April 15, 1941
ANALYTICAL EDITION TABLE11. RECOVERY OF
Reducing Sugars in 10 Cc. -Before Inversion--After GalGalvanom3Ig. vanometer 2-cc. per eter 1R I elo n readali- 10-cc. readKO. ings quo$ juice ings
of Juice InversionMg. per Mg 2-cc. per ali10-w. quot juice
SUG.4RS
ADDEDTO
251
WATERMELOX
JUICE
Reducing Sugars in 10 Cc. of Supplemental Juice Sugar -4dded. t o --Before Inversion-After Inversion10 Cc. of Juice GalMg. Mg. GalMg. Mg. Suvanomper per vanomper per crose eter 2-cc. 10-cc. eter 1-00. 10-cc. Inas r.eadaliSamreadaliSamvert invertb ings quot ple ings quota ple
Mg.
Recovery ofa Invert Total sugarc sugard
Me.
Mg.
%
Jfg.
70
1
1 7 . 0 0 0 , 5 2 8 528 20.75 0.661 661 250 263 24.75 0.763 763 18.75 0.594 1188 238 95 518 101 17.00 0.528 528 20.75 0 . 6 6 1 661 25.00 0 . 7 6 8 768 1 8 . 5 0 0.585 1170 *3 *I 19 *2 2 2 3 . 7 5 0 . 7 4 0 370 21.25 0.675 675 250 263 19.50 0.621 621 18.75 0.594 1188 248 99 511 100 603 18.50 0.585 1170 A14 A6 *l6 +=3 2 2 . 7 5 0.717f 359 20.75 0.661 661 19.00 0.603 3 1 3 . 7 5 0 . 3 6 4 364 21.25 0 . 6 7 5 675 500 526 30.00 0.857 857 28.00 0.825 1650 500 100 996 97 21.50 0 . 6 8 2 682 32.25 0.887 887 29.50 0 , 8 4 9 1698 *15 *o *28 *3 1 4 . 0 0 0.380 380 a * values represent maximum deviations from average recoveries. b Mg. of s u c r o ~ e added divided by 0.95 t o convert t o invert sugat formed after hydrolysis. c Obtained by subtracting mg. of reducing sugars per 10 00. of juice before inversion (column 4) from t h a t observed after supplementing with known quantity of invert sugar (column 12). d Obtained b y subtracting mg. of reducing sugars per 10 cc. of juice after inversion (column 7) from t h a t observed after supplementing with known quantity of invert sugar plus eucrose (column l o ) . e Owing t o high total sugar content, half of usual aliquot supplemented with 1 cc. water was oxidized. i 10 cc. of clarified juice, rather t h a n 5 cc. as usually taken, were diluted t o 100 00. before oxidation of 2-00. aliquot.
error, due to the logarithmic relation between concentration and galvanometer deflection; at the 1.1-mg. glucose level, for example, the observed variation of 52.25 to 58.25 (the greatest galvanometer variation observed in Table I) results in a n error of only *2 per cent. I n the routine analysis of large numbers of plant samples, usually involving great variation in sugar concentration, the sacrifice of a certain degree of precision in the extremely dilute range is well repaid by the saving of time and material consumed in reanalyzing samples requiring greater dilution.
Summary A rapid photocolorimetric method for determining reducing sugars in aliquots containing up to 1.2 mg. has been described. The method is a n extension of the procedure developed by Forsee for plant materials and represents a threefold increase in concentration range. The decrease in yellow color Droduced bv heating reducing- sugars - with alkaline ferricyani2e was Il&Sured wit' an Evelyn photoelectric colorimeter. Test-tube absorption cells were employed, permitting
complete serial analyses without transfer. A reference curve for glucose and invert sugar has been prepared. The method was developed for rapidly analyzing large numbers of watermelons during a short harvesting period. Good recoveries were obtained when various sugars were added to watermelon juice.
