Preparation and thermal decomposition of Cu(NH3)4CrO4

I imp

nucf. Chem. 1971.Vol 31. pp 1367-1170. PergamonPres\

Printed in Great Britan

PREPARATION AND THERMAL DECOMPOSITION OF Cu(NH3)4Cr0, C. STUART KELLEY,* Chemical Laboratory, Edgewood

LOUIS L. PYTLEWSKIt Arsenal. Aberdeen Proving

(First receined 26 July 1974; in revisedform

and LESLIEENG Ground. MD 21010, U.S.A.

3 September

1974)

Abstract-The preparation, characterization, and thermal decomposition of CU(NH,),Cr04 are reported. The decomposition of the complex is monitored by thermal analysis. Decomposition is by two steps: the first near 150°C: the second near 210°C. Each step corresponds to the release of two moles of NH, per mole of complex. The end product is identified by elemental analysis and magnetic susceptibility measurements as CuCrO, and is stable and water insoluble. Activation energies for the low temperature and high-temperature reactions are 18 kcal/mole and 39 kcal/mole, respectively. At 2Sl”C the two reaction rates are equal and the reactions proceed as though the conversion were’from &(NH&CrO, to CuCrO,.

INTRODUCTION

salts are known to dissolve in aqueous solutions of ammonia to form, principally, the solvated, deep blue, copper tetramine cation from which crystals of copper tetramine salt can be isolated with relative ease[l]. The thermal decomposition process for Cu(NH&S04 occurs in steps with the loss of one mole of ammonia per mole of complex at each step until CuS04 is recovered[2]. Thermal analyses of a substantial number of 3d transition metal ion complexes with nitrogen-containing ligands (including ammonia and ethylenediamine) have been conducted by numerous investigators[3]. In general, the ligands are evolved either singly or in pairs until the ligand-free compound is recovered. Only when strong oxidizing anions are used (such as NO,-, C104-, Cr04’-) does the thermal decomposition process deviate from those just described[4]. For these cases, the ignition temperature is reached before removal of the last molecule of nitrogen base. Motivation for the present study was twofold. The first concerns the preparation and chemical characterization of solid, pure Cu(NH&Cr04; the second concerns the thermal properties of the resulting compound. COPPER

EXPERIMENTAL

Sample preparation

It wasrecognized that, in the copper-chromate system (which requires solutions on the basic side for preservation of the CrO,‘-), tetraminecopper(I1) chromate could not be prepared by adding CuCrO, to NH,OH because there was no genuine commercally-available copper compound of that formulation. Items labeled “copper chromate” were, in fact, the basic copper salt-written variously as 2CuCuCr0,.HZ0 or [Cu(OH)],CrO,. Upon attempts at crystallization, any combination of a copper salt *Permanent address: General Research Corporation, Westgate Research Park, McLean, VA 22101 U.S.A. tpermanent address: Department of Chemistry, Drexel University, Philadelphia, PA 19104, U.S.A.

,.I N.C.. Vol 37. No 6-C

and a chromate salt in NH,OH solution always yielded a red-brown solid that was identified from spectral measurements and elemental analyses as Cu(OH)NH,CrO, Attempts to obtain CuCrOa by solution and solid state reactions using commercially available compounds were unsuccessful (this includes the direct addition of CuO to 00,). We found that pure Cu(NH&CrO, is easily recovered from an NH,OH solution of the red-brown Cu(OH)NH,CrO, which is extremely soluble in NH,OH. The resulting Cu(NH,),CrO, is a green-black crystalline solid which readily precipitates from solution. The crystal itself is then very insoluble in concentrated NROH solution but dissolves in water with subsequent hydrolytic decomposition

Procedure The Cu(NH,),Cr04 crystals prepared by the above method and used in this study were taken from a common lot stored in a vacuum desiccator. Samples, in the form of long needles, were powdered immediately before thermal analysis. Measurements were taken of the mass loss and mass loss rate vs temperature using a Fisher Thermal Gravimetric Analyzer with a Cahn Time Derivative Computer. The temperature range Fpanned ?.(-~a 500°C. The platinum sample pan, enclosed in a quartz hangdown tube, was connected to one arm of a Cahn electrobalance and could be lowered into an oven having a programmable heating rate. The output of the electrobalance and that of a thermocouple measuring the sample temperature were fed to a strip chart recorder. All thermal measurements were performed in air. The thermocouple was calibrated at 0°C and 100°C and positioned below and within 0.5 mm of the sample pan. The sample pan was capable of accepting up to 20 mg of powdered solid. Typical runs. however, involved I O-l 5 mg. Preliminary ma?\ IOS~ measurements were performed with \ariou$ heating rates between I.0 and lO”C/min. Two major mass loss steps were observed in all runs at temperatures that decreased with decreasing heating rate. This effect was moct likely due to diffurion of NH, through the bulk of the sample. At the highert heating rate (IO”C/min). the rate of diffusion of NH, from the cample lagged far enough behind the heating rate that ignition occured via oxidation of NH1 by CrO,‘-, and the ignition W;I\ violent enough to blow powder from the pan. Because there appeared to be no limiting line shape for the mass vs temperature curve at theve heating rates, the following procedure was adopted:

