A SOURCE O F ERROR IN THERMOCHEMICAL MEASUREMENTS MAKING USE OF COMMERCIAL OXYGEN L. J. P. KEFFLER Department of Chemistry, University of Liverpool, Liverpool, England Received November SO, 1938
I n a previous paper (1)it has been shown that the apparent value obtained for the water equivalent of a calorimeter is dependent not only upon the small amounts of combustible impurities usually present in commercial oxygen, but also upon the pressure in the cylinder at the time of delivery of its contents into the bomb. With the particular samples of oxygen (denoted G and K) by means of which this phenomenon was first brought to light, there was a gradual decrease in the apparent value for the water equivalent; this decrease ultimately reached seven and five parts per ten thousand for G and K, respectively, after the pressure in the cylinder had dropped from about 125 to 35 atmospheres. It will be the object of the present paper to investigate whether compressed oxygen does always show the same behavior under the particular conditions prevailing in bomb calorimetry. I. THERMOCHEMICAL DATA I N SUPPORT O F T H E CONTENTION THAT T H E
PURITY OF T H E OXYGEN DELIVERED INTO A BOMB IS DEPENDENT UPON T H E PRESSURE I N T H E OXYGEN CYLINDER AT T H E TIME O F DELIVERY
Two series of combustions were carried out with a constant amount of standard benzoic acid in the bomb and at a fixed initial temperature; for the &st series, oxygen (sample J) from unknown origin was used throughout, while for the second series, the oxygen (sample M) had been obtained from liquid air by fractionation. I n both cases the oxygen was delivered into the bomb to a constant pressure of 35 atmospheres.
A . Data relative to oxygen J I n order to estimate from the figures in table 1 the amounts of the combustible impurities present in the oxygen delivered at the various pressures, it is necessary to compare them with those obtained from the head portions of four different cylinders of oxygen, namely, L, K, G, and E (see table 2), by means of which the value of a constant heat capacity had been determined in an earlier series of combustions with the same preparation of standard benzoic acid and under exactly the same conditions as in 277
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L. J. P. KEFFLER
the case of oxygen J (or later on oxygen M). In the case of oxygen E, all traces of combustible impurities had been removed by a previous purification. It is difficult to suppose that such a remarkable agreement between so many individual values and especially between the means, which are identical for all practical purposes, is entirely due to chance. When it is further taken into account that there is a similar kind of agreement between the heats of combustion found a t various periods with several different cylinders of oxygen, when the precaution was taken to determine the water equivalent with the same oxygen as for those heats of combustion and as simultaneously as possible with the latter, it must be concluded that the influence of chance must obviously be ruled out altogether. TABLE 1 Apparent variation of the water equivalent for decreasing pressures of the oxygen in cylinder J PRESIURE
W A T E R EQUIVALENT
MEAN WATER EQUIVALENT
at tnospheres
ca1.u per degree
cal.ia per degree
122 i 1
4274.7 4273.3 4272.8
4273.6
90fl
4273.6
4273.6
71 f 1
4275.5 4276 3
4275.9
4275.4 4274.8
4275.1
,
I
38 f 3
!I
B. Data relative to oxygen M The results obtained for the apparent value of the water equivalent with some new oxygen (M) were found to be strikingly similar to those just recorded for oxygen J, as will appear from an examination of tables 3 and 4. The similarity is so complete, indeed, that the values obtained with sample J may be used for sample M as well for all pressures recorded, a t least within the very small margin of a few tenths of 1 calorie for the experimental error. I n order to simplify the problem raised by the above data, it was felt advisable to arrange the experimental conditions so as to obtain results which could correspond only to one impurity at a time. Such a result was not impossible to attain by purely thermochemical means, namely, by preparing or purifying some oxygen and then adulterating it with a known
SOURCE OF ERROR IN THERMOCHEMICAL
MEASUREMENTS
279
amount of one single and well-defined impurity. But as it meant purifying compressed oxygen a t a temperature of about 800°C. in order t o free it from any combustible impurity possibly contained in it, or alternately purifying oxygen a t ordinary pressure and afterwards compressing it t o a t least 100 atmospheres, it introduced considerable technical difficulties. Such a solution will be tried in the future, as soon as some conditions required for its successful fulfillment will have been realized. For the present a TABLE 2 Values obtained f o r the water equivalent for pressures above 100 atmospheres in the cylinder and 56 atmospheres in the bomb, with oxygen f r o m the cylinders L, K , G, and E OXYQEN
PRESSURE IN CYLINDER
BENZolC NO.
lJEIED
WATER EQUIVALENT
ca1.16
per degree
MEAN WATER EQUIVALENT
cal.15 per degree
48a
4277.6 4276.9
4277.2
48a
4276.3 4276.5 4277.8 4277.4
4277.0
112/109
39b
4276.8 4277.0 4276.5 4278.7
4277.2
123/121
39b
4277.7 4277.0 4276.6
4277.1
39c
4277.4 4278.0 4276.3 4275.6 4278.0
4277.1
121/102
116/113
Preheated
simpler, though quite efficient, procedure has been devised, which is based upon the fact that in the combustion inside the bomb, under well-defined conditions, of a constant amount of a definite substance in the presence of oxygen containing a small amount of nitrogen as impurity, a constant proportion of the latter is converted to nitric acid; by means of a simple titration of the acid with standard alkali the percentage of nitrogen impurity present in any one bomb filling may thus be determined. TEE JOURNAL O F PHYBICALCHEMISTRY, VOL.