Literature Cited (1) Assoc. Official Agr. Chem., Official and Tentative Methods of Analysis, 4th ed., p. 473, 1935. (2) Evelvn. K. A.. J . BioZ. Chem.. 115.. 63 (1936). ~ . Folin, O., Ibid., 77, 421 (1928). Forsee, W. T., IND.ENQ.CHEM.,Anal. Ed., 10, 411 (1938). Hagedorn, H. C., and Jensen, B. N., Biochem. Z.,135,46 (1923). Hanes, C. S., Biochem. J., 23, 99 (1929). (7) Hassid, W. X., IND.ENQ.CHEM.,Anal. Ed., 8, 138 (1936). (8) Ibid., 9, 228 (1937). (9) Hawkins, J. A., and Van Slyke, D. D., J . Biol. Chem., 81,459 (1929). (10) Hoffman, W. S., Ibid., 120, 51 (1937). CONTRIBUTION KO. 7 from U. S. Regional Vegetable Breeding Laboratory, Charleston, S. c.
Decomposition Temperatures of Some Analytical Precipitates Calcium Carbonate and Lead Sulfate RI. L. NICHOLS
0
AND
B. E. WHITE, Cornell University, Ithaca, N . Y.
NE of the essential steps in many gravimetric analyses is the ignition of a precipitate either to remove a volatile
impurity or to change the composition to a more desirable weighing form. The higher the temperature used, the less time will be required to fulfill the purposes of the ignition. However, since all compounds will decompose if heated to a high enough temperature, it is necessary to set a maximum temperature a t which it is safe to ignite a precipitate without danger of decomposition. I n analytical chemistry we are concerned mainly with the decompositions which result in a loss of weight or, in other words, in which at least one of the decomposition products is a gas or a volatile substance. Only for those substances whose gaseous decomposition products are normal constituents of the atmosphere will it be possible to establish definite decomposition temperatures. For the class of substances whose gaseous decomposition products are not constituents
of the atmosphere, it is not possible to state with exactness the ignition temperature below which no decomposition is possible. The statement of the maximum permissible ignition temperature will necessarily depend upon the conditions under which the ignition is carried out and upon the degree of accuracy desired in the determination. The aim of these studies is to work out a satisfactory method of determining these dissociation pressures and of correlating the dissociation pressures with the maximum allowable ignition temperatures. The classes of precipitates which have been selected for study are the carbonates, sulfates, phosphates, and chromates. I n deciding which individual compounds in each class to study, the authors have selected the list of weighing forms of various elements given by Lundell and Hoffman (34). The need for studying these decomposition temperatures becomes evident if we consider the great variation in the ignition temperatures for various precipitates
252
INDUSTRIAL AND ENGINEERING CHEMISTRY
which are recommended by different investigators and in the standard texts on quantitative analysis. I n reviewing the various investigations pertaining t o this subject, two types of results are found. The first are merely q u a l i t a t i v e statements, such as "ignition at
Vol. 13, No. 4
Method and Apparatus For the determination of the dissociation pressures of the different precipitates at various temperatures i t is desirable to have a single
FIGURE 1. DIAGRAM OF APPARATUS
red heat produces partial decomposition" or "no decomposition results on heating with a blast lamp". Many of these statements were published before the advent of the thermocouple which made Dossible exact determination of high temperatures. The second type is more of a quantitative nature, either measurements of the dissociation pressures or loss in weight experiments. One of the compounds which have been the most extensively and probably the most accurately studied is calcium carbonate, formed by the ignition of calcium oxalate a t the proper temperature. Willard and Boldyreff (40) state that it is advisable to carry out this ignition above 450' C. and that decomposition results above 550' C. The methods which have been used for this study include the determination of dissociation ressure (8, 16, SO, 83, 86, SO,34-36,38, e l ) , loss in weight (6, 167, rate of gas evolution ( I ) , as saturation (10, Bb), "differential" method ( 7 ) , emanation f.1, 42), and the use of a high-temperature vacuum balance (4, 38, 97). Probably the most accurate results are those of Johnston (20) who used a static method. His values are slightly lower than those of Pott or Zavriev (42) and he explained t.his discrepancy by the fact that in using a very small sample he was able to obtain a uniform temperature throu hout the charge which cannot be done with a larger charge. Tfis is a necessary precaution, since the pressure developed A;iU be that in which the carbon dioxide is in equilibrium with the calcium carbonate at the lowest temperature. The precipitate selected for this first investigation was lead sulfate. Its decomposition was first mentioned by Berthier (9) who in 1822 stated that it decomposes slight1 a t white heat. Erdman (11) in 1854 found that this substance Lst weight when heated in an open or loosely covered crucible. According to Boussingault (6) a loss of weight occurs on ignition at white heat. Percy (according to Mellor, 25) in 1870 stated that lead sulfate was permanent a t a low red heat. It was found by Bailey (8) that no decomposition results on igniting at 500" C. Friedrich and Blickle (IS)gave the decomposition temperature as 850" C., while Hofman and Wanjukow (17) gave 637" C. as the temperature of the beginning of decomposition and 705' C. as the temperature at which dissociation becomes marked. Both these investigations were made by the thermal method. Mostovich (87) found that up to 800' C. lead sulfate does not change when heated in a current of dry air but at this temperature sulfur trioxide begins to be liberated. In 1906 Doeltz and Grauman (9) found no loss in weight between 700" and 800' C. and only a very slight loss at 900' C. but a big loss at 1000' C. Schenck and Rassbach (39) in 1908, Grahmann (14) in 1913, and Jaeger and Germs ( I S ) in 1921 confirmed the decomposition at between 900' C. and 1000" C.
