Proceeding from KHSO4, K2S2O8, NaHSO4 and Na2S2O8, potassium- and sodium-pyrosulfates were prepared and their thermal decomposition was investigated by a T.G.A. method. This was done in order to clarify contradictory values found in the literature. The presence of an intermediate with composition K2SO5 was detected

Gellings, P.J.

Vries, K.J. de

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de Vries, K.J.

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The thermal decomposition of potassium and sodium-pyrosulfate

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J, inorg, nucl. Chem., 1969, Vol. 31, pp. 1307 to 1313. Pergamon Press. Printed in Great Britain T H E  T H E R M A L  D E C O M P O S I T I O N  O F  P O T A S S I U M  A N D  S O D I U M - P Y R O S U L F A T E  K. J. DE VR1ES and P. J. G E L L I N G S  Twente, University of Technology, Enschede, The Netherlands (First Received 16August 1967" in revised form 20 September 1968) Abstract-Proceeding from KHSOa, K2SzOs, NaHSO4 and Na2S208, potassium- and sodium-pyro- sulfates were prepared and their thermal decomposition was investigated by a T.G.A. method. This was done in order to clarify contradictory values found in the literature. The presence of an inter- mediate with composition K2SO5 was detected. I N T R O D U C T I O N  IN A m~CENT literature survey we found a wide variety of data on the decomposi- tion temperatures of dry, powdered potassium- and sodium-pyrosulfates. The starting materials used by different investigators were either freshly prepared pyrosulfates or pyrosulfates obtained from the thermal decomposition of the corresponding dry, powdered hydro- and peroxydi-sulfates. Table 1 lists the values found. The thermal decomposition reactions are considered to be: 2KHSO4 --* K2S207+ HzO (1) K2S2Os --~ K2S207 + ~rO2 (2) K2S207 ---> K2SO4 + SO3 (3) NaHSO4. H20 ---> NaHSO4 + H~O (4) 2NaHSO4 ---> Na2S207 + HzO (5) Na,2S2Og ---> Na,~S207 + ~O2 (6) N a ~ S z O 7  ---> N a 2 S O  4 q-- SO3 (7)  In order to clarify the contradictory literature values we re-investigated the decomposition ranges of the reactions (3) and (7) between 20-1000°C using the corresponding hydro-, peroxydi- and pyrosulfates as starting materials. E X P E R I M E N T A L  The compounds used were analytical grade: KHSO4(E. Merck), K2S208 (E. Merck), K2SEO7 (Brocades-Stheeman), NaHSO4. H~O (E. Merck), Na2S208 (Riedel-De Hahn). The samples were powdered and dried by heating at 56°C in air for about 72 hr. This was im- mediately followed by the thermogravimetric analyses (T.G.A.) between 20 and 1000°C. These analyses were carried out in a Stanton TR-01 thermobalance in an atmosphere of air. Samples of about 100 mg of the pre-dried compounds were placed in glazed alumina crucibles pre- heated in air to 500°C for a day. No buoyancy corrections were applied. Heating rates used were 2/3 °, 4[3 ° and 4°C/min. The decomposition products were quenched by simply removing the furnace of the Stanton TR-01. Their composition was checked by analytical methods. No evidence was found that a reaction of the products with CO2 of atmospheric air had taken place. From the diffraction patterns formed in a Guinier 11 camera (Nonius) using Cu Ka radiation we obtained d values of the decomposition products. These values were corrected using AINH4 (SO4)2. 12I-t20 as a standard and were compared with the A.S.T.M. X-ray data file. 1307 1308 K.J .  