C.S. KELLEY,L. L. PYTLEWSKIand L. ENG

1368

the oven was set to a predetermined temperature, and, once thermal equilibriumwas established, the sample in the hangdown tube was lowered into the oven. In all cases, the sample temperature equilibratedto the oven temperature before measurable sample mass loss occurred. The resulting mass vs time curves were recorded for temperatures that spanned the two major mass loss ranges. The specific oven temperatures for the mass vs time curves were 130, 164, 171, 173, 189, 217 and 222°C. Differential thermal analyses were preformed also in the standard fashion at a heating rate of 5°C/min.

and then reduced by 14 per cent due to the loss of 2 mole of NH3 for each mole of Cu(NH3)4CrO4 via the low-temperature reaction. A plot of - I n [(mA + mB)/moMB/MA] vs t is shown in Fig. 1 for the low-temperature I

I

I

b

Results The preliminary measurements indicated that two decomposition steps occured: one in the vicinity of 150°C and the other near 210°C. A mass loss corresponding to 2.6 per cent water content in the original sample occurred in the range of 70-100°C. This initial water content was subsequently confirmed by water content determination of the original sample batch. The mass of the sample after the decomposition reactions at 150 and 210°C corresponded to 88 and 74 per cent, respectively, of the starting material. The molecular weights for Cu(NH3)2CrO4 and CuCrO4 are 86 and 72 per cent that of Cu(NH3)4CrO4. The preliminary mass loss rate vs temperature curves indicated that the two major mass loss rates were at a maximum at 150 and 210°C. Other mass loss rate maxima occurred near 120, 135 and 197°C, but these had less than 10 per cent of the intensity of the two dominant peaks.

171"C

5"0 t

Oo|I"

INTERPRETATION In the reaction A ~ B + 2 C t , we record the sample mass m,~ + ms. The mass of compound A is mA ; that of B is ms. The identity of C is gaseous NH3 for both the lowand high-temperature reactions. The identity of A is Cu(NH3),CrO4 for the low-temperature reaction and Cu(NH3)2CrO4 for the high-temperature reaction. The indentity of B is Cu(NH3)2CrO4 for the low-temperature reaction and CuCrO4 for the high-temperature reaction. We assume that the reaction A ~ B + 2C~ proceeds as mA

=

moeTM

(I)

and that, for the low-temperature process, only a small amount of the initial mass, too, is in the B form. The moles of A converted in the reaction equals the moles of B produced. Therefore, (too- mA)/MA = ms/Ms

,6,.c

0

20

I

40

I

60 to rain

I00

Fig. I. Sampleweight (see Eqn 3) vs time for the low-temperature reaction at 130, 164 and I71°C. The slopes of the curves are the values of k. reaction and in Fig. 2 for the high-temperature reaction. The data points lie on straight lines except for the induction period at short times and for long times when data resolution from the experimental curves is made difficult by the small mass changes. The slopes of the curves increases with increasing temperature and correspond to the values of k at the temperatures measured. It is assumed that k varies as

(2) k = koe-aE/RT

where MA and Ms are the gram molecular weights of compounds A and B. Inserting Eqn (2) in Eqn (1)and rearranging, ma + ms = moe-k' + (MB/MA)mo(1 - e % or

kt =ln(1 -MB/Ma)-ln[(mA +m,)/mo-MB/Ma].

