ANALYTICAL CHEMISTRY
142 n-BUTENE ANALYSIS IN ISOBUTENE SAMPLES
In special cases it is desired to determine a 1-butene and 2butene split for samples containing approximately 95% isobutene. Although the hydrogen chloride treatment described eliminates isobutene as such, the resulting tert-butyl chloride interferes seriously in the resolution of the small concentration of 1-butene and 2-butene. To increase the relative partial pressure of 1-butene and 2-butene compared to the partial pressure of tert-butyl chloride in the hydrogen chloride-treated sample, a simple partial condensation is made bv cooling the reaction bulb to -40' C. prior tointroduction to the mass spectrometer. .4t the reduced temperature and pressure existing in the reaction bulb practically all of the tertbutyl chloiide is condensed while only a relatively small amount of the butenes is dissolved by the liquid phase. I t was found that this process increased the butene concentration in the vapor phase by a factor of about 10. A sample may then be removed from the reaction bulb at -40" C. and introduced directly into the mass spectrometer. The mass spectrum vhich is obtained from the 'vapor in the reaction bulb at -40" C. (record C) is used in conjunction with the two mass spectra already described from the untreated sample and the hydrogen chloride-treated gas sample for computation of the analysis Records A and B are computed in the manner outlined in Method 2, with the exception that the more stable 57 peak is used as the base peak for tert-butyl chloride instead of the 77 peak. Record C is used only to indicate the relative amounts
of I-butene and 2-butenes by solving two simultaneous equations based on the 39 and 56 peaks. This ratio may then be applied to the results calculated from records A and B to complete the analysis. A typical analysis of a sample containing more than 95% isobutene which was treated by this latter method is shown in Table
IV. ACCURACY OF METHOD
To determine what advantage the method might have in reducing errors due to changes in sensitivity caused by fluctuations of the instrument, a mathematical analysis was made of a represrntatire mixture. Error breakdovns, calculated by an inverse method (1) for an assumed sensitivity fluctuation of 1% are shown in Table V. I t is apparent that the uncertainties for a mixture containing tert-butyl chloride are smaller than for the mixture in n-hich isobutene is present; hence greater accuracy should result .from the use of hydrogen chloride treatment than would be possible from the conventional mass spectrometer analysis. LITERATURE CITED
(1) Consolidated Engineering Corp., Pasadena, Calif., private com-
munication. (2) McMillan, ISD.ENG.CHEM.,AKAL.ED.,9, 511 (1937) (3) Washburn, Wiley, and Rock, Ibid., 15, 541 (1943). RECEITED J u n e 9. 1947. Presented before the Division of Petroleum Chemistry at the 112th Meeting of the AMERICAN CHEMICAL SOCIETY, New York
PI'. Y .
Determination of Sodium, Potassium, Aluminum, and Zinc In Derivatives of Carboxymethylcellulose E. P. SAR'ISEL, SPENCER H. BUSH', ROBERTA L. WARREN, AND A. F. GORDON* The Dow Chemical Company, Midland, Mich. &lethodsare described for the determination of sodium, potassium, aluminum, and zinc in derivatives of carboxymethylcellulose. They were developed on the basis of accuracy and rapidity of analysis for the application to research and control purposes.
T
HE rapidly increasing commercial importance of the metallic salts of carboxymethylcellulose makes desirable the development of analytical methods for the accurate determination of the specific metal ions contained therein. Procedures have been devised for the determination of sodium, potassium, aluminum, and zinc in their respective salts of carboxymethylcellulose. Rlost of the methods presented in this paper are satisfactory for the analysis of varying amounts of one element in the presence of the other elements mentioned. For example, small amounts of sodium are likely t o be found in other salts of carboxymethylcellulose and it would be useful to analyze for sodium and aluminum in B mixture of the sodium and aluminum salts of carboxymethylcellulose. In so far as possible, rapid methods of analysis have been developed, utilizing colorimetric and volumetric techniques. Considerable investigation was required t o find a suitable rapid method for the digestion of the salts of carboxymethylcellulose, in order that the cellulosic residue might be destroyed and leave the metallic constituents in a soluble state. Wet-ashing with a sulfuric acid-nitric acid mixture is commonly used for the digestion of organic material and is satisfactory for this type of 1 ?