DE VRIES and P. J. GELLINGS Table I. Thermal decomposition data of potassium- and sodium-pyrosulfate into the corresponding normal sulfates proceeding from the hydro-, peroxydi- and pyrosulfates respectively as found in the literature Decomposition data* for Starting Reactions reactions (3) and (7) respec- Authors Reference material involved tively Spitsyn, Ill KHSO4 (1) (3) Meerov Tomkova, Jiru, [2] KHSO4 (1) (3) Rozicky Wendflandt, [3] KHSO, (1) (3) Sturm Duval [4] KzS~Os (2) (3) Wickert [5] K2S~O~ (3) Duval [6] K~S~O7 (3) Spitsyn, [1] NaHSO4 (5) (7) Meerov Duval [7] NaHSO4 (5) (7) Tomkova, Jiru, [2] NaHSO4. H~O (4) (5) Rozicky Boros, [8] NaHSO4 (5) Lorant Duval [4] Na~S~O7 sets in between 370--4200C occurs between 395 and about 800"C? occurs between 470 and 700"C occurs between 488 and about 900°C occurs between 321 and 8000C starts progressively above 210*C and is completed around 870°C¢; heating rate used 5*C/min sets in between 320-3700C (7) occurs between 430 and 620"C; heating rates used are between 2.5 and 5°C/rain occurs between 370 and 780°C? (7) finishes at 910°C (7) occurs between 197 and 8000C *achieved by various Thermogravimetric Analyses (T.G.A.). ?the authors used steady and increasing temp. methods. ~the author's T.G.A. curve flattens near 680"C. RESULTS The Figs. 1, 2 and 3 show the T.G.A. curves, obtained at two heating rates, for potassium hydro-, peroxydi- and pyrosulfate. The Figs. 4 and 5 show the comparable T.G.A. curves for NaHSO4 and Na2S2Os respectively. The parts of the curves between Na2S2Or and Na2SO4 were not reproducible. The Tables 2 and 3 show the observed data for the conversion of potassium- and sodium-pyrosulfates into the normal sulfates. 1. V. I. Spitsyn and M. A. Meerov, J. Gen. Chem. USSR. 22 963 (1952). 2. D. Tomkova, P. Jiru andJ. Rozicky, Colin Czech. chem. Commun. 25 957 (1960). 3. W.W. Wendtlandt and E. Sturm, J. inorg, nucl. Chem. 25, 535 (I 963). 4. C. Duval,Analytica chim. ,4 cta. 20, 20 (1959). 5. K. Wickert, Brennst-Wiirme-Kraft. 11, 110 (1959). 6. C. Duval, Microchim. lchnoanaL ,4cta 348 (1963). 7. C. Dural, Inorganic Thermogravimetric Analyses. Elsevier, Amsterdam pp. 198,257,258 (1963). 8. M. Boros and B. Lorant, Se(fen-Ole-Fette-Wachse. 89, 531,555 (1963). The thermal decomposition of potassium and sodium-pyrosulfate 1309 9C 8 ¢  Q. '~, KHSO 4 q . . . . . . .  4 "C I m i n  = ° C  ' ~ '  ' , ~ o  ' ~ o  ' ~ o  ' ~x~'  ' Fig. 1. T.G.A. curves of KHSO4 between 20 and 1000°C taken at different heating rates. 9¢ 8 ¢  e~ ,., 7¢ K 2 S 2 0 8  -~- "C I m i n  3 . . . . . . .  4 "C  I m i n  L oc Fig. 2. T.G.A. curves of K~S2Oa between 20 and 1000~C taken at different heating rates. 1310 K.J .  DE VRIES and P. J. GELLINGS 90 80 y: 70 K 2 $207 °C 400 Fig. 3. T.G.A. curves of K2S~O~ between 20 and 1000*C taken at different heating rates. 10C 9C 8C O, " ' " ' ' ' " "  I~ H 504 \\  'C,m,n ~6o ' 4bo ' e~o ' ~ o  ' ~ o  ' Fig. 4. T.G.A. curves of NaHSO4 between 20 and 1000°C taken at different heating rates. The thermal decomposition of potassium and sodium-pyrosulfate 1311 100 90 ,, ' ', N a2S208 2. "Clmin 8O 3 , ........ "-~ *C Imin i I 6G ~ ' "  i a C  i i • i i i Fig. 5. TG.A.  curves of Na2S208 between 20 and 1000°C taken at different heating rates. Table 2. Thermal decomposition ranges of the various potassium pyrosulfates into normal potassium sulfate Heating rate 4/3°C/min Heating rate 4°C/min Starting Decomposition range Decomposition range material Reaction (°C) (°C) KHSO4 (3) 390--650 400-700 650-840 700-920 K2S208 (3) 400--650 400--690 650-840 690-900 K2S~O7 (3) 400-650 400-710 650-840 710-910 Table 3. Thermal decomposition ranges of sodium-pyrosulfate obtained from Nail  SO4 and N%S2Oa Heating rate H eating rate Heating rate 2/3°C/min 4/3°C/min 4°C/rain Decomposition Decomposition Decomposition Starting range range range material Reaction (°C) (°C) (°C) N aH SO4 (7) 330-820 340-850 390-900 N a~S208 (7) 330-800 340-830 370-900 1312 K . J .  