method which will be applicable t o all, T h e vacuum static method of Johnston (20) and others, found so satisfactory for studying the decomposition pressures of carbonates, would not be equally satisfactory for studying sulfates and other' substances whose gaseous products are not permanent gases at room temperature and would condense in the cooler part of the apparatus. Therefore the dynamic or gas saturation method was adopted. This method was first used for the determination of vapor pressures of solids by von Wartenberg (39) although Regnault (29) and others had used i t earlier to determine the vapor pressure of liquids. This method consists of passing a measured volume of an inert gas over the heated substance, determining the amount of vapor in the gas, then from Dalton's law calculating the dissociation pressure. The method has also been used by Keppeler and d ' h s @ I ) , Dutoit ( I O ) , Isambert (18), and Finkelstein (la) for determining the vapor pressures of various substances. Since these authors give insufficient details of their work and their results are not in agreement with those obtained by other methods, this method was first tried on calcium carbonate, as the dissociation pressures of this substance are probably the best known of any materials. The a paratus used is shown in Figure 1. The nitrogen, which was usefas the inert gas, passes from the cylinder, A , to a pressure regulator, B , throu h a bubble counter, C, and then through a series of tubes, D, with Ascarite and Dehydrite to remove all carbon dioxide and water. From these tubes it passes to the reaction tube, E,a microcombustion tube with a side arm, made of Pyrex or transparent quartz, depending upon the temperature being used. The front part of the tube is filled with small pieces of quartz which serve as baffles, making certain that the nitrogen is heated to the temperature of the sample. Following the quartz granules is the latinum boat containing the sample. The boat is 5 cm. long witi the other dimensions such that it will just fit inside the combustion tube. The combustion tube i5 heated by an electric furnace, E. From the combustion tube the nitrogen passes to the gas buret, F , going through either the absorption train, G, or the by-pass, H . The absorption train used with calcium carbonate consisted of a Stetser-Norton absorption bulb filled with Dehydrite for removing any water and a Wesson absorption bulb filled with Ascarite and Dehydrite. I n several experiments Pregl micro absorption tubes were used. In all cases the microprocedure of wiping with moist flannel and chamois was used and a similar tube was used as a tare in weighing. The by-pass was used to permit flushing after introduction of the sample and to make it unnecessary to stop the flow of gas between the 5ushing and the beginning of the experiment.
fried
253
ANALYTICAL EDITION
April 15, 1941
TYPE K POTENTIOMETER I'
f -7-
II
U W
increasing this volume seemed to have no effect. It is only necessary to flush sufficiently to remove all air and to give time enough for the sample to begin to dissociate and to saturate the gas with carbon dioxide. When the level of the water in the buret reached the top graduation, the stopcocks were turned to pass the gas through the absorp tion train. After the desired volume of gas had been aspirated through the system, the flow of water from the buret was stopped and the absorption train disconnected. I n many cases several runs were made with the same sample by merely refilling the buret to the top graduation, inserting another L I absorption train, and omitting any flushing. At the end of the run the temperature of the nitrogen was noted, the barometric pressure was read, and corrections for gravity, temperature, and capillary depression were applied, and the time required to carry out the run was recorded. The weighings were made as previously described. Any changes in barometric pressure during the experiment were neglected. The pressure in the tube was found to be within 0.2 mm. of the barometric pressure. Since this difference in pressure would have no significant influence on the results, it was also neglected.
