_ _ _ _ _ _ _ _ _ ~
EDTA under the conditions of the procedure. Trivalent iron is also masked with this reagent in amounts up to the molar content of zinc present. I n amounts greater than this, high results are obtained. To prevent the interference of iron, 2,4-pentanedione is added. The 2,Cpentanedione will mask iron in the absence of fluoride but the dark red color of this chelate prevents observation of the end point. The complex formed between iron, 2,4pentanedione, and fluoride is yellow and thus allows observation of the end point color change from orange to green. I n the presence of 2,4-pentanedione and fluoride, the iron content can be 20 times that of zinc without interference. Higher amounts have not been tested. The zinc cannot be back-titrated with 0.01hl Zn+2 using ;an excess of EDTA if Fe+3, A l + 3 , or Ti+' are present using the above procedure. When Ca+2,Mg+:!, and F- are present, Zn+Zcannot be titrated directly in the absence of Al+3. A back-titration of excess EDTA with Zn+2 yields good results. This is attributed to the precipitation of CaZnR and MgZnF4 because low results and unstable end points were obtained in the direct titration. The back-titration (cannot be used as a general method because Fe+3, Al+3, and Ti+4 which are almost always found in rubber products interfere. By adding Al+3 before the titration all interferences from Ca+2 and Mg+2 are prevented, because of the greater solubility of the zinc salts than Ca3(AlF& and M a (A1F6)2. Lead ions interfere by reacting slowly with EDTA during the titration which causes poor and unfitable end point and
Table I.
~
Determination of Zn+* in Rubber Using NH4F Only as Masking Agent
No. of runs 3 6 6
6 6 3
Table II.
CaO
~~
FetOI
Contents, % TiOs
1.24 1.13
1.13 1.12
1.12
ZnO found, %
Std. dev.
1.01 1.06 10.2 10.2 10.1 10.0
1.01 1.07 10.2 10.0 10.0 10.0
0.00 0.01 0.06 0.06 6.00
Determination of Zn+2 in Rubber Using the Procedure Presented
MgO
Present, 90 TiOt Fe20s 1.12
8.48
ZnO
1.24 1.12
5.92
high results. Lead can be removed by precipitation as PbS with hydrogen sulfide (ASTM, D297-61T). Sulfide ion interferes with the dithizone indicator but can be removed by oxidation with bromine to elemental sulfur and reduction of excess bromine with a small amount of sodium sulfite or hydroxylamine hydrochloride. Rubber samples to which known amounts of zinc oxide were added were analyzed for zinc using ammonium fluoride alone as masking agent. The results are shown in Table I. Additional samples were analyzed using the procedure as written. The results are shown in Table 11. The relative standard deviation was 1.0%. Each sample was analyzed in duplicate.
ZnO
ZnO found, %
Recovery, %
1.06 10.0 2.29 1.77
1.10 9.98 2.29 1.76
104 99.8 100 99.5
LITERATURE CITED
(1) Am. SOC. Testing Materials, D29761T, sec. 40f (1964). (2) . , Blenkin. J.. Trans. IRZ 40. T123 (1964). (3) Macdonald, A. M. G., Analyst 86, 3 (1961). (4) Miksch, R., Hinteneder, L., Kautschuk. Gummi 15, WT358 ,( 1962). (5) Milner, G., "The Principles and Application of Polarography, ' p. 440, Longsmans, Green & Co., New York, 1957. (6) Roberts, C. M., Trans. Inst. Rubber Znd. 33, 97 (1957). T. L. HUNTER ,
I
Research Division The Goodvear Tire & Rubber Co. Akron, OGo 44316 PRESENTED at the Division of Rubber Chemistr ACS, meeting in Miami Beach, ay 4-7, 1965.
2
Detection of Gases Based on Entropy Considerations SIR: The ( W ) v d u e for a gas is the number of electron volts of energy required to create an ion pair in the gas. Recently Lovelock pointed out that the ( W ) value for gases could be exploited for the detection 'of gases in air (9). He indicated that this property has not previously been exlploited for detection purposes. This is not precisely true because (W) values are one variable which determines response of an ionization chamber or a proportional counter. Saturation currents for pure gases are a linear function of the pressure of many gases (6) and the slope of the saturation current us. pressure plot is related to the (W) value for the particular gas. The alphameter is based on the difference in the slopes of saturation current us. pressure plots and
thus on the ( W ) value (b), although i t was not called a ( W ) value detector. Thus, the alphameter has been used to measure the composition of binary mixtures of gases at known pressures. It has been used for the analysis of mixtures of hydrogen and oxygen, carbon dioxide and oxygen, and nitrous oxide and nitrogen. The alphameter utilizes a source of 0.5 mc. of radium. Lovelock utilizes a source of 200 mc. of tritium in his ( W ) value detector, but suggests that an CY source would also be suitable. Lovelock indicates, however, that his ( W ) value detector is independent of temperature and pressure changes. A number of gas mixtures have been studied in a parallel plate ionization chamber with a Pu2*9 (Y source. A linear function of the
reciprocal of ( W ) value vs. an empirical function of pressure has been derived (11).
