power supply limited the standing curtent which could be used. The value selected (ca. 1.5 ma.) represented a compromise between high current (within the normal regime) and loss of output when the external resistor was reduced in value. With discharges running a t about 250 volts it became possible to supply the tubes and the impedance changer required to feed the recorder from the same stable 400-volt source instead of from two separate power packs. This was not only more economical but gave greater stability. A disadvantage of the lower supply voltage was the increased probability of extinction of the discharge by very high vapor concentrations and subsequent difficulty in relighting. The use of closely spaced electrodes improved the performance but did not eliminate irregular response to large peaks. In this case discontinuities were associated with erratic movements of the glow and departure from the normal regime. The use of pentodes, mentioned above but untried as yet, may improve the behavior in this respect. The data on sensitivity to paraffin hydrocarbons (Table I and Figure 6) and baseline noise (Figure 5) show a satisfactory signal-to-noise ratio for concentrations down to 1 in 10,000,000 and indicate that the detector is capable of the analysis of mixtures in quantities between the microgram and millimicrogram levels. Calibration is required but the linearity of response to paraffins appears to be good at levels below a few micrograms. With regard to the variation of response with molecular weight of hydrocarbons (Figure 6), little can he said as yet. Although the ionization potential of the gas or vapor is usually considered to be an important factor governing the potential drop in a discharge, it is evi-
dent from the data so far obtained that it is certainly not controlling: A progressive rise in molar response was observed between Ca and CS, whereas the ionization potentials show little change beyond C4. Corresponding data on the tendency to electron attachment of hydrocarbons and their effect on the work function of metals is meager, but these factors are no doubt important in determining the response of this type of detector. An instrument consisting essentially of a vibrating reed electrometer has been used to measure changes in contact potential of metal surfaces resulting from the presence of vapors in a gas stream (3,8),but failed as a detector for elution gas chromatography because of slow desorption of the vapors. It is interesting that no serious difficulty of this type has been encountered with the discharge tubes, although some tailing of peaks has been observed; no carbonaceous deposits accumulated in several weeks of daily use. A possible explanation for the difference between the two detectors is that the discharge itself tends to clean the surface and assists removal of adsorbed materials. FUTURE DEVELOPMENTS
The three most striking characteristics of this detector are its simplicity, its high output level, and its great sensitivity. Future developments which take advantage of these points can be foreseen. Firstly, it should be possible to design sensitive detectors which are robust both in the detector and in the ancillary equipment. Metal equipment with st& bility and sensitivity comparable to the glass apparatus described has been constructed. Secondly, the low concentration levels a t which the detector operates will
permit most interesting developments in gas chromatography, such as the use of new stationary phases with very low absorptive capacities. Increased column efficiencies and the possibility of analyzing materials of low volatility a t r e l a tively low temperatures can be expected. Results already indicate that mass transfer effects in the stationary phase are much reduced. For example, the columns used in this work maintained their normal efficiency although they were operated under conditions of low pressure and high gas velocity which, with existing detectors, would have been inefficient. A direct application of practical use is the analysis of hydrocarbons present in low concentration in hydrogen or other gas streams. ACKNOWLEDGMENT
The author thanks the chairman and directors of the British Petroleum Co. for permission to publish this paper. LITERATURE CITED
(1) Emeleus, K. G., ‘‘Condu$ion of Electricity through Gases, p. 14, Methuen & Co.. Ltd.. London. 1951.
Francis, V. J., Jenkins, H. G., Repts. Progr. in Phys. 7, 230 (1940). Griffiths, J., James, D., Phillips, G., Analyst 77, 897 (1952).
Harley, J., Pretorius, V., Nature 178, 1244 (1956).
James, A. T., Martin, .4. J. P., Biochem. J . 50,679 (1952). Lion, K. S., Rev. Sci. Instr. 27, No. 4, 222 (1956).
Lunt, R. W., von Engel, A,, Repts. Progr. in Phys. 8,338 (1941). Phillim. G.. J. Sci. Instr. 28, 342 RECEIVED for review August 26, 1957. Accepted February 8, 1958. Division of Analytical Chemistry, 132nd meeting, ACS, New York, N. Y., September 1957.
Determination of Germanium by the HeteropoIy Blue Method ELWOOD R. SHAW and JAMES
F. CORWIN
Department of Chemistry, Antioch College, Yellow Springs, Ohio ,The cause of the up to 100% variation in absorbancies of identical samples of germanium dioxide, analyzed by the heteropoly blue-molybdate complex method, was traced to different interpretations of the literature recommended time interval between the addition of the molybdate and the reductant. The optimum interval was selected by a time study, and
1 3 14
ANALYTICAL CHEMISTRY
the best acid and molybdate concentrations were determined. Different operators obtained results which agree within 5%.
N
methods of colorimetric analysis using organic reagents have been deireloped, but the most widely used and convenient method for routine analysis is the molybdenum blue EW
method. Several modifications of the method reported in the literature have been summarized by Krause and Johnson (7‘). Most publications refer to the instability of the yellow molybdogermanic acid complex; however, no specific study was made of its rates of formation and decomposition under the conditions of the recommended procedures. The literature (1, 8) rec-
0.400
3
oloo~
f
O 7 5 m l 4N H,S04 2 m l 5 %
4 @ I mi 4N H2SG+
o1
-~
.