I

80

(3)

In applying Eqn (3) to the low-temperature reaction, mo was corrected for water content. For the hightemperature reaction, mo was corrected for water content

(4)

where ko is the frequency factor, hE is the activation energy for the reaction, and R is the universal gas constant. Because the value of ko was not known for either the low-temperature reaction or the high temperature reaction, data were reduced via k = k, exp [ - A E ( I / T - 1 / T t ) / R ]

(5)

where the subscript 1 refers to one of the experimental measurements. Figure 3 is a plot of In k vs I/T for both the low- and high-temperature reactions. For the equa-

Preparationand thermaldecompositionof Cu(NH3)4Cr04

6'0 I -

1369

tions presented, the temperature is in degrees Kelvin, and k in rain -t. The activation energies, AE, are proportional to the slopes of the curves in Fig. 3:18.111 kcal/mole for the low-temperature reaction and 38.506 kcal/mole for the high-temperature reaction (or, to the degree of accuracy here, 18 and 39 kcal/mole, respectively). From the values of AE. the values of ko were calculated by Eqn (4). The reactions are described by

i

!22"C

4'0!

kl-2"425×lO~exp(-9"l15×lO3/T) k , - 7-780 ~ 10~ exp( - 1.938× 10'/T)

(6)

Jc, 3.0-

where subscripts L and H refer to the low- and high-temperature reactions. Notice that k,,, > ko, and AE, > AEL. This means that + 173=C at some temperature (251°C, see Fig. 3) the mass of Cu(NH~)2CrO4 does not vary with time, and the reaction _c I proceeds as though the conversion were from I Cu(NH3hCrO4 to CuCrO4. Using the above values of kL and k,, we plotted theoretical curves of ma +mB vs t at i temperatures that correspond to one of the lowtemperature measurements and to one of the highI J5O temperature measurements. The curve for the highO' 0 1() 20 30 4() temperature reaction virtually reproduced the experiment,, mln tal curve exactly. The low-temperature reaction was also Fig. 2. Sampleweight(see Eqn 3) vs time for the high-temperature well fitted, except for the time delay associated with the reactionat 173, 189,217 and 222°C.The slopesof the curves are the low temperature induction period mentioned above. values of k. DISCUSSION

Although the thermal measurements indicate that 2 mole of NH3 are expelled in each of the two steps from 1 mole of Cu(NH3)4CrO4, the resulting solid is very sensitive to continued heating. Ignition occurs in all cases if the temperature increases above 5°C following evolution of the last mole of NH3. The ignition products are always copper chromite and molecular oxygen. If the furnace is dropped away from the sample just as the fourth molecule of NH3 is removed, a jet-black solid is recovered. This solid is insoluble in water and does not decompose under ambient conditions. Although it is amorphous to X-rays, elemental analysis and,magnetic susceptibility measurements (run in situ) indicate that the material is CuCrO,.

kH= 7" 700xlO~6ex p [-I. 938 xlO4/T] 251"C

-5

TC E . -I0

CONCLUSIONS

kH=2.425 xlOSex p [-9.115 × 103/T]

-15

-20

-

O

2 I'O

5

L 2'0

~ 3"O

4'0

5'0

T I , iO-3.C-I Fig. 3. Low- and high-temperaturereaction rates as functions of temperature. At the crossover point (251°C)the concentrationof Cu(NH3)~CrO4 is constant.

We report a method of preparation for obtaining the complex Cu(NH3)4CrO,. This method was used to obtain samples for subsequent thermal analyses that monitored the thermal decomposition. The decomposition takes place in two steps, occurring near 150°C and 210°C. Each step corresponds to the evolution of 2 mole of NH3. We identified the end product as CuCrO4 based on elemental analysis and magnetic susceptibility measurements. The CuCrO4 is stable and water insoluble. If the CuCrO4 is heated beyond the temperature at which all NH3 is evolved, ignition occurs with copper chromite and molecular oxygen as end products. The activation energies for the low-temperature and high-temperature reactions that evolve NH~ are 18 kcal/mole and 39kcal/mole, respectively. The equations governing these two reaction rates are obtained, and are such that the reaction rates are equal at 251°C. At this temperature the reactions proceed as

C. S. KELLEY, L. L. PYTLEWSKIand L. ENG

1370

though the conversion were from Cu(NH3)4CrO, to CuCrO4 because the amount of Cu(NH3):CrO4 is constant with time. REFERENCES

1. F. A. Cotton and G. Wilkinson, Advanced Inorganic Chemistry, 3rd Edn, p. 917. Interscience, New York (1972).

2. W. W. Wendlandt, J. inorg, nucl. Chem. 25, 833 (1963). 3. W. W. Wendlandt and J. P. Smith, Thermal Properties of Transition Metal Ammine Complexes. Elsevier, New York (1967). 4. W. W. Wendlandt and C. Y. Chou, J. inorg, nucl. Chem. 26, 943 (1964).