I n a previous paper (12) pulse attenuation data were reported for a number of gases and gas mixtures mixed with 1.3% butane-98.7% helium. An attempt has been made to correlate the pulse attenuations with the known properties of the gases, ( W ) being one of these. The correlation between ( W ) values has not proved to be as fruitful as that between pulse attenuations and the absolute entropy in the hydrocarbon series. EXPERIMENTAL
The apparatus has been described previously (12). A new cylinder of the nucleonics gas 1.3% butane-98.7% VOL 37, NO. 1 1 , OCTOBER 1965
1437
helium was obtained and all data were taken using the first 25% of the gas in the cylinder to minimize errors caused by the changing composition of the gas. The hydrocarbons used in this study were all of 99% purity and were obtained from The Matheson Co., Inc. No attempt was made to purify the gases. RESULTS
Pulse amplitudes in the Geiger region of gas amplification were normalized to 100% for the butane-helium mixture and pulse attenuations were expressed as a per cent of the normalized value. The ( W ) values for each of the hydrocarbons were obtained from tabulated values (8). A general downward trend was observed when per cent pulse attenuation was plotted us. ( W ) for the particular gas at a constant amount injected. The correlation with standard entropies, So, however, is rather startling. Linear plots were obtained for methane, ethane, propane, and n-butane when per cent pulse attenuation was plotted us. So of each gas. The plots of per cent pulse attentuation us. So are shown in Figure 1. Similar linear plots were obtained when using other cylinders of the butane-helium mixture, the only difference being in the magnitude of the pulse attentuation for a specific gas. Composition of the counting gas determines the actual per cent pulse attenuation for each gas. As a cylinder of counting gas is exhausted, the composition of the gas changes. If all measurements are taken within a short time interval, the same relative order of pulse attenuations is obtained. Absolute values for pulse attenuation require precisely controlled counting gas composition, a variable which cannot be controlled with gas mixtures from a cylinder. DISCUSSION
Many attempts have been made to relate the performance of gas-filled radiation detection devices to the value of ( W ) of the gas filling. These values represent the contribution to energy loss by excitation,. primary ionization, and secondary ionization caused by a delta ray of kinetic energy greater than the ionization potential of the gas ( 4 ) . Because ( W ) values appear to be relatively independent of the energy of the ionizing particle, the discrepancies in values reported in the literature for specific gases may be caused by the
1438
ANALYTICAL CHEMISTRY
ABSOLUTE ENTROPY, S*
Figure 1 . Per cent pulse attenuation vs. entropy
presence of impurities in the gases which were investigated or to the uncertainty in the measurement. The presence of impurities is of particular importance if the gas has a metastable state which is excited by impurities. Numerous examples of this factor have been reported ( I , 2, 7, 9, IO). Determination of ( W ) values is made by ionization chamber and proportional counter measurements. Diethorn has derived an expression relating the dimensional parameters of a proportional counter to the work done on an electron between each ionizing event (3). The relation of ( W ) values to the gas amplification in a proportional counter has been suggested. As a chamber is operated at higher gas amplifications, the apparent ( W ) value decreases because photoionization plays a progressively more important role. Whether a gas is a good proportional gas with a long proportional plateau or a good Geiger gas is determined by the impor \Lance of the photoionization process. In the Geiger region of gas amplification the pulse amplitude is the same regardless of the source of ionizing radiation because the discharge spreads completely over the anode by a photoionization mechanism. In the process of photoionization, photon energies are order of magnitude less than those of the electrons accelerated by the high voltage field in the counter. As the gas
mixture becomes richer in molecules with increasing complexity, more modes of excitation become available and excitation takes place at the expense of ionization. This manifests itself as a pulse attenuation which is a linear function of the entropy of the molecule if one considers a homologous series of compounds. Entropies at 25” C. are not representative of the actual entropy of a gas in the counter; however, the same general relationship should hold for the gases with the entropy they possess in the discharge. The gas amplification is thus decreased as a linear function of the entropy. The same linear relation holds if one plots the pulse attenuation us. the number of degrees of freedom of the molecule. The values for the entropies of the gases are handbook values (6). Tabulated ( W ) values are less precise than the thermodynamic values listed for the hydrocarbons. This correlation is strictly empirical and may be fortuitous; nevertheless, it is reasonable until more information is available. A detailed investigation of the hydrocarbons through C-10 is in progress and will be the subject of a later communication. LITERATURE CITED
( 1 ) Bertolini, G., Bettoni, M., Bisi, A., Phys. Rev. 92, 1586 (1953). (2) Bertolini, G., Bettoni, M., Bisi, A., Nuovo Cimento 11, 458 (1954). (3) Diethorn Ward, “A Methane Proportional kounter System for Natural
Radiocarbon Measurements,”
NYO-
6628, Office of Technical Services, Department of Commerce, Washington 25, D. C., 1956. (4) Fano, U., Phys. Rev. 70, 44 (1946). (5) Gimenez, C., Labeyrie, J., J. Phys. Radium 12, 64A (1951). (6) “Handbook of Chemistry and Physics,” C. D. Hodgman, ed., Chemical
Rubber Publishing Co., Cleveland, Ohio, 1964. (7) Jesse, W. P., Sadauskis, J., Phys. Rev. 88, 417 (1952);,
(8) Lind, S. C., Radiation Chemistry of Gases,” p. 274, Reinhold, New York, 1961. (9) Lovelock, J. E., ANAL. CHEM. 37, 583 (1965). (10) Melton, C. E., Hurst, G. S., Bortner, T. E., Phys. Rev. 96, 643 (1954). ( 1 1 ) Moe. H. J.. Bortner. T. E.. Hurst.
ARTHUR F. FINDEIE FREDERICK W. WILLrms University of Alabama School of Chemistry University, Ala.
