ANALYTICAL CHEMISTRY
1184 Table 11.
Influence of Aluminum Content on Accuracy and Precision Aluminum, hlg.
Present 4.46
Av. d e r . Std. dev. Coefficient of variation
.4v. 8.92
AT. dev. Std. dev. Coefficient of variation .4v. 22.30
Av. dev. Std. dev. Coefficient of variation Av.
Found 4.47, 4.46, 4.45. 4.45 0 . 2 2 part per hundred 0 . 0 1 mg. 0 . 2 2 part per hundred 4 . 4 6 f 0 . 0 7 mg. [99.9% confidence limits (1) I
8.96, 8.94, 8.87, 8.88 0 , 4 5 part per hundred 0.05 mg. 0,50part per hundred 8 . 9 1 f 0 . 3 7 m g . (99.9% confidencelimits) 21.78, 2 1 . 8 0 , 22.78, 22.21 1 . 8 part8 per hundred 0 . 4 7 mg. 2 . 1 parts per hundred 2 2 . 1 4 f 3 . 5 mg. (99.9% confidence limits)
basic as before and then boiled until a flocculent preci itate of chromic and/or ferric hydroxide is formed. The s o f h o n is filtered while still hot, and the residue is washed. The normal procedure is then followed with the combined filtrate and washings. When ammonium ion is the only interfering substance present, the basic solution need only be boiled to remove the ammonia before carrying out the normal procedure. Care must be taken in handling the strongly alkaline solutions so that no carbon dioxide is introduced. I n order t o prevent reaction with glass and the formation of soluble silicates, the strongly basic solutions should not be boiled or stored in glass for excessive periods of time. RESULTS
The method was developed t o provide a simple determination for aluminum a t low concentrations in certain solutions used in
the surface treatment of aluminum alloys. Accordingly, an acidic fluoride solution used for the chemical brightening of aluminum, and in which control of the aluminum concentration is required, was used as a test solution. There is less than 1% error due t o fluoride ion when the unknown sample contains less than 75 mg. of fluoride ion. Weighed amounts of foil (99.83y0 aluminum) were dissolved in the acidic fluoride solution and aliquots of the resulting solution, chosen to contain aluminum in amounts ranging from 4 to 22 mg., were analyzed by the mixed indicator method described above. The results shown in Table I1 illustrate the effect of sample size on the accuracy and precision of the method. The average deviation, the standard deviation, and the coefficient of variation increase significantly with the aluminum content of the sample. Bushey ( 2 )has shown that the pH a t which the excess alkalinity is neutralized increases as the concentration of aluminum in solution increases. For larger aluminum concentrations, therefore, the first color change a t 9.80 in the prevent method is beyond the neutralization of the excess alkalinity However, for an aluminum content approximating that used in the standardization-i.e., 4.5 mg.-the accuracy and precision of the method are of the same order of magnitude as the method developed by Bushey. The simplicity of the present method permits the determination of aluminum without instrumentation by relatively untrained personnel. The major disadvantage of the method is the necessity for a preliminary determination to fix the sample size for optimum precision. ACKNOWLEDGMENT
The authors wish to thank the Kaiser Aluminum and Chemical Corp. for permission to publish this paper. LITERATURE CITED
(1) Brownlee, K. -4., “Industrial Experimentation,” pp. 33-4, Chem-
ical Publishing Co., Brooklyn, 1949. (2) Bushey, A. H., ANAL.CHEM.20, 159 (1948). (3) Kolthoff, I. hl., Rosenblum, C., “.Zcid-Base Indicators,” p. 109, hlacmillan, New York, 1937.
RECEIVED for review April 14, 1955. Accepted .4pril 13, 1956
Methylmagnesium Chloride as Reagent for Determination of Reactive Hydrogen GEORGE
D. STEVENS
Ansul Chemical Co., Marinette, Wis.
An improvement in the Zerewitinoff method for determining active hydrogen has been made by using methylmagnesium chloride in tetraethylene glycol dimethyl ether as the Grignard reagent. The new reagent has the advantage of low vapor pressure and excellent solubility for most organic compounds. Preparation of the reagent is described and results of active hydrogen determinations on several compounds are discussed.
