reagent grade) nor to that for any lead or nickel oxide listed in the A.S.T.M. Powder Index. It appears therefore that the product of this decom-

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Table 3. Spacings, d and intensities, I for the X-ray powder diffa'action pattern of waterinsoluble products of decomposition of K2PbNi(NO~)e Spacings, d (pm)

Intensities relative to the strongest line given 100

466 37O 332 266 225 215 207 188 170 164 157

13 47 69 100 30 42 32 21 30 13 12

position represents a new mixed oxide which may possibly be "nickel plumbite". An attempt was made to prepare such a compound using a method originally given by Berzelius[12] and described by Mellor[13]. However the X-ray powder diffraction pattern of this product showed it to be tetragonal lead(II) oxide. Potassium tetranitromercurate( I I ) nitrate The i.r. spectrum of the products after the first thermal decomposition reaction of this compound is evidently that of a mixture of potassium nitrate and nitrite. The product is a white solid with very small amounts of a red solid also present. The melting point of potassium tetranitromercurate(II) nitrate is 168°C. Thus decomposition occurs above the m.p. The thermogravimetric curve for the reaction under nitrogen is similar in appearance to that in air. Using the i.r. spectra data and the weight-loss, we may conclude that the reaction under nitrogen is K3Hg(NO2)4NO~ ~ 2KNO2 + KNO3 + Hg + 2NO2. The mass 10ss in air is much less than the theoretical value for this equation. A reasonable explanation for this would seem to be that some of the mercury is being oxidised in air and is involatile at the temperature of the reaction. This is supported by the presence of a further small loss in weight between 330°C and 470°C amounting to 4%, probably due to the decomposition of mercury(II) oxide. The second decomposition corresponds to the decomposition of a mixture of potassium nitrite and potassium nitrate. CONCLUSION

Accepting Wendlandt's report that under certain conditions sodium hexanitrocobaltate(III) decomposes to give nitrite as the only nitrogen containing 12. J.J. Berzelius, Schweigger'sJourna132, 156 (1821). 13. J. W. Mellor, A Comprehensive Treatise on Inorganic and Theoretical Chemistry, Vol. VII, p. 669. Longmans, London (1961).

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M.B. DAVIES and J. W. LETHBRIDGE

solid product, we conclude from our work that changes in sample particle size and overall sample size can result in a change in reaction stoichiometry with the formation also, or even exclusively of nitrate. Nitrate is also a product of other heavy metal nitro complex salts. We consider that nitrate is formed through the oxidation of nitrite by oxides of nitrogen whilst still intimately associated with the decomposing solid. It is possible that the heavy metal ions are involved in this oxidation. Leaving aside the mercury complex where the low m.p. of the complex and metal volatility introduce additional factors, we find that the lead complex is wholly oxidised to nitrate, in the cobalt complex oxidation to nitrate could be partial or complete, but in the nickel complex only partial oxidation to nitrate was observed. The order of ease of nitrate formation for the complexes containing these metals therefore appears to be Pb > Co > Ni which is also the order of ease of formation of uncomplexed states Pb(IV), Co(III) and Ni(III). We conclude therefore, that the oxidation of nitrite to nitrate by oxides of nitrogen is catalysed by the heavy metal ion present in the matrix and is favoured where the metal ion can most readily be oxidised to a higher oxidation state. Acknowledgements-We thank Dr. D. DoUimore for use of Stanton thermobalances, Prof. H. Lipson for camera facilitiesfor X-raypowder data on cobalt oxideand S. M. Middletonfor technical assistance. This work was started in the Universityof the West Indies,and one of us (M.B.D.)wishes to thankthe LeverhulmeTrust for a Fellowshipand Prof.G. C. Lalorfor use ofa thermobalance.