250
INDUSTRIAL A N D ENGINEERING CHEMISTRY
Vol. 16, No. 4
stopcock to an evacuated filter h s k . With the stopcock closed, a small amount of purified acetone is poured over the millings in COl the tube; the stopcock is then opened and Evolved the acetone slowly sucked away. After a (Pressure Temin Volume peraCarbon Dioxide minimum of three such washings the sample CaCOa Weighed on 251.9 M1.) ture Observed Calculsteda Difference is placed in a vacuum desiccator and kept IVQ. Mm. C. Mole Mole Mole overnight a t a pressure of about 1 micron mm. of mercury). In earlier work, 1.9 Analytical 1.460 30 1.95 X 10-6 1.90 X 10-6 + O . O r X 10-6 balance ether was used instead of acetone, but its use 1.041 Microbalance 0.784 29 1.05 1.04 +0.01 was discontinued when it was found that in 0.918 28 1.23 1.25 -0.02 1.250 Microbalance humid weather evaporation of the ether 1.93s Microbalance 1.393 28 1.87 1.93 -0.06 chilled the metal enough to cause an undesirMean 0.03 ably copious condensation of moisture. The a 0.5 gram of ateelcohtaining 0.05% carbon gives 2.08 X 10-6 mole COZ. question has also been raised as to whether some trace of acetone may not remain adsorbed or occluded on the sample, but direct comparison of the carbon content of a number of washed samples with that of samples carefully cleaned but not it is necessary to show that the material collected in the liquid washed indicates that when the washing is properly carried out nitrogen trap contains nothing other than carbon dioxide. That the acetone remaining is not sufficient to increase significantly this is in fact true has been demonstrated by analysis of the conthe carbon present. densed material by means of a fractional vaporization method based upon the fact that, as the temperature of the trap is slowly raised, there is for each condensed substance a characteristic temperature a t which it vaporizes to cause a marked increase in the gas pressure of the system. Table 111.
Absolute Standardization with Iceland Spar (Calcium Carbonate)
-
To carry out such a test, the trap was fitted with a copper shield, which helped to equalize the temperature, to which was attached a thermocouple. After combustion of a sample the excess oxygen was evacuated and the mercury levels were raised to isolate the calibrated volume. The trap was then quickly surrounded by an empty Dewar vessel which allowed it to warm only very slowly, and a t suitable intervals the pressure was measured by means of the McLeod gage. For the materials condensed from the combustion of Bureau of Standards Sample 558, which contained 0.02% sulfur and might have been expected to yield a little sulfur dioxide, the result of these measurements is shown by curve A , Figure 1. The pressure, hence the number of moles of gas in the system, remained low up to about - 120" C., a t which temperature there was a relatively sudden increase to a pressure corresponding to the production of about 4.5 X 10-6 mole of gas; further increase of temperature up to 0" C. produced no further significant increase in the amount of material vaporized. The corresponding behavior of the condensate from the Iceland spar is shown by B, Figure 1. Had any sulfur dioxide or trioxide, or any other condensable substance, been present there would have been a t a somewhat higher temperature another knee in these curves, as Wooten demonstrated by intentionally adding a little sulfur dioxide to the system. Curve A indicates a slight evolution of gas between 0" C. and room temperature, an increase which is not evident in B . This increase, which was noted in all such runs on steel, indicates the evolution of less than 2% of some other gas, presumably water vapor. It is evident therefore that the gas condensed in the trap during combustion of a steel is virtually pure carbon dioxide. The authors have also investigated the influence of two steps in the procedure upon the accuracy of the result. The first is the matter of washing the sample to remove superficial dirt and grease, which must be taken into account, since the surface of the sample is usually large and may therefore hold a relatively large quantity of carboniferous material.
If the sample is in the form of a thin strip, the surface can be cleaned by abrasion, after which the metal can be cut into small pieces with a pair of snips, avoiding any contact of the sample with the fingers or other material likely to contaminate it. If, however, the specimen is more massive, as, for instance, the fractured end of a tensile specimen, it is necessary to do some machining and in such case washing is desirable. In the authors' procedure washing is carried out in a small Pyrex tube with a sintered crystalline Alundum bottom, the lower part of which is ground into a holder connected through a
TEMPERATURE
Figure 1.
T.
Fractional V a orization of Condensed Carbon bioxide
The other question considered is whether the true blank valuethat is, with the high crucible temperature resulting from the burning of a sample-is appreciably different from that obtained in the normal way. To gain some information on this point a number of check runs were made on the same material after a preliminary heating of the crucible in oxygen and without opening the system, on the basis that if the higher temperature due to combustion of the sample did cause an increased blank, this blank should decrease on successive combustions, so that the apparent carbon content would successively decrease. The results indicated no significant trend in this direction. LITERATURE CITED
(1) Murray and Ashley, IND.ENG. CHEM.,ANAL.ED.,16,242 (1944). (2) Wooten and Guldner, Ibid., 14, 835 (1942).
Photoelectric Photometet-Correction In the caption of Figure 4 of the article "Photoelectric Photometer for Determining Carbon Disulfide in the Atmosphere" [IND.ENG. CIIEM.,ANAL.ED., 15, 593 (1943)l the caption should have read: Re. 250,000 ohms. RI.15,000 ohms. Ra. 50,000 ohms. SHIRLEIGH S~LVERMAN