39, NO. 1
280
L. J. P. H E F F L E R
11. PREPARATION AND ANALYSIS O F A MIXTURE O F OXYGEN AND NITROGEN
I N SUITABLE PROPORTIONS AND UNDER A TOTAL PRESSURE
100 ATMOSPHERES A mixture of oxygen and nitrogen containing approximately 0.1 per cent of nitrogen was first prepared. This mixture was then used for combustions of a fixed amount of 1.72 g. of pure salicylic acid, under a constant O F ABOUT
TABLE 3 Apparent variation of the water equivalent for decreasing pressures of the oxygen in cvlinder M PRESSURE I N CYLINDER
atmospheres
~ 0 1 . 1 5per
135/132
80/77
WATER EQUIVALENT
1
47/44
degree
IEAN WATER EQUIVALENl
MEAN ERROR FOR MEAN I N PARTS PER THOUSAND
ca1.u per degree
4273.3 4273.1 4275.2
4274.0
0.05
4275.6 4275.7 4275.6
4275.6
0.00
4274.2 4275.7 4275.5
4275,l
0.03
TABLE 4 Comparison between the variations in the water equivalent when determined with oxygen J and oxygen M , respectively NUMBER OB COMBUSTIONS
PRES8URE IN CYLINDER
atmospheres
135 120 80 70 40
Oxygen J
Oxygen M
cal.16 per degree
ca1.u per degree
4274,O 4273.6 4275.6 4275.9 4275.1
4275.1
pressure of 35 atmospheres in the bomb and with a calorimeter a t a constant initial temperature of 20.5"C. The successive amounts of nitric acid found as the pressure in the cylinder decreased from 110 t o 15 atmospheres have been collected in table 5. The results recorded for pressures in the cylinder which were lower than the final pressure in the bomb have been obtained by means of the following artifice: the bomb was first filled to a pressure of
SOURCE OF ERROR IN THERMOCHEMICAL MEASUREMENTS
281
15 atmospheres with the oxygen-nitrogen mixture; it was then connected with the particular cylinder of nitrogen-free oxygen already used for making up the oxygen-nitrogen mixture, until the pressure had risen to the usual mark of 35 atmospheres, in order to allow for the lesser amount of nitrogen impurity introduced in the bomb as the result of this procedure. Three blank combustions were also carried out with the nitrogen-free oxygen. A consideration of table 5 shows that there is first an unmistakable increase in the amount of nitric acid formed during the successive combustions, as the pressure in the cylinder decreased from 110 to 55 atmospheres; indeed, for a pressure of 81 to 74 atmospheres, the amount of nitrogen converted to nitric acid was found to be just double of that obtained at the
TABLE 5 Mean correction to be applied to the water equivalent for different ranges of pressure in the cylinder in order to allow for the formation of nitric acid PRESSURE I N CYLINDER
NITRIC ACID CORRECTION
MEAN NITRIC ACID CORRECTION
atmospheres
calories
calories
106/98
0.3 0.8 0.7
0.6
81/74
0.5 1.3 2.1
1.3
61/55
1.9 2.6 2.7
2.4
20/18
0.0 0.5 0.1
0.2
higher pressure; and for a further drop of pressure down to 61 to 55 atmospheres, the amount of nitric acid formed was four times as large as the original amount found. When, however, the pressure in the cylinder had fallen to about 20 atmospheres, the results obtained for the nitric acid were just below those found a t the very start (after the difference between the mean for these results and the mean for the blanks had been multiplied by the factor 15/35 in order to allow for the lesser concentration of the nitrogen impurity in the three final bomb fillings made with the nitrogen mixture and thus to render the final data entirely comparable with the original ones).
282
L. J. P. KEFFLER CONCLUSIONS
Whatever the true cause of the phenomenon recorded in the preceding pages, the following conclusions will apply : (1) The statement made in a previous paper that the concentration of a combustible impurity eventually present in compressed oxygen may vary appreciably with the pressure in the cylinder at the time of delivery into the bomb, has been confirmed with three more cylinders of oxygen, one of unknown source and the other two derived from liquid air. (2) The apparent value for the water equivalent of a calorimetric system, when determined by means of oxygen from any of these cylinders, all received from the British Oxygen Company, showed a total variation of the order of five to seven parts in ten thousand, which therefore cannot be ignored if a precision of the order of one or two parts per ten thousand is to be secured. (3) The character of the curve, water equivalent against pressure in the cylinder, as revealed by the investigation of four different cylinders, may take one or other of two different forms, presumably depending upon the nature of the combustible impurity causing the contamination. I n conclusion, the author wishes to tender his thanks to Prof. E. C. C. Baly, Head of the Inorganic Department, for the facilities provided for this research. REFERENCE (1) KEFFLER, L. J. P.: J. Am. Chem.
SOC.66, 1259 (1934).