6C6
LAMP
RES1STANCES
FURNACE
1110 A C
FIGURE 2. DIAGRAM OF REGULATOR During preliminary runs, it was noted that the temperature of the furnace varied as much as 20' to 25' C. during a day, owing to variations in the line voltage. The furnace was therefore equipped with a temperature regulator with which it was possible to keep the temperature constant to within * l . O o C. for any length of time. The regulator is shown in Figure 2. I t was operated by a chromel-alumel thermocouple, the hot junction of which was placed outside the reaction tube directly beneath the boat containing the sample. The thermocouple was connected in the usual manner to a Leeds & Northrup Type K ometer, balance being indicated by a Leeds & Northrup KO. E reflecting wall galvanometer. The beam of light reflected from the galvanometer struck a photoelectric cell which, by means of amplification and two relays, switched on an auxiliary heating current. By adjusting the main heating current so that it was not quite sufficient to maintain the desired temperature, and by having the hotoelectric cell in such a position that as the temperature felp the light beam struck the cell and turned on the auxiliary current, it was possible to keep the temperature practically constant, The proper key of the potentiometer was kept permanently depressed by a clamp. The operating current was supplied by a 3-cell, 2-volt storage battery and was adjusted frequently with a standard cell. The cold junction was kept at room temperature, adjusting for any changes. The temperature readings of the chromel-alumel thermocouple were calibrated by a platinum, platinum-rhodium thermocouple, calibrated by the National Bureau of Standards, the junction of which was placed in the reaction tube in the same position that the platinum boat occupied in the experiment. The temperatures recorded in all the experiments are believed to be accurate within * 2 . 0 ° C. It was found that a space about 4 cm. long in the tube was a t the same temperature and for a distance of 1 cm. on each side of this space it was not more than 1" C. lower. Therefore, in all experiments, the entire contents of the boat were within 1" C. of the same temperature.
?;;tk
Decomposition of Calcium Carbonate Pure calcium carbonate was prepared from Baker and Adamson reagent grade calcium carbonate by double precipitation as the oxalate, drying, and then igniting for 12 hours in an electric muffle a t 500" C. This temperature converts (40)all the oxalate to carbonate. The calcium carbonate was pulverized in an agate mortar and kept in a desiccator. Spectroscopic analysis of this calcium carbonate showed that it contained less than 0.01 per cent of magnesium and no appreciable amounts of any other impurity. The platinum boat containing 0.5 gram of calcium carbonate was placed in the center of the reaction tube where the temperature was constant to within * 1' C. After replacing the stopper in the mouth of the tube, the flow of nitrogen through the tube and the by-pass was started by opening the stopcock at the bottom of the buret. Generally 500 ml. were used for flushing and
The results for the determination of the dissociation pressures at various temperatures are tabulated in Table I and are shown in Figure 3 by plotting the logarithm of the pressure against the reciprocal of the absolute temperature. Only the values obtained a t the faster rates were used and for comparison the values obtained by Johnston (20) and Andrussow (7) have also been plotted on the same graph. To show the effect of varying rates of gas flow on the dissociation pressures, a series of experiments at 636' c. with different rates was made. The results are shown in Table I1 and Figure 4. The results at the slowest rate at 640" C . were IOM
lo75
1100
112:
115c
1175
1201
121:
-1.00
-0.50
0
0.50
1.00
FIGURE 3. EFFECTOF TEMPERATURE
INDUSTRIAL AND ENGINEERING CHEMISTRY
254
rithm of the pressure 0.05. The value for zero rate in Figure 4 is the equilibrium pressure determined by Johnston (2@. It is evident from these results that the slower the rate, the nearer the Dressures determined will approach the equilibrium values. This was shown by von Wartenberg (39) in his studies of the vapor pressure of the metals. H e determined the vapor pressure a t three different rates, and by plotting the pressure against the rate was able to make a straight-line extrapolation to zero rate. However, the rates he used were much faster, ranging from 11 to 100 ml. per minute. Such a procedure would not be justifiable in the authors' experiments, as shown by the rapid increase in the pressure a t rates approaching the zero rate. Equation 1
TABLEI. DISSOCIATION PRESSURES No.