T
H E so-called Zerewitinoff method for the determination of reactive hydrogen in organic compounds has been the subject of much investigation. A comprehensive review by Olleman (3) describes most of the literature concerning the method u p t o 1952. After the work of Zerewitinoff (9, IO), the majority of the literature concerned modification of the apparatus and procedure for the analytical technique. Few investigators used a Grignard reagent other than methylmagnesium iodide. Terent’ev, Shcherbakova, and Kremenskaya (6) used methylmagnesium chloride and reported that it lost titer on standing and was
generally less reactive than the bromide or the iodide. Huckel and Wilip ( 2 ) report,ed the use of methylmagnesium bromide in isopentyl ether in the Zerewitinoff determination, as did Petrova and Perminova ( 4 ) . Because many organic compounds containing active hydrogen are insoluble in the solvent ordinarily used for the preparation of the Grignard reagent (pentyl ether), a number of secondary solvents have been required (3). Use of these additional solvents requires exacting purification and necessitates blank determinations for precise results. The choice of the secondary solvent may influence the amount of methane produced by the reaction, because of undesirable precipitation and other factors. 4 discussion of inconsistencies in the determination caused by different solvents has been reported by Kright (8). Hill ( 1 ) found that many alkyl and aryl magnesium halides could be prepared in good yields by reaction in dialkyl sthers of glycols. The author has found t’hat a preparation of methylmagnesium chloride in tetraethylene glycol dimethyl ether is an excellent reagent for the determination of active hydrogen by the Zerewitinoff method, Using an apparatus developed by Siggia ( 6 ) , the reagent was tested with several alcohols and phenols using two of the common secondary solvents, pyridine and di-
V O L U M E 28, N O . 7, J U L Y 1 9 5 6 oxane. Results were generally good, with many of the problems of solubility previously encountered having little effect on the determinations. The extremely low vapor pressure of tetraethylene glycol dimethyl ether was found to be an added advantage in preparation and subsequent use of the reagent. PREPARATION OF METHYLMAGNESIUM CHLORIDE
I n order to use a minimum of methyl chloride during preparation of the reagent, it was necessary to provide for maximum contact of the vapor with the magnesium. The problem was solved with a reaction vessel consisting of a glass column about 20 mm. in diameter and 150 mm. in length sealed to the bottom o r a standard 250-ml. round-bottomed flask. The flask was fitted with a tFo-hole rubber stopper through which a drop tube made of 6-mm. glass tubing penetrated to the bottom of the column, The exit hole in the stopper permitted escape of excess methyl chloride and was fitted with a drying tube to prevent moisture from entering the system (Figure I).
M E - k Y L 1-7 Ci.iLORIDE. INLET
1185 became too vigorous from a high rate of methyl chloride addition. I n such cases, sufficient cooling was provided by directing a stream of air toward the hot parts of the column. After completion of the reaction, the reagent was decanted through a filter of dry glass wool into a bottle fitted with a stopper and drying tube. The bottle was then heated on a steam bath for several hours to remove the excess methyl chloride in solution, after which it was fitted with a rubber serum cap through which portions of the reagent were subsequently drawn with a hypodermic syringe. Activating Magnesium. After the Grignard reagent has been prepared, a small amount may be saved for initiating the reaction in future preparations. The magnesium for the first preparation was activated by an adaptation of the method of Underwood and Gale ( 7 ) . A small quantity of magnesium turnings and several crystals of iodine were placed in a 10-ml. test tube and covered with 2 ml. of anhydrous ethyl ether. The reaction was allowed to proceed for 5 to 10 minutes, after which the ether was decanted and the test tube carefully heated to redness in a free flame. After cooling, about 5 ml. of tetraethylene glycol dimethyl ether, previously saturated with methyl chloride, was added to the test tube. The reaction started rapidly and proceeded until all of the methyl chloride was consumed. The magnesium and the Grignard reagent prepared in this manner were then used to initiate the reaction in the larger apparatus. DETERMINATION O F ACTIVE HYDROGEN
DRV'NG TUBE
ST4\IDARD 250-UL ROUND -BOTTOMED
Apparatus. The apparatus suggested by Siggia ( 5 ) was used for all active hydrogen determinations. The equipment appears to he the most simple and reliable of the many variations developed for this analysis. A detailed explanation of this apparatus and the manipulative procedure may be found in the original publication by Siggia. The only modification was the substitution of a 25-ml. buret for the original 7-ml. gas buret, permitting a slightly larger sample size for analysis.
Table I.
