1182
ANALYTICAL CHEMISTRY
I n operation, an appropriate length of tubing was placed in the flanging mold (Figure 4, B). The magnesium (or aluminum) cap was slowly heated with a microburner until a temperature of 260' =t5" C. was reached. (The plastic used corresponded to M. W. Kellogg Co.'s grade 270. For grade 300 a maximum temperature of 290" C. is desirable.) During the period from 225' to 260' C. pressure was maintained upon the tube by turnin the stud a t the top of the yoke (Figure 4, -4). The end of the t u t e in contact with the heated cap was thus made to flow into the flanging mold. The 260" temperature was maintained for approximately a half hour, during which time the pressure was kept a t a maximum. To prevent the upper m-all of the tube from softening and collapsing, powdered dry ice was placed around the tube in the lower part of the yoke. The excellent insulating properties of the Teflon gaskets permitted the maintenance of a temperature gradient of about 200" between the heated cap and the upper part of the mold body. Finally, the entire assembly m s rapidly quenched in ice water. The mold (Figure 4, A ) for forming the bottom of the vessel was used next. This was handled in much the same way as the flanging mold. however, it was necessary to employ an electrically heated Lip on the steel piston. The heater consisted of a 0.5-ohm Nichrome wire embedded in Sauereisen cement and it was operated a t 4 volts. When set up initially, the end of the plastic tube extended far enough below the piston tip to provide sufficient material for completely filling the mold when the position s h o m in the diagram had been reached. I t was very important to clean the mold thoroughly after use and also to maintain the temperature below 270" C. Discoloration and decomposition resulted from excessive temperatures. (The darkening of Fluorothene is usually due to contamination by other organic materials or from plastic sources other than virgin material. The authors have felt that there may be an effect due to the magnesium surfaces of their mold.) I t was equally important to keep the final temperature above 250' C. to prevent ridging and cracking of the plastic during its flow through the mold. Vessels so constructed were capped by a Fluorothene (or hlonel) disk, which was held tightly in place by a slightly modified pipe union fitting (see Figure 3, D). The vessels showed no sign of cracks, leaks, or deterioration after repeated immersions in liquid air under pressures in the range 0 to 2 atm. Some have been in continuous use for 3 years. Capillaries. Capillaries of fine bore could not be prepared by drilling Fluorothene rod. However, it was possible to prepare capillaries of less than 0.1 mm. bore by the following method.
A 0,018-inch hole was drilled in a a/s-inch rod to a depth of 1 inch. The rod was then rotated slowly above a niicroflame until the plastic became transparent and pliable. A capillary was then drawn out in the same manner as with glass tubing. The plastic must be drawn very slowly and the center portion of smallest diameter should be permitted partially to harden before the drawing is completed. Otherwise, the softened rod may be broken before it has been stretched sufficiently. Small biilhs may be bloivn by first drilling and softening~. the plastic rod as for capillary preparation. Sealing. Electrode Leads. Electrode leads of datinum wire or other metal may be sealed into Fluorothene b> first drilling a hole in the plastic a few thousandths of an inch undersize. By welding a wire of smaller diameter to the end of the electrode lead, one has a means of pulling the larger diameter wire into the plastic. The smaller diameter wire is slipped through the hole in the plastic, and by carefully heating the lead wire while pulling the mire is forced into the hole as the plastic softens. A wire above 0.050 inch in diameter may be successfully sealed into Fluorothene by employing a tapered Teflon packing ring, which is squeezed around the wire by a packing nut in a manner similar to the seal around the glass tubing as shown in Figure 1, B. Test tubes and beakers of Fluorothene are available comniercially for laboratory use. Furthermore, there is on the market a Kel-F grease which is a lower molecular weight polymer of trifluorochloroethylene. This can be used for lubrication of stopcocks and other surfaces exposed to corrosive reagents. ACKNOWLEDGMENT
The authors wish to acknowledge the financial support of this work by the U. S. Atomic Energy Commission under Contract KO,AT(l1-1)-90, Project No. 3, Chicago Operations Office. LITERATURE CITED
(1) Brown, R. A., Skahan, D. J.. ANAL.CHEM.26, 788 (1954). (2) Kaufman, H. S., Kroncke, C. O., Jr., Giannotta, C. R., Modern Platies 31, 234 (October 1954). (3) Kellogg Co., M. W., Jersey City 3, N. J., Tech. Bull. 1-1-55. (4) Rlonk, J. W., "Outgassing of Naterials in a Vacuum," Atomic Energy Commission MDDC-1307. ( 5 ) Schildknecht, C. E., "Vinyl and Related Polymers," p. 487, Wiley, New York, 1952. RECEIVED for review November 18, 1955. Accepted February 29, 1956. Abstracted in part from the M . S. thesis of George Balog, Illinois Institute of Technology, February, 1954.