4 m l 5%
I'
---L--pp-
100 200 300 400 500 600 Minutes between Molybdate and Reductant
700
Figure 1. Effect of slight variations in acidity and amount of molybdate
111 0
ommended that the yellow complex be promptly or immediately reduced to the heteropoly blue complex. This led to variations in the results as high as 100% if the glassware was changed or different analysts attempted to use the same calibration curve. As somewhat reproducible results had been obtained using the procedure of Boltz and Mellon (f), the shape of the formation-decomposition curve of the )-ellow molybdogermanic acid complex with time was determined. The effect of variations in pH on the maxiinurn of the curve was also determined. A modified procedure is recommended which gave results on 100 to 200 routine analyses for germanium dioxide that agree within 1 to 3%. APPARATUS AND REAGENTS
A Beckman Model DU spectrophotometer with 1-cm. quartz cells, which checked to within 10.005 absorbance unit, was used with distilled water in the reference cell. All solutions and reagents were stored in polyethylene bottles and all distilled water was run through a Crystalab Deeminizer (Crystal Research Laboratories, Inc., Hartford, Conn.) before use. Reagents were of analytical grade unless otherwise stated. Germanium dioxide, 99.99% pure (A. D. Mackay Co., Inc., New York), was used as a primary standard. The stock solutions contained 0.9660 and 0.3228 gram of germanium dioxide per liter. The germanium dioxide mas dissolved with the aid of dilute sodium hydroxide and then neutralized with 4N sulfuric acid. The air-dried commercial germanium dioxide, checked gravimetrically using the method of Hecht and Bartelmus ( Q , contained 99.89% germanium dioxide. Ignition of a commercial germanium dioxide sample to melting caused a loss of 0.088%, which is at-
1-
I
___
I
4 Minutes between Molybdate and Reductant 2
3
_.-I
5
Figure 2. Effect of time between addition of molybdate and reductant on absorbance
tributed t o absorbed water. For comparison, sodium germanate was prepared by fusing 0.9912 gram of germanium dioxide with a carefully measured equimolar amount of sodium carbonate to a clear melt in a platinum crucible. An excess of 200 p.p.m. of carbonate mas found by Kitson and MelIon (6) to cause an error of 2%. Ammonium Molybdate. A 5% aqueous solution of ammonium molybdate [(-1H~)J10~02~.4H~O] was made by dissolving 5 grams of the salt in 80 ml. of warm distilled water. After cooling, 2.8 ml. of concentrated sulfuric acid was added, followed by water to make 100 ml. Reductant. A 2y0 stock solution of ferrous ammonium sulfate, FeSO4.(NH4)2S04.6H20, was prepared by dissolving 5 grams of the salt in 250 ml. of distilled water containing 1 ml. of 4N sulfuric acid. The reductant was mixed just prior to use in the ratio of 5 ml. of the 2oj, stock to 20 ml. of 4N sulfuric acid. The molybdate and reductant may be kept 2 t o 3 weeks without noticeable deterioration if stored in a refrigerator. Because of temperature effects, the molybdate should be brought to room temperature before use. RECOMMENDED PROCEDURE
Dissolve the sample, using any method necessary to assure complete solution. If separation from interfering ions is necessary, distillation (2, 4 ) may be used. Measure the aliquots so that the final concentration a t the end of the analysis will be 0.1 to 0.3 mg. of germanium dioxide per 100 ml. Neutralize an appropriate aliquot in a 100ml. volumetric flask, using silica-free
ammonia water or dilute sulfuric acid. Dilute to the mark and mix thoroughly. Transfer an appropriate aliquot of this to a 100-ml. volumetric flask containing 2.0 ml. of 4N sulfuric acid, and wash d o m the sides with enough distilled water to make 36 ml. Add 8 ml. of 5% ammonium molybdate reagent and mix thoroughly. In 1.0 f 0.2 minute add rapidly 50 ml. of the reductant, dilute to the mark. and mix well. After 20 to 30 minutes, pour out the solution from the neck of the flask, and read the absorbance of the remainder at 825 mu.