DE VRIES and P. J. GELLINGS We found that in all the T.G.A. curves of the various potassium pyrosulfates there was a knee just dividing the decomposition (3) into two parts which corres- pond with the following reaction scheme: K2S2Or ~ K2SO5 + SOs (3a) KzSO5 ~ K~SO4+½02. (3b) In order to find whether the intermediate step could be connected with a separate phase the reaction was stopped at the temperature where the intermediate step appeared. The diffraction pattern of this product was compared with the diffraction patterns of the neighbouring phases K2S207 and K2SO4. Both these latter phases were detected but there were more diffraction lines present in the diffraction pattern of the intermediate and these were ascribed to belong to "KzSOs". In Table 4 we have listed the d values found from the diffraction pattern of the intermediate in the first column. The d values of KzS2Or and K~SO4 as taken Table 4 Powder pattern A.S.T.M. A.S.T.M. d K~S~O~ K2SO4 "K2SO5" (A) (A) (A) (A) 3"93 3"85 3"85 3"69 3"65 3"65 3"40 3"40 3"20 3"15 3-09 3"O7 3"07 2"12 2"12 2"11 2"10 1"76 1"76 1"72 1 "69 3"93 3"09 2"10 1 "69 3"69 3"20 3"15 2"11 1 "72 from the A.S.T.M index and coinciding with the d values found by us for these materials, are given in the next two columns. In the last column of Table 4 the remaining d values are listed which were assumed to be due to the intermediate K2SO5. DISCUSSION From the weight losses taken from the T.G.A. curves, the X-ray diffraction patterns and chemical analyses we may conclude that the thermal decomposition of the potassium-and sodium-, hydro-, peroxydi- and pyrosulfates occurs in definite steps which agree with the reaction scheme given by the reactions (1)-(7). Comparing the decomposition data listed in Tables 1, 2 and 3 with each other we The thermal decomposition of potassium and sodium-pyrosulfate 1313 can match, within the limits given in Tables 2 and 3, the thermal decomposition data mentioned in the literature with exception of reaction (3) proceeding from K2S207 for which Wickert[5] and Duval[6] gave lower values for the initiating temperature. From Tables 2 and 3 we find that for the same heating rate the decomposition ranges of potassium pyrosulfates of different origin hardly differ. The behaviour of the sodium pyrosulfates is similar. From the results using different heating rates, it appears that both the initial decomposition temperature and the decomposition range at which the pyro- sulfates convert into the sulfates depend upon the heating rates used. Consider- ing the complete conversion, (see Figs. 1-5), from the starting material to the normal sulfate it is noticed that the higher the heating rate the less sharp the inter- mediate step in the T.G.A. curve. Hence the variation in decomposition data found in the literature, (see Table 1), can be well explained to be due to different heating rates and procedures while such parameters as crystallite size and specific surface are as yet not considered. We found that in the T.G.A. curves of the potassium sulfates of various origins a clear intermediate step appeared at which the observed loss in weight agreed exactly with the calculated loss of SOe. The evolution of SOs was proved quali- tatively by an iodometric analysis procedure. The weight loss from the inter- mediate step towards K, SO4 agreed well with the calculated loss of oxygen. The occurence of a flattening in the thermal decomposition curve of K2S20r at about 680°C as measured by Duval[6] seems to be similar with the one found here. The existence of a separate phase was established by means of the X-ray diffraction experiment. In the T.G.A. curves of the sodium sulfates of various origins we could not detect any reproducible flattening of the curve that could be associated with an intermediate stage in the decomposition. Hence it might be concluded that a phase of composition "Na~SOs" must be less stable than a phase of composition "K2SO~", a conclusion which might also be expected from a consideration of the stability rules for salts with an easily polarizable anion. Acknowledgement-The authors are pleased to thank Mr. J. Boeijsma for his help with the X-ray diffraction experiments. 

https://doi.org/10.1016/0022-1902(69)80241-1

The thermal decomposition of potassium and sodium-pyrosulfate

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