Present address, University of Michigan, Ann Arbor, hlioh. Present address, Purdue University, Lafayette, Ind.
product, but it is time-consuming. Lindner (8, 9) and others (3) have reported that 3oY0hydrogen peroxide aids in the digestion of plant tissue. The use of 30% hydrogen peroxide subsequent t o a preliminary treatment with sulfuric acid greatly speeds up the digestion process. Dry-ashing for the destruction of cellulose residues is usually more rapid than n-et-digestion but suffers from the fact that at elevated temperatures insoluble oxides may be formed with some metals, which gives low analytical results. This difficulty may be overcome and most of the benefits of dry-ashing retained bv ashing at some moderate temperature such as 400" C. The ashing of the sample should be only partially completed and at a moderate temperature it should be sufficient to remove all of the volatile constituents and to reduce the bulk of the original sample to a carbonaceous residue. Subsequent to dry-ashing, the residue is taken up in concentrated sulfuric acid with continued additions of 30% hydrogen peroxide until the solution becomes clear. When digestion is complete, the clear liquid remaining in the calibrated 50-ml. Kjeldahl digestion flask (Figure 1) is diluted to the mark with distilled water and suitable aliquots are removed for metal analysis by the various procedures described belon. Khen it is knovin that sodium is present alone in an organic compound, a convenient method of analysis (7) is t o
V O L U M E 20, NO, 2, F E B R U A R Y 1 9 4 8 evaporate the sample with sulfuric acid and weigh as the sulfate. Sodium can be determined in the presence of other metals ,directly as the triple salt ( I , IS), sodium magnesium uranyl acetate. Potassium present in amounts up to ten times the amount of sodium does not interfere. Moderate amounts of magnesium, ammonium, calcium, barium, strontium, ferric, aluminum, chromium, zinc, and many other metal ions can be tolerated. Two methods are preM r n . 0D sented for the determination of aluminum, one a gravimetric procedure in which the aluminum is precipitated with 8-hv7I T 7 -~ drosyquinoline (6) and the I 50 m / mark other a c o l o r i m e t r i c method (11) involving the use of ammonium aurintricarboxylate (illuminon) to form a red colored I complex. The 8-hydroxyF quinoline procedure is well established and reliable, but time-consuming. The authors have modified the colorimetric Aluminon method from a proeedure which was designed Figure 1. Kjeldahl Flask for trace amounts t o one which gives satisfactory results on samples ranging up to 1270 aluminum. This is both rapid and convenient t o carry out. Potassium is a rather difficult element t o determine. The two common methods, both slow and tedious, involve precipitation of potassium as the chloroplatinate and the perchlorate. In 1941 Willard and Boyle ( I d ) developed a method in which potassium is quantitatively precipitated as the periodate. This precipitate is dissolved in a boric acid-borax buffer, potassium iodide is added, and the resulting free iodine is titrated with standard arsenite solution. Potassium mag be separated from aluminum, zinc, sodium, and calcium by this scheme of analysis. It is possible t o separate as little as 0.4 mg. of potassium from seventv times as much sodium. This procedure has been adapted to the wet-ashing technique and has proved both rapid and accurate for determining potassium in carboxymethylcellulose. Zinc is *easily determined in carboxymethylcellulose by reaction xyith a standard solution of potassium ferrocyanide to form potassium zinc ferrocyanide:
1, l i
+
2 K4Fe(C?\T)8 3 Z n + + +
KaZna [Fe(Cx)e]*-I- 6K'
SPECIAL R E 4 G E h T S
All reagents are of reagent grade unless o t h e r n G noted. Determination of Sodium by Magnesium Uranyl Acetate Procedure. It is best to make magnesium uranyl acetate up in 4liter quantities, since such a large amount is used. Dissolve 1200 grams of magnesium acetate tetrahydrate or 800 grams of anhydrous magnesium acetate and 120 grams of glacial acetic acid in distilled water and dilute to 2 liters. Heat on a hot plate to dissolve, stirring occzsionally to promote solution. Prepare another solution by dissolving 180 grams of uranyl acetate dihydrate and 120 grams of glacial acetic acid in distilled water, dilute t o 2 liters, and heat until in solution. Mix the two solutions, cool to 18" to 20" C. and hold there 1.5 to 2 hours. The solution should be cloudy with some precipitate. If clear, add 25 ml. of distilled water containing 1 gram of C.P. sodium chloride to form pieeipitate. Filter through a large funnel, using 32-cni. fluted filter paper. When paper clogs and filtration slows too much,