Vol.
MI.
Baro- Volume metric at Temp, Pressure 8. T.P. Rate Wt. of COS p c. Mm. H e M1. Ml./min. Gram Mm.
log p
Temp. O C.
1 2 3 4
2031 2031 2031 2031
22.8 23.0 23.3 24.0
727.3 727.3 733.2 737.3
1744 1741 1754 1755
14 17 10 10
0.0004 0.0010 0.00155 0.00092
0.08 0.21 0.33 0.20
-1.08 -0.68 -0.48 -0.71
560 560 564 564
5 6 7
2031 2031 2031
22.3 22.8 23.5
728.7 724.2 723.3
1750 1735 1736
14 17 9
0.0018 0.0026 0.0021
0.38 0.55 0.45
-0.42 -0.26 -0.35
591 591 591
8 9
21.8 22.4
733.2 733.7
0.19 0.17
615 615
739.4 739.0 738. 9
1.5 5.8 17 17 23
1.54 1.49
23.5 23.8 24.0
1575 877 1765 1762 1759
0.0065 0.0035
10 11 12
1811 1011 2031 2031 2031
0.0081 0.0085 0.0065
1.72 1.81 1.39
0.23 0.26 0.14
624 624 624
13 14 15 16 17 18 19 20 21 22 23
1011 1011 1011 1011 1256 1026 1011 1011 1011 1011 1011
21.0 20.8 20.8 20.7 20.7 20.4 20.9 21.2 21.8 22.3 20.8
1.86 2.06 2.32 2.32 1.97 2.45 1.87 2.20 1.70 2.13 2.74
0.27 0.31 0.36 0.36 0.29 0.39 0.27 0.34 0.23 0.33 0.44
636 636 636 636 636 636 636 636 636 636 636
22.5 22.8 22.0 21.6 20.5
10 5.7 5.7 5.7 5.7 5.7 3.5 1.7 1.8 1.5 1.7 1.5
0.0044 0.0050 0.0055 0.0055 0.0058 0.0059 0.0045 0.0052 0.0040 0 0050 0.0065
1856 506 506 1624 575
870 872 873 877 1092 922 905 903 886' 884 905 1613 439 440 1431 508
10
24 25 26 27 28
723.0 724.1 725.1 727.9 727.9 751.4 751.7 751.1 739.7 739.5 750.8 735.3 735.3 734.5 741.9 739.4
0.0130 0.0040 0.0046 0.0132 0.0046
3.08 3.40 3.88 3.47 3.32
0.49 0.53 0.59 0.54 0.53
640 640 640 640 640
29 30 31 32
1011 1011 1011 1011
24.0 24.0 24.3 24.7
746.2 746.2 746.4 745.2
885 885 883 881
18 18 13 13
0.0074 0.0092 0.0093 0.0083
3.16 3.93 3.97 3.56
0.51 0.69 0.60 0.55
651 6R1 651 651
33 34 35 36 37 38 39
2031 2031 2031 2031 2031 2031 2031
22.0 22.5 22.5 22.8 24.5 24.9 25.0
732.5 735.6 737.8 736.0 740.6 738.7 738.7
1762 1765 1770 1773 1759 1760 1750
17 17 16 14 14 12 14
0.0320 0.0280 0.0293 0.0338 0.0324 0.0364 0.0288
6.70 5.38 6.16 7.07 6.87 7.83 6.20
0.83 0.73 0.79 0.85 0.83 0.89 0.79
679 679 679 679 679 679 679
10 10 10
corrected graphically to 636' C. I n Figure 3 the rate of change of the logarithm of the pressure with the temperature is 0.0137 per degree, which agrees well with the corresponding Values obtained from the Curves Of Johnston and Andrussow. Thus a change of 4' C.would Change the lo@-
Vol. 13, No. 4
0.043 y = -
2637
(1)
( z + l ) 2 + m + 2
where x is the rate and y is the pressure, was found to fit this curve but seems to have no theoretical significance.