-4s no significant change in absorbance could be attributed to the order of adding acid, germanium-containing solution, and water, they were added in that order. Larger flasks were later used, because Kenyon and Bewick (6) noted that agreement was considerably improved with larger ~ o l u m e s of solution in a similar analysis. DISCUSSION AND RESULTS
Figure 1 shows the general shape of the curves when absorbance is plotted against the time interval between the addition of molybdate and reductant for three concentrations of germanium dioxide. Although the readings newr became constant, the slope of the curves after the maximum was essentially the same. This similarity indicated that reproducibility could be improved if an optimum time interval were used. Reproducible results would be unlikely if too short an interval is used. VOL. 30, NO. 8, AUGUST 1958
1315
Figure 2 shows the results of varying the acidity and amount of molybdate reagent. The 10% molybdate solution was prepared without sulfuric acid ( 1 ) ; the 5% solution contained 1 equivalent per ml. Curves 1, 2 , and 3 of Figure 2 represent the result of varying the acidity before addition of the molybdate reagent, with the amount of molybdate held constant. At the higher pH the maximum n a s reached 0.2 to 0.4 minute after addition of molybdate; however, the results are dficult to reproduce because of the sharp and variable peak. An increase in the amount of molybdate reagent, with the acidity held constant, also increased the rate of approach to the maximum, but the sharp peak was not present (curves 1 and 4, Figure 2). As all curves were stabilized in the 1-minute region and small differences in p H had minor effect, 1 minute was selected as the best time interval. The effects of large excesses of molybdate or of higher acidities on the formation of the yellow molybdogermanic acid complex noted by Kitson and Mellon (6) also apply to the forniation of the blue complex. Color intensities were very low and variable
Table I. Determination of Germanium Dioxide in Typical Samples
Concn.,
hIg. GeOg/ 100 M1. 0.0483 0.0965 0.104 0.106 0.145 0.145 0 193
o.24i 0.290 0.338
70
Error 3.4 - 4.3 1.5 2.8 - 0.6 0.6 - 2.7 - 0.06 - 6.8 -10.7
+ + + +
if the solution mas neutral to litmus when the molybdate was added. Above p H 7, the intensities were again high, but results were not reproducible in this range. The readings did not vary significantly when samples contained equivalent germanium dioxide, from either pure germanium dioxide or carefully prepared sodium germanate. Under the conditions recommended in the general procedure, the optimum concentration of germanium diovide at the end of the analysis is between 0.1 and 0.3 mg. per 100 ml. of solution.
The results were variable below 0.1 mg. per 100 ml. and fell off rapidly above 0.3 mg. per 100 ml., with an error of -10.7% a t 0.338 mg. per 100 nil. (Table I). LITERATURE CITED
(1) Boltz, D. F., bfellon, M. G., ANAL. CHEM.19, 873 (1947). ( 2 ) Geilman, LV., Brunger, K., Biochem. Z. 275, 375 (1933). Xikro( 3 ) Hecht, F., Bartelmus, chemie ver. Mikrochtm. Acta 36/37. I , 466 (1951). i 4 ) Hoffman, J. I., Lundell, G. E. F., J . Research iVatl. Bur. Standards 22 , 465 (1939). Kenyon, 0. A., Bewick, H. A., ANAL. CHEM.25, 146 (1953).
e.,
Kitson, R. E., Mellon, M. G., IKD. EKG. CHEM.,A 4 ~ED. ~ 16, ~ . 128 (1944).
Krause, H. H., Johnson, 0 . H., ANAL. CHEM.25, 134 (1953). Snell, F. D., Snell, 0. T., "Colorimetric Methods of A4nalvsis." Vol. II, 3rd ed., p. 233, 'i'an"Sostrand, Yew York, 1949. RECEIVEDfor review July 15, 1957. Accepted February 24, 1958. Supported by U. S. Air Force through the Air Force Office of Scientific Research, Air Research and Development Command, under contract No. A F lS(600) 1490.
Determination of Traces of Water in Hydrocarbons in Gasoline Boiling Range Sample Handling and Interferences J. WEST LOVELAND and THOMAS B. WEBSTER' Sun Oil Co., Marcus Hook, Pa. CHARLES P. HABLITZEL and GEORGE W. REED Sun Oil Co., Toledo, Ohio )Reliable water contents in the part per million range in hydrocarbons in the gasoline boiling range can b e obtained by titration with Karl Fischer reagent. Contamination by adsorbed and atmospheric moisture is avoided in the recommended technique by using ovendried glass sample bottles and neoprene gaskets and by making transfers of sample into the closed titration vessel with a hypodermic syringe. Mercaptan interference is corrected for by determining apparent water before and after drying the sample on a column of Drierite. For naphtha samples from the same crude source, a constant correction factor for mercaptan interference can be applied;
13 16
ANALYTICAL CHEMISTRY
this considerably reduces the time required for a water determination.
I
of reformer unih for producing aromatics or high octane gasoline, the amount of water present in the naphtha feed stocks is important because of possible deactivation of catalyst. Generally, water contents are in the range of 10 to 50 p.pm Of the various methods available for determining water in naphtha, the Karl Fischer technique (3) is the most suitable, Hanna and Johnson (5) determined water contents of hydrocarbons by first extracting the water with dry ethylene glycol and titrating the glycol N THE OPERATION
extract with Karl Fischer reagent. Most of their work was on samples containing more than 50 p.p.m. of water. Peters and Jungnickel ( I d ) extracted water-saturated hydrocarbons with glycol and titrated in the same vessel. Wiberley (15) used a Karl Fischer micromethod with color end point in a two-phase solvent system for gasoline and kerosine. In most cases a singlephase electrometric titration is preferable. The literature is meager on t8hesubject of the effect of sample handling on the !determination of water in hydrocarbons 1 Present address, Esso Research Ltd., England.