143 change filter papers. Keep the beaker below 20 a C., preferably a t 18" C., and do not allow solution in funnel to become too warm. Sodium Wash Solution. Mix 35 ml. of glacial acetic acid, 405 ml. of absolute ethyl acetate, and 460 ml. of absolute ethanol. Add magnesium uranyl acetate reagent to this until a precipitate forms a t 20" C. Filter. Preparation of Jones Reductor. To 300 grains of pure 20- to 30-mesh zinc add 300 ml. of 2 7 , mercuric nitrate (or chloride) and 1 to 2 ml. of concentrated nitric acid. Stir thoroughly for 5 to 10 minutes, decant solution, and wash 2 to 3 times by decantation Fill the reductor tube with distilled water, and add zinc slowly until column is completely packed. Wash with 500 nil. of distilled water, using gentle suction; leave reductor full of mater after Tvashing. The level of liquid should never drop below the zinc. Before adding zinc, place a plug of glass wool in reductor t o prevent zinc from plugging the stopcock. Place another plug a t top weighted with glass beads to keep zinc well packed. These columns will last for several months with reasonable care. In time the zinc dissolves and must be replaced. Diphenylamine Indicator. Dissolve 1 gram of diphenylamine in 99 grams of Concentrated sulfuric acid. Keep in a darkcolored dropping bottle. Determination of Aluminum by 8-Hydroxyquinoline Procedure. Dissolve 50 grams of 8-hydroxyquinoline in 880 ml. of distilled water and 120 ml. of glacial acetic acid. Agitate t o ensure solution, and filter into a dark bottle. This reagent is good for 2 weeks, after which it should be discarded. ALURIIKOX COLORIMETRIC PROCEDURE
hluminon Solution, 0.2%. Dissolve 0.2 gram of ammonium aurintricarbosylate in 100 ml. of distilled water. Standard Aluminum Solution. Dissolve 1.0000 gram of pure aluminum metal in dilute hydrochloric acid and dilute to 1 liter. Working Standard. Dilute 5 ml. of the standard to 500 ml. (1 ml. = 0.01 nig. of aluminum). Determination of Potassium. Aldehyde-Free Ethanol. Reflux 95% ethanol 2 to 3 hours after adding 0.5 gram of sodium hydroxide and 2.5 grams of silver nitrate per liter. Distill. Anhydrous Ethyl Acetate. Add magnesium perchlorate t o 99% ethyl acetate and distill. Add equal parts of the ethanol and the ethyl acetate for the reagent. Periodic Acid. Dissolve 1 gram of periodic acid in 3 ml. of water just before use. Boric Scid-Borax Buffer. Dissolve 40 grams of boric acid and 40 grams of sodium tetraborate in 1 liter of distilled mater. Determination of Zinc. Potassium Ferrocyanide, KrFe(CN)s. Dissolve 80 grams of potassium ferrocyanide analytical reagent in 100 ml. of distilled water and heat on a hot plate until solution is complete. Filter rapidly through a Buchner funnel, using medium filter paper, to remove insoluble residue from the reagent. Transfer immediately to a beaker and cool with occasional stirring. Complete cooling in an ice bath, stirring to assure small crystals. Filter through Biichner funnel. Wash with 2 to 3 10ml. portions of distilled water a t least as cold as the sample and preferably colder to prevent too great a loss of precipitate. Transfer the precipitate to a large watch glass, spreading it thin, cover Tyith another glass, and dry in 105' C. oven for 3 to 4 hours. This yields anhydrous potassium ferrocyanide. To prepare the reagent dissolve 18.78 grams in distilled water and dilute to 1liter. Standardize against zinc standard until four to six samples are tested. All should fall within a few tenths of a milligram of another. Procedure for Standardizing. Pipet 20-ml. aliquots of the zinc standard into a 400-ml. beaker. Make ub six samples. To the standard add 100 ml. of distilled water, 2 grams of ammonium sulfate, 3 drops of diphenylamine indicator, 5 drops of 1% aqueous potassium ferricyanide, and 15 to 20 ml. of 6 N sulfuric acid. Titrate to a green end point, proceeding very slowly wit,h constant agitation near end point. If the end point is passed, backtitrate with zinc standard to first permanent blue coloration. S o t e temperature of potassium ferrocyanide and milliliters necessary. Make the six runs. Determine the mean, and divide milliliters of titration into 0.005 x ml. of zinc standard for calculating the factor. Preparation of Zinc Standard. Weigh out 2.500 grams of powdered zinc (reagent grade), making a correction if necessary for the per cent insoluble in sulfuric acid. Dissolve in 50 ml. of dis. acid to allow tilled water and sufficient concentrated c . ~sulfuric the reaction to proceed smoothly but not too rapidly to completion. \Then all zinc is in solution, dilute to 500 ml. Diphenylamine Indicator, same as in sodium determination.
A N A L Y T I C A L C i~p. LVL I a
144
nL
a clean filtration tlask, and pass through the sodium solution a t a rate not to exceed 75 ml. per minut,e. Follow this with three 25-
3fPotassiu-m and ZJncAdded to Sodium C1BiboxymethylOelllllose (0.25 gram of sodium osrbmymethyleellulose digested) Potassium or zinc ZhD potassium in Sample Added. Addedb Found Recovered M.7. MO. Mo. MO. % .. 50.0 49.98 99.9 None None 100.0 99.78 99.8 None None Nome Nolle 99.9 150.0 149.96 49.8 99.7 None 49.9 None None 99.8 None 99.4 99.5 149.7 100.0 None 149.7 NOle Added BS ZnClr. b Added as KCI.
I
-1.
,111.
WILDIIL.b
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distilled water.
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