Decomposition of Lead Sulfate Pure lead sulfate was prepared by adding, slowly and simultaneously, a solution of thrice recrystallized lead nitrate and dilute sulfuric acid to 5 liters of hot water, the lead nitrate being kept in slight excess. After thorough digestion, the lead sulfate was separated, washed, dried, and ignited at 500" C. Spectroscopic analysis showed that the lead sulfate contained no significant amount of impurity.
The decomposition of sulfates has two complicating factors. First, the decomposition product, sulfur trioxide, dissociates into sulfur dioxide and oxygen, 2503
2502
----f
+
(2)
0 2
Thus both oxides of sulfur mustbe absorbed and determined, Although it would be possible to determine these separately, it is not convenient and i t would have no significance, as the reverse reaction 2501
+
0 2
----f
2SOa
(3)
may occur as the gases are cooled down on leaving the furnace. In one experiment a t the highest temperature, after
TABLE11. EFFECTOF RATEON DISSOCIATION PRESSURBS Run N o . 13 14 15 16 17
(Temperature 636O C . ) Rate M1 ./min. 10 10 10 10 10
log P 0.27 0.31 0.36 0.35 0.29
Av. 0.32 5.7 5.7 5.7 5.7 5.7
0.39 0.27 0.34 0.23 0.33
Av. (of runs 18, 20, and 22) 0 . 3 5 23 24 25 26 27 28
3.5 1.7 1.8 1.5 1.7 1.5
0.44 0.44 0 48 0.54 0.49 0.48
Av. (of run8 25, 26. 27, and 28) 0 . 5 0 Johnston's value for equilibrium or zero rate 0 . 6 7
FIGURE 4. EFFECTOF RATEOF GAS FLOW
I
ANALYTICAL EDITION
April 15, 1941
usually outside the furnace. Since this could not be prevented and as indicated previously, the amount of sulfur trioxide was probably much less than that of sulfur dioxide, this was neglected. However. i t is a Dossible source of error.
TABLE111. DECOMPOSITION OF LEADSULFATE Volume
NO. of N I
M1. 1 2 3 4 5 6 7 8
2031 2031 1528 1518 2031 2031 1868 1668
9 10 11 12 13 14
1578 2081 1518 1908 2031 1618
16 17 18 19 20 21 22 23
3355 1956 1883 2031 1688 1518 1110 1518
24 25 26 27 28 29 30 31 32 33
1478 1728 1981 1518 518 1201 1036 1056 1011 1011
Bar- Volume Temp. metric at of N~ Presaure S.T.P. Rate wt. of BaSOi C. Mm. Hg M1. Ml./min. Gram Liter 1745 4 0.00265 0.0015 24.5 734.6 1770 4 0.00264 0.0015 2 3 . 5 740.5 23.0 739.5 1333 1 . 4 0.00252 0.0019 1 . 4 0.00268 0,0020 1316 2 3 . 5 737.2 1 . 3 0.00308 0.0017 1755 24.0 736.6 1743 1 . 3 0.00277 0.0016 734.8 24.8 1535 1.1 0.00261 0 0017 26.6 733.8 1410 1.4 0.00220 0 0016 27.0 733.0 Averages at 762' C. 0.0017 1.5 0.00432 0.0032 26.8 730.2 1330 1719 4 0.00512 0.0030 730.4 26.2 730.8 1298 1 . 4 0.00486 0.0037 25.6 1 . 3 0.00531 0.0032 1639 2 5 . 0 736.0 1740 4 0 00551 0.0032 733.7 25.0 1.4 0,00486 0.0035 1375 728.1 25.0 Averages a t 809' C. 0.0032 2.5 0.02866 0.0099 23.0 731.7 2897 1720 1 . 3 0.01850 0.0108 22.5 743.7 1620 1 . 6 0.01 586 0,0098 24.8 736.5 1740 4 0.01851 0.0106 734.5 25.0 1 . 4 0.01537 0.0106 25.5 1453 739.7 0.01233 0.0095 1.3 1298 734.2 26.0 26.0 0.00991 0.0105 1.4 734.2 946 26.8 1 . 3 0.01311 0.0102 1282 730.9 Averages at 857' C. 0.0102 0.06625 0.0532 27.3 730.6 1245 4 1446 1 . 5 0.07772 0.0533 27.2 726.6 0.09160 0.0550 1666 4 26.4 726.9 1297 1 . 4 0.07002 0.0540 732.1 25.0 444 1 . 2 0.02232 0.0503 24.6 732.9 1034 4 0.05999 0.0581 24.5 735.8 1 . 2 0.04974 0.0548 742.1 908 23.0 4 922 742.5 0.04999 0.0643 23.5 4 742.2 0.04831 0.0548 882 23.5 0.04795 0.0543 883 4 23.4 742.7 Averages a t 904' C. 0.0550
p
log p
0.11 0.11 0.14 0.14 0.12 0.11 0.12 0.11 0.12 0.23 0.21 0.27 0.23 0.22 0.25 0.23 0.69 0.77 0.69 0.75 0.75 0.67 0.74 0.72 0.72 3.72 3.73 3.81 3.78 3.52 4.07 3.89 3.85 3.89 3.85 3.81
