indicator. A second crucible is prepared similarly, the 0.5 ml. of sample being substituted by an equal volume of water. During titration the crucibles are illuminated by the filter-equipped tungsten lamp; extraneous artificial light should be excluded. The calciumfree blank then appears pink and the calcium-containing solution greenish yellow. The sample is titrated with 0.002M EDTA while being stirred continuously. The end point is reached when the last trace of yellow is gone from the sample and the latter matches the blank in hue and saturation. The titration requires less than 5 minutes. When several aliquots of a particular specimen are to be titrated, all the sample crucibles are prepared a t the same time and are shielded from dust until used. A single blank suffices for such a batch of successive titrations. However, because the color of the indicator in the crucible deteriorates in time, a given blank preparation should not be used with samples prepared appreciably later than itself. RESULTS
A standard solution about 0.001M in calcium was prepared by dissolving Iceland spar in hydrochloric acid. A portion of this solution was then diluted to 0.0004M and used to standardize a 0.002M solution of EDTA. Measured volumes of the 0.0004M calcium solution were diluted four-, sixteen-, and fortyfold, respectively. Samples of the three resulting solutions were analyzed for calcium; the results are shown in Table I. This investigation n-as undertaken to facilitate calcium determinations in small quantities of biological material. Hence, a check was made of the recovery of a known increment of calcium introduced into a sample of biological origin. A 4-ml. quantity of a solution of dry-
Table 1.
Ca present, y Ca found, y
a
Ave,. Y Std. dev., y Std. dev., % Recovery, yo Calculated t,o be 1.014 y.
Results of Calcium Determinations
Std. Ca 1.966 1.95 1.93 1.89 1.87 1.94 1.92 0.034 1.7 97.5
Yerve Ash FTith Ca addeda
Solution Aliquots 0.494 0.198 0.49 0.21 0.44 0.22 0.49 0.20 0.51 0.28 0.50 0.20 0.49 0.22 0.027 0.033 5.5 17. 98.4 112.
ashed lobster nerves in dilute hydrochloric acid was treated with Amberlite IRA-401 ion exchange resin (acetate form) and then made up to 10 ml. with rinses of the resin. [This procedure, following in principle that of Hahn et al. (S), was explored briefly as a means of eliminating possibly troublesome phosphate from samples to be analyzed for calcium. However, it has not been established that phosphate interferes with the titration.] Four 0.5-ml. samples from this resin-treated solution were analyzed for calcium. To a second 4-ml. portion of the same nerve ash solution, 0.5 ml. of the nominal 0.001M standard calcium solution was added. This was also passed over Amberlite and analyzed in the same way as the first portion. The results of these tests are given in Table I. The recovery of the added 1.014 y of calcium was calculated to be lOO.S%, with a standard deviation of 0.043 y, or 5.2%. The standard deviation of an individual determination, calculated from the pooled data of Table I, is 0.032 y. MAGNESIUM INTERFERENCE
Magnesium did not interfere when 0.5-7 calcium aliquots were titrated in the above manner, even when a hundredfold molar excess of magnesium
...
...
4.04 4.10 4.03 4.07
5.03 5.13 5.07 5.10
4.06 0.032
5.08 0.043
...
...
...
...
wis present. Only when the magnesium content was so high as to give visible precipitates upon addition of hydroxide did recovery of calcium appear significantly reduced. While these cases were not probed further, it seems plausible that significant quantities of calcium might be occluded in such precipitates. ACKNOWLEDGMENT
The authors acknowledge gratefully the helpful interest of Julian M. Tobias and of Thomas B. Coolidge. LITERATURE CITED
(1) Buckley, E. S., Jr., Gibson, J. G., 11, Bortolotti, T. R., J . Lab. Clin. Med. 38,
761 (19511.
(2) Dikhl, H., Ellingboe, J. L., ASAL.
CHEW28, 882 (1956). (3) Hahn, R. B., Backer, C., Backer, R., -4nal. Chim. Acta 9, 223 (1953). (4) Keynes, R. D., Lewis, P. R., J . Physiol. (London) 134, 399 (1956). (5) Kingsley, G. R., Robnett, O., Am. J . Clzn. Pathol. 27, 223 (1957). (6) Natelson, S., Penniall, R., A S A L . CHEM.27, 434 (1955). (7) Tucker, B. M., Analyst 82, 284 (1957). RECEIVED for review July 9, 1958. - 4 ~ cepted October 21, 1958. Work supported in part by a grant from the United States Public Health Service and in part by a grant from the Wallace C. and Clara A. Abbott Memorial Fund of the University of Chicago t o Julian M. Tobias.