Ill I
I
-2ouuc
Figure 1. Apparatus for preparation of methylmagnesium chloride
Reagents and Procedure. I n a typical preparation, about 7 grams of reagent grade magnesium turnings were made to react with excess methyl chloride in 200 ml. of tetraethylene glycol dimethyl ether. The commercially available material from Ansul Chemical Co. was found to be satisfactory. The final strength of the reagent is then about 1.4iM.A higher proportion of Grignard reagent to solvent was found to cause some precipitation of magnesium salts. I n contrast, a solution only 1M in methylmagnesium iodide in isopentyl ether causes continued objectionable precipitation on standing. The magnesium turnings were introduced into the dry apparatus, filling the column around the drop tube. About 10 ml. of Grignard reagent from a previous preparation was added, along with enough tetraethylene glyrol dimethyl ether to fill the column and cover the magnesium (about 30 ml.). Anhydrous methyl chloride vapor was then allowed to bubble slowly through the column. At the beginning of the reaction, the ether became cloudy from the reaction of the reagent with small amounts of impurities and water on the magnesium. When all of the magnesium became activated, the reaction became more rapid and exothermic, with subsequent clearing of the solvent. The remaining 170 ml. of ether was then added. The reaction was easily controlled by the rate of addition of methyl chloride. Because of the high boiling point of tetraethylene glycol dimethyl ether, it was not found necessary to cool the apparatus unless the reaction
Active Hydrogen Indicated bv Alcohols and Phenols
Compound Methanol
Solvent Pyridine
Methanol
Dioxane
2-Butoxyethanol
Pyridine
2-Butoxyethanol
Dioxane
Ethylene glycol
Pyridine
Ethylene glycol
Dioxane
Resorcinol
Pyridine
Resorcinol
Dioxane
Hydroquinone
Pyridine
Pyrogallol
Pyridine
Sample, Mg. 17.3 17.7 17.5 22.3 22.6 25.8 62.4 61.1 29.3 28.3 25.2 25.1 18.3 18.5 18.9 24.0 23.6 25.1 25.8 22.8 28.4 27.8 26.1 26.0
Corrected Val. of Methane, Hydrogen/Mo1e Cc. Calcd. Found 13.3 12.8 12.8 16.9 16.4 17.6 12.4 12.4 6.3 6.2 17.9 19.0 12.6 13.6 14.2 9.8 9.6 10.1 10.6 9.6 11.7 12.1 13.6 14.0
1 1 1 1 2 2 2 2 2 3
1.10 1.04 1.05 1.08 1.04 0.97 1.04 1.07 1.13 1.15 1.97 2.10 1.91 2.04 2.09 2.01 2.00 1.98 2.02 2.07 2.03 2.13 2.92 3.02
Procedure. About 3 ml. of the Grignard reagent was transferred by means of a dry hypodermic syringe from the reagent bottle to the serum-capped reaction flask on the apparatus. The reagent was then stirred and heated in a steam bath for 10 minutes, after which it m-as allowed to cool with stirring for 20 minutes. This blank determination on the reagent permitted reaction Kith small amounts of water that may have remained in the apparatus. After sweeping with dry nitrogen, the gas buret was leveled and prepared for addition of the sample. The sample for determination was weighed in a 2-ml. hypodermic syringe and introduced through the serum cap on the reaction flask in the same manner as the reagent. After stirring and initial leveling of the mercury in the gas buret, the reaction flask was heated for 10 minutes and cooled to room temperature with stirring as be-
ANALYTICAL CHEMISTRY
1186 fore. The observed volume of methane was then corrected for temperature and volume of sample added and calculated as moles of active hydrogen in the sample. RESULTS
Tetraethylene glycol dimethyl ether is an excellent solvent for many organic compounds containing active hydrogen. However, in order to compare the results obtained using the niethglpyridine and magnesium chloride reagent with previous data (8), dioxane were chosen as solvents for several compounds containing active hydrogen, as shown in Table I. K n o m samples were prepared by weighing appropriate amounts of the glycolr and phenols into small, serum-capped flasks and diluting with the solvent. There was very little precipitation of the rextion products in pyridine. Dioxane gave no precipitate with the alcohols of low molecular weight, although there was some precipitate with the phenols. H>tlroquinone and pyrogallol gave erratic results accompanied by the formation of large amounts of precipitate when dioxane wa? used as the solvent. It was felt that a t least part of this difficulty was due to the fact that thorough agitation of this precipitate was not achieved, and as a result the reaction was incomplete. I n the author’s laboratory, the reagent has been widely used in conjunction with the Karl Fischer reagent for the determination of both water and active hydrogen compounds in several ethers of glycols. The water found by the Karl Fischer reagent may be calculated as active hydrogen and subtracted from the total determined with the Grignard reagent, the difference then being the active hydrogen other than that due to water. I n contrast to the usual properties of chloride Grignard reagents in ethereal solvents, the reagent prepared in tetraethylene glycol dimethyl ether shoved !ittle tendency t o precipitate
magnesium salts when stored for at least 1 month. No loss in titer was observed a t the end of such storage. However, during storage the reagent must be kept free from moisture in a container such as that described so that it can be withdrawn without contamination. The reagent has not as yet been applied to the determination of compounds which react n i t h Grignard reagent by addition or coupling. It is evpected to be of further value for this type of analysis. ACKYOW LEDGMENT
The author wishes to thank the Ansul Chemical Co. for permission to publish this m-ork and Philip Ehman for his valuable help in preparing the manuscript. LITERATURE CITED
Hill, J. S. (to Cincinnati Milling llachine Co.), U. S. Patent 2,552,676 (May 15, 1951). Huckel, W., Wilip, E., J . praht. Chem. 156, 95-6 (1940). Olleman, E., ANAL.CHEW24, 1425 (1952). Petrova, L., Perminova, E., J . A p p l . Chem. (U.S.S.R.) 4, 722-3 (1931).
