Chemical Analysis Based on X-Ray Absorp Measurements with a Multiplier Phototul Gases E. H. WINSLOW, H. M. SMITH, H. E. TANIS, AND H. A. LIEBHAFSKY Rese arch Labomtory , Geneml Electric eo., Scheneetarly, N. Y. X-ray absorp Lents with a multirdier phototube, which am Izseful in the analysis of sohas ana liquids ( Z ) , show pmrnise also lor worx with gases. Results obtained o n six gases are reported.
. . "
. .~.
I
..
< m aanalysis or gases by x-ray absorption amem in at least
L four wrtys from the work previously reported (8):
1. Since the mass cIf the sample is likely t o he small, it is ad visahle t o work with softer x-rays in order t o ensure reasonabe! abSor,ptiop ,_I . 2. uwing GO ( I ) , mghly transparent windows are desirable. 3. If longer wave lengths are used, particular attention must be paid to changes in the effective wave length. The (Or density) Of the be culatcd from the gas laws, which means that the pressure and the temperature must he known.
.
-
ULLKL~YU
METHYL CHLORIDE
To test the performance of the apparatus under the most favorable conditions, i t was decided to measure the absorption of methyl chloride a t a series of pressures in a ?ell capable of
*'
The symbols of (8)are used here where apphcable. the absorption equation is conveniently written:
lated from Equation 1 and were ohtai from tabulated values of absorption C O ~ L L U ~ ; U U SIUL wavelengths (1).
withstanding reduced pressures.
For gases,
where p is the density of the gas and 1 the cell length in centimeters. That beryllium is the best material for cell windows is clear (Figure 5, 8). Thin sheets of a suitahle plastic, such as polystyrene, me more easily obtained, but they will fail at pressures removed from atmospheric since they do not have the strength of beryllium. ORIENTING DATA
X-ray absorption measurements were made on severd of the em. long, and 3.8 cm. in diameter, with windows of polystyrene The results for five settings of the x-ray tube voltage are given in Table I; only one reading of the output ourrent, i, was taken at each setting. The second column of Table I contains io values corresponding t o the five voltage settings. These io values were calculated from the output currents for hydrogen (third column) because a measurement of i o in the evacuated cell could not he risked with the plastic windows. On this calculation, which was done by Figure 1. Metal Cell for Gas Analysis by Equation 1, p!, = 1.00 was used, this arbitrary procedure being X-Ray Absorption permissible because hydrogen is almost transparent. A . Mons1 cell Table I shows that setting 1, which gives the softest radiation, B . Valve is the best for analytical work, since it yields the greatest ratio C. Expansion tube (6000 to 1) between the output current for hydrogen and that for chlorine. Effective wave lengths have SetTable I. ExPI'oratory Experiments with Various Gasesa C been included in Table I t o WI: , HZ i show the changes that occur 1.52 2560 1.26 100 1.17 1 603,000 600,000 530 '0 1.47 320,000 1.59 258.000 4100 1.26 195 1.16 1.56 in this important variable 2 704,500 701.000 620 ,O 1.50 385000 o 1.37 48o:ooo 1.52 4oi;doo i . 4 5 7050 1.22 448 1.13 836,000 760 when one gas is substituted 3 S~Q,~OO 8600 1.21 598 1.11 904,400 soo.ooo 805 0. 1.45 520000 1.51 4 f o r another. These wave 3 i,oio.ooo ~ 0 0 5 , o o o 903 o 1.43 593:ooo 1.49 sob:ooo i . 4 3 11.ooo 1 . 1 8 930 1.10 lengths correspond to mass a ovtput ourrentsin miorosmpere, iffeotive FWB lengths Angstrams. absorption coefficients calcu0.010 em. thick.
86
NOVEMBER 1947 The results for two series of eupan&ionswith methyl chloride are given in Figure 2; the voltage across the x-ray tube was slightly different in the two series. ,The abscissas were calculated with the aid of Equation 2. The ordinates are logarithmic increments of the average output currents, twelve 0.601 individual readings, 5 seconds apart, being taken to establish each average. 0.501 Each series has been broken arbitrarily into three sections for convenience in plotting; the corresponding pressure limits are shown. The ordinate values given apply to the two lowest curves, but the scale of the graph is the same for all. For each curve, the radius of the circles represents l/100 of the pressure at the midpoint of the curve. The precision of the results in Fig# ure 2 is gratifying. Within each series, the gradual increase in slope O K ' 40 ' 60 ' do ' IO0 ' o ;i ' I40 ' I60 ' I60 ' 200 220 ' 4 0 -AP(rnrn Hg) with decreasing pressure is doubtless due to an increase in effective wave Figure 2. Methyl Chloride Expansion Results length. The following values of hf. Owing to alight differences i n settings, coincidence of t h e t w o s e t s of curves i s n o t t o be expected. were obtained by the use of aluminum foil (!ee 2, Equation 3) atter different This cell (Figure 1) waz made by cementing beryllium windows nunib:rs of expansion!: 1.17 A., n = 0; 1.17 A,, n = 6; (0.05 cm. thick) with Apiezon wax into a 60-cm. length of Monel 1.18 A., n = 11; 1.20 A,, n = 40; 1.28.4., n = 61 tubing, 1.0 cm. in internal diameter. Attached to the cell is a Measurements by the expansion method were also carried out glass tube closed by a stopcock to n-hich gas can be admitted on gases other than methyl chloride, with results very similar to from the cell through the valve for the purpose of reducing systematically the pressure of the gas in the cell. The equation those in Figure 2. governing thi? pressure reduction is: 0.90
d
,
LITERATURE CITED
(1
- a ) ' L=
P./P
(2)
where a is the fractional pressure change in a cell for a single expansion into the evacuated glass tube, and n is the number of expansions in which the pressure was reduced from p , its initial value, to p,,. The value a = 0.02890 was carefully'determined by making pressure measurements at the beginning and at the end of a series of n expansions.
(1)
Compton, A. H., and bllison, S. K., "X-Rays in Theory and Experiment," 2nd ed., Chap. Y I I , New York, D. Van Nostrand
Co., 1943. ( 2 ) Liebhafsky, H. A , , Smith, H. M,, Tanis, H. E., arid Winslow, E. H . , ANAL.C H E M .19, , 861 (1947). RECEIVED April 23, 1947. Presented before the Division of Analytical CHEMICAL and Micro Chemistry at the 111th Meeting of the AMERICXX SOCIETY, Atlantic City, N . J .
Determination of Small Amounts of Water in Gases and Liquids by Infrared Spectrometry A. F. BENNING, A. A. EBERT,
AND
C. F. IRWIS
Jackson Laboratory, E. 7. d u Pont de Nemours & Company, Wilmington, D e l .
A method is presented for the rapid determination of minute quantities of water in various solvents by measurement of the infrared absorption of a sample contained in a pressure absorption cell. Concentrations of w-ater as low as 1 p.p.m. can be determined in a number of commercial refrigerants.
A
S ACCURATE knowledge of the water content of a gas or
liquid is frequently desirable. In the case of refrigerants, this is of special importance because moisture in a refrigerating system mag result in one or all of the following effects: plugging of the expansion valves or capillary tubes by ice, corrosion of metallic parts, and copper plating on bearings and other rubbing surfaces,
These undesirable effects can be kept at a practical miiiiniuni if the water concentration of the refrigerant is not alloir-ed to eljceed 10 p.p.m. This critical control requires a rapid and accurate analytical method applicable to the fluorochloremethane refrigerants, particularly difluorodichloromethane. The conventional method for determining very small quantities of water in refrigerants is based on absorption of the water