-0.96 -0.96 -0.86 -0.85 -0.92 -0.96 -0.92 -0.96 -0.92 -0.64 -0.68 -0.57 -0.64 -0.66 -0.61 -0.64 -0.16 -0.11 -0.16 -0.12 -0.12 -0.17 -0.13 -0.14 -0.14 0.57 0.57 0.58 0.5s 0.55 0.61 0.59 0.59 0.59 0.59 0.58
255
Temp.
' C. 762 762 762 762 762 762 762 762
The determinations were made the same as with calcium carbonate except for the absorption of the decomposition products. The oxides of sulfur were absorbed in dilute sodium hydroxide and determined gravimetrically, so no desiccant was needed. An excess of sodium hydroxide was used, about 10 to 15 ml. of 0.01 N at the lower temperatures and double this amount at the highest temperature. The alkali was placed in a 50-ml. Erlenmeyer flask, fitted with two delivery tubes. The tip of the inlet tube was drawn out to a capillary and dipped below the surface of the liquid. A second flask showed that all the oxides of sulfur were absorbed in this flask. The analysis for the oxides of sulfur was made by the standard microprocedure.
809 809 809 809 809 809 857
857 857 857 8,57 857 857 857
The results are given in Table I11 and a graph of the averages, plotting the logarithm of the pressure against the reciprocal of the abso904 lute temperature, is shown in Figure 5. Only 904 two rates were used and, as is evident from 904 904 Table 111, no significant differences are apparent 904 at the two rates in this case. 904 904 Figure 6 is a graph of the same results showing 904 904 the weight of sulfur trioxide per liter of nitrogen 904 plotted against the centigrade temperature. It would seem that below 710" C. no appreciableloss in weight should occur when lead sulfate is heated in a r&f€le furnace, while at 730" C. a small but definite loss should occur. This prediction was tested by determining the loss in weight of lead sulfate on heating.
A platinum-wound muffle furnace (28) of about 700-ml. volume was used. The temperature regulator previously described was used and the temperature was measured with a platinum, platinum-rhodium thermocouple. The temperatures are believed to be accurate t o within h5' C. Two samples of lead sulfate were weighed into small &ml. platinum crucibles and placed in the center of the muffle. One crucible was covered with a loose-fitting lid while the other was left uncovered. After heating the desired length of time, they were removed from the muffle, allowed to cool for 20 minutes in a desiccator, and weighed. The same samples were used repeatedly.
The results are given in Table IV. It can be seen that a small loss in weight occurred during the first 2 hours at 680' C. This is probably due to loss of moisture rather than decomposition, since further heating at this temperature and 1040
1000
960
9 00
840 20
FIGURE 5 15
the alkali used t o absorb the oxides of sulfur was made just neutral to methyl red and hydrogen peroxide was added to oxidize the sulfur dioxide, i t required almost as much alkali as had been originally used to bring the solution back to neutral. This indicates that practically all of the sulfur trioxide was converted t o sulfur dioxide and that the reverse reaction (Equation 3) is unimportant. The other disturbing factor is the possibility of the sulfur trioxide condensing on the cooler parts of the apparatus before reaching the absorption flask. T o prevent this the absorption flask was placed as near the reaction tube as conveniently possible. No visible signs of condensation mere noted; but after a series of ten to fifteen runs, a small amount of sulfur trioxide fumes was visible on heating the part of the tube
10
5
0 650
700
750
800
FIGURE 6
850
900
INDUSTRIAL AND ENGINEERING CHEMISTRY
256
TABLEIV.