Siggia, S., “Quantitative Organic Analysis via Functional Groups,” pp. 41-8, Wiley, New York, 1948. Terent’ev, A. P., Shcherbakova, K. D., Kremenskaya, N. Y., J . Gen. Chem. (U.S.S.R.) 17, 1 0 0 4 (1947). Underwood, H. TI’., Gale, J. C., J . Am. Chem. SOC.56, 2117 (1934).
Wiight,‘ G. F., “Organic Analysis,” pp. 155-95, Interscience, New York, 1953.
Zerewitinoff, T., Ber. 40, 2023 (1907); 4 1 , 2 2 3 3 (1908); 4 2 , 4 8 0 6 (1909); 43, 3590 (1910).
Zerewitinoff, T.. 2. anal. Chem. 50, 680 (1911); 52, 729 (1913); 68, 321 (1926). R E C E I V Efor D review December 12. 195.5. Accepted February 27, 1958.
Improved Spot Test for Boron and a Quantitative Estimation of Boron in Very Dilute Solutions T. S. BURKHALTER and DIXON W. PEACOCK Agricultural and Mechanical College o f Texas, College Station, r e x .
The standard spot tests for boron have been evaluated experimentally with particular attention to their reproducibility and to the substances that interfere with the tests. Most of the standard spot tests for boron were unsatisfactory for practical use with very dilute solutions. A superior spot test for boron, using sorbitol, combines ease of procedure, reproducibility, and high sensitivity. Based on the mechanism of this spot test, and using a series of polyhydric compounds, a method for the quantitative estimation has been developed.
B
ECAUSE of the recent developments in the iise of boron in the atomic energy program, and because of tLc criticdl level of boron content in agricultural soils, a rapid and sensltive method for the detection of boron in very dilute solutions (less than 1 p.p.m.) is most desirable. Several relatively sensitive spot tests for boron are described in the literature. I n all these tests, however, the limit of concentration is too high to permit a reliable detection of boron in very dilute solutiona. I n an attempt t o extend the limit of concentration of some of these spot tests so that the tests could be applied to very dilute
solutions, a survey of the more applicable of the available tests has been made. A study of the quinalizarin test (5, 4 ) revealed that when the volume of the test solution is increased to 1 ml. (instead of the recommended few drops) the quantity of base and/or salts present, as well as the degree of drying, becomes critical. The degree of drying is easily controlled, but the quantity of base piesent varies as the volume of solution, at constant pH. The piesence of an excess of either base or sodium salts causes a red coloi ation which masks the blue of the boron complex. Similarly i t was found that when chromotrop2B ( 3 )is used as the color-developing reagent, an orange color, which obscures the test for boron, is developed in the presence of even relatively low concentrations of sodium salts. Although no interfering color is developed by sodium salts in the curcurmin ( I ) test for boron, the sensitivity of the test is so decreased by the presence of salts that the test is of little value in working with very dilute solutions. The spot test with the lowest limit of identification described in the literature is the mannitol test ( 2 , 6). N o common substances interfere. It seemed logical, therefore, to begin the search for a superior test n i t h an investigation of this reaction. Mellon and Morris ( 7 ) determined the titration curves of boric