Solubility of Cesium Metaperiodate and its Coprecipitation in the Determination of Potassium as the Metaperiodate SIR: Recently Jentoft and Robinson (2) reported the determination of potassium by precipitation of potassium metaperiodate from a 3301, (by volume) ethyl alcohol solution buffered to p H 3.5 with lithium acetate, followed by iodometric estimation. The present authors have investigated the possible application of this method to the determination of cesium, because no very successful method is available a t present. Measurement under the conditions demanded by this method 474
0
ANALYTICAL CHEMISTRY
has shown that the solubility of cesium metaperiodate is too large for a successful determination. Following the procedure of Jentoft and Robinson (1) it was established the minimum solubility occurs in a 20y0 ethyl alcohol medium. In such a solution 2.79 mg. of cesium as the metaperiodate are soluble in 5 ml. of solution a t 0" C. and buffered to pH 3.5. Further details may be found in a thesis ( 3 ) . The degree of the interference of cesium in the determination of potas-
sium by the metaperiodate method has not been previously ascertained. Potassium solutions containing cesium in varying amounts were analyzed by the method of Jentoft and Robinson. Samples containing I, 5, 10, and 25 nig. of potassium were prepared with 1, 2.5, and 5 mg. of cesium present for each concentration level of potassium. Solutions containing 10 mg. or less of potassium were also analyzed with only 0.5 mg. of cesium present. I n earlier experimental work on the solubility of cesium
MILLIGRAMS OF CESIUM PRESENT
Figure 1 . Determination of potassium metaperiodate in the presence of cesium nitrate
metaperiodate, it n a s shown that 3.11 mg. of cesium are soluble in 6 ml. of 33y0 ethyl alcohol, 0.1111in respect to excess periodic acid. nhich are the solution conditions for the potassium determination. The concentrations of cesium selected for investigation are thus above and beloJy this solubility level. The amount of cesium coprecipitated, determined as the amount of periodate found in excess of the amount of potassium metaperiodate present, but reported as milligrams of potassium, is plotted as ordinate (Figure 1).
The results are somewhat variable, as it is difficult to maintain constant the actual conditions of precipitate formation, but it is apparent that the amount of coprecipitation is a function of the concentration of cesium present. Although 3.11 mg. of cesium should be soluble under these conditions, through coprecipitation with potassium metaperiodate less than this amount remains in solution. Five milligrams of cesium, which reported as potassium are equivalent to 1.47 mg., are completely carried down Ivith potas-
sium metaperiodate eyen when as little as 5 mg. of potassium are precipitated. When 2.5 mg. of cesium are present, which normally should be completely soluble, part or all of the cesium is precipitated, depending on amount of potassium metaperiodate precipitating. The amount of coprecipitation with potassium metaperiodate is dependent upon the rate of precipitate formation as well as the amount of cesium present. As is true of coprecipitation in general, the slower the rate of precipitation the lesser the amount of coprecipitation. At lower concentration levels of potassium, precipitation takes place slowly and little coprecipitation oceurs. LITERATURE CITED
(1) Jentoft, R. E., Robinson, R. ANAL. CHEM.26, 1156-8 (1954). (2) Ibid., 28, 2011-15 (1956).
J.,
(3) Snyder, E. R., "A Study of the Determination of Cesium as the Metaperiodate and the Interference of Cesium
in the Determination of Potassium Metaperiodate," M. S. thesis, University of Kashington, Seattle, Wash, 1956. ELOISESNYDER REXJ. ROBINSON Chemistry Department University of Kashington Seattle 5, Wash.
High Temperature Gas Chromatography of Aromatic Hydrocarbons SIR: A gas chromatograph capable of successful operation a t temperatures up to a t least 445" C. has been evaluated in these laboratories. The instrument used for these high temperature studies was a n especially modified design based on the Loe Engineering Co. (237 North Fairoaks -4ve.. Pasadena, Calif.) Chromat-0-Flex. A high resistance filament-type four-element detector was placed in the coluiiin oven. Other important modifications include the use of 2-inch-thick Maranite insulation, two 300-13 att strip-type base load heaters controlled by a variable autotransformer. and one 250-matt coil-type trim heater controlled by a Hallikainen Thermotrol Model 1053A n-ith platinum resistance thermometer detector. Some exploratory studie- in high temperature gas chromatography of aromatic hydrocarbons performed n ith this instrument have led to the divoi-ery of a new type of immobile phase material with good high temperature ztahility. SCREENING OF IMMOBILE PHASES
Probably the most important single
hindrance to the development of high temperature gas chromatography has been the lack of suitable immobile phase materials. Several recent papers (1-3) have reported the suitability of silicone grease and silicone gums as high temperature-immobile phases. Ogilvie, Simmons, and Hinds (6) report the use of asphaltenes for aliphatic hydrocarbon separations. Such materials should possess both good partitioning ability and high temperature stability and preferably be readily available a t reasonable cost. K i t h these factors in mind, a number of materials n-ere selected and initially tested at 300" C. These are shomm in Table I, along with their relative ratings as partitioning agents for a test mixture consisting of biphenyl, and 0-, m-, and p-terphenyl. The partitioning ability is expressed in terms of relative peak separation, S,relative peak sharpness, &, and resolution, R, as given by Jones and Kieselbach ( 5 ) . Because the m- and p-terphenyl isomers are the most difficult to separate, the calculations were done on this pair in all cases. Of the commercially available materials,
only the asphaltenes afford complete separation of m- and p-terphenyl. Also, in all cases, additional experimental work with higher boiling aromatic hydrocarbon samples established the loss of peak symmetry due to decomposition and/or elution of these immobile phase materials. The behavior of the asphaltenes was rather unexpected on the basis of information cited by Ogilvie, Simmons, and Hinds (6). Initially the material was an excellent partitioning agent, but it deteriorated after 48 to 50 hours. Several factors are possible explanations of the apparent differences. The origin of the asphalt may influence its performance. Ogilvie et al. make no statement as to stability for prolonged periods, and we did not observe tailing prior to 48 hours a t 300" C. We used the total asphaltene fraction and this could account for the different results. [Simmons ( 7 ) disclosed that less than 20% of the pentane-insoluble fraction was used, and that column life mas several weeks at 320' C.] When tailing developed at 300" C., we reasoned that perhaps volatiles were VOL. 3 1 , NO. 3, MARCH 1959
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