Temp.
c. 680
T -m iEaoh ignition Hours
2 4.5
12
700
720 730 800
3 3 5 11 12 10.5 29 16 7 42 1 1 1 1 1
Loss IN WEIQHTOF LEADSULFATE
Sample 1 (Covered).
Original Weight 1.5469 Mg. in WeightWeight after Lossin each Total a t each Loss per each ignition ignition temperature hour (av.) -LOSS
Total a t each temperature Hours
2 6.5 18.5 3 6
11 22 12 22.5 51.5 16 23 65 1 2 3 4 5
Vol. 13, No. 4
Mg.
Mg.
Mo.
1.5465 1.5465 1.5464 1.5463 1.5463 1.5463 1.5463 1.5463 1.5462 1.5462 1.5460 1.5460 1.5453 1.5449 1.5449 1.5447 1.5445 1.5444
0.4 0.0 0.1 0.1 0.0 0.0 0.0 0.0 0.1 0.0 0.2 0.0 0.7 0.4 0.0 0.2 0.2
0.4 0.4 0.5 0.1 0.1 0.1 0.1 0.0 0.1 0.1 0.2 0.2 0.9 0.4 0.4 0.6 0.8 0.9
0.1
at 700" C. caused no further appreciable loss. At 720' C. the sample in the open crucible showed a slight loss in weight on each ignition while the weight of the sample in the covered crucible remained almost the same. At 730" C. the decomposition is slightly more marked, although heating for 1 or 2 hours a t this temperature would not introduce a noticeable error when igniting samples as large as 1 gram. The loss in weight of 800" C. is appreciable in the course of 1 hour. It is of interest to note that both samples turned slightly yellow a t the temperature a t which the loss in weight became noticeable. To be certain that the loss in weight reported is not due to volatilization of the platinum, the weights of the crucibles before use were compared with those after all the ignitions were completed. The total loss in weight of the covered and uncovered crucibles after 162 hours' heating was 0.1 and 0.2 mg., respectively. Summary The dissociation of calcium carbonate has been studied by the gas saturation method of determining vapor pressures and the results have been compared with the values obtained by Johnston with a static method. The effect of the rate of gas flow in this method has been studied and the results indicate that equilibrium values are not attainable a t rates which are convenient to use. The slower the gas rate, the nearer the pressures approach equilibrium values. Since this method more nearly duplicates the conditions in a n actual ignition than the static method, the dissociation pressures obtained should be of more value in predicting the maximum possible ignition temperatures for analytical precipitates. The dissociation pressures of lead sulfate a t four temperatures have been determined. A prediction of the temperature a t which a noticeable loss in weight will occur when lead sulfate is ignited has been made and experiments to determine the loss in weight have borne out the prediction. Under the conditions described the loss in weight of lead sulfate due to decomposition first becomes appreciable a t 730" C.
Acknowledgments The authors are indebted to Drs. Lawrence and Curtiss of the Department of Mathematics for the derivation of Equation 1, and to Dr. Gartlein and Mr. Higinbotham of the Physics Department and Dr. Machlillan for the design of the regulator.
% 0.0018
0.0003 0.0001 0.0009
0.019
Sample 2 (Uncovered).
Original Weight 1.1671 Mg. Loss in Weighh Weight after Loss in each Total a t each Loss er each ignition ignition temperature hour &Y.) Mg. Mo. Mo. % 1.1668 0.3 0.3 0.2 1.1669 (;:;I 1.1668 0.3 0.0019 1.1667 0.1 0.1 1.1667 0.0 0.1 1.1667 0.0 0.1 1.16665 0.05 0.15 0.0008 1.1665 0.15 0.15 1.1664 0.1 0.25 1.1660 0.4 0.65 0.0011 1.1655 0.5 0.5 1.1653 0.2 0.7 1.1635 1.8 2.5 0.0033 1.1628 0.7 0.7 1.1626 0.2 0.9 1,1621 0.5 1.4 1.1618 0.3 1.7 1.1616 0.2 1.9 0,033
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