Simple Indicator Method for Determination of Aluminum R. V. PAULSON
and
J.
F. MURPHY'
Kaiser Aluminum and Chemical Corp., Spokane 69, Wash.
A simple volumetric method was needed for control of aluminum concentration in certain solutions used in finishing aluminum. Such a method was developed, which is applicable in the presence of fluoride and affords accuracy and precision for dilute solutions.
T
H E method for aluminum developed by Bushey ( 2 ) involves the titration of an alkaline solution (Region A , Figure 1) to the point a t which the free hydroxyl is neutralized (Region B, Figure l), using pH measurements to determine the end point. Acid is then added through the region of precipitation of aluminum hydroxide to the point a t which the precipitate is just redissolved (Region C, Figure 1). Potassium fluoride is added to precipitate cryolite in the acid solution, and the excess acid is titrated with standard base using phenolphthalein to the appearance of a pink color which remains for 15 seconds. The volume 1
Present address, General Electric Co., Schenectady 5 , N. Y.
of hydrochloric used in the determination corresponds to the distance from B to D in Figure 1. Because this determination is not affected by the presence of fluorides, an attempt was made to simplify it without undue loss in accuracy or precision. Preliminary experimental work with mixed indicators led to the conclusion that a determination based on the titration of an alkaline aluminate solution with standard acid to an end point in the pH range above 9 was feasible. The mixed indicator described by Kolthoff and Rosenblum (S), which has two color changes in the chosen pH range, was tested. The first method attempted involved the use of two samples. Potassium fluoride solution was added to one sample to precipitate aluminum as cryolite. Both samples were then titrated to the green end point of the mixed indicator a t pH 9.80. HOWever, it was found that better precision and greater sensitivity were obtainable by using a single sample, titrating to the green color (pH 9.80), adding potassium fluoride solution a t this point, and then titrating the hydroxide liberated from the aluminate ion by the fluoride to a yellow color a t pH 9.35. The volume of
V O L U M E 28, NO. 7, J U L Y 1 9 5 6 Table I.
1183
Effect of Foreign Ions
(8.60 mg. of aluniinum present)
Foreign Ion COI--
Mg. 12.49 12.49
Cr+++
5.44 5.44 5.44
FFe+++
SH,
18.9 28.4 37.8 5.43 2.71 5.43 2.71 3.08 6.16
+
6.16 SO*--
3.53 17.6 35.3
Treatment of Soiution None 13.3 mg. B a C h added
None Boiled Boiled and filtered None None None None Kone Boiled Boiled and filterzd None None Boiled None None iYont?
AluminunL Found, Error,
yellow end point a t D. T h u s in the example given, which is for the standardization of acid against a known quantity of aluminum, the aluminum content is equivalent to 12.65 ml. of 0.092,V hydrochloric acid.
47c
Mg. 8.71 8.59
1.28 0.12
6 32 8.23 8.59 8.57 8.55 8.50
14.9 4.32 0.12 0.35 0.58 1.16
7.95 8.44 8.57 8.60
7.58 1.85 0.35
10.5
0.00
8.90 9 29 8.59 8.36 7.44 5.72
3.51 7.90 0.20 2.80 13.5 33.4
11.5
1
9.0 9.5
I
I
I 10.0
n
11.0 0
2 4
6
8
1 0 1 2 14
MI S t a n d a r d Figure 2.
HCI
Standardization curve
0.603 mg. of alurninuiii; 1 nil. of hydrochloric acid = 0.6855 Ing. of aluminum
4.0
A summary of the effect of foreign ions which interfere in this determination is listed in 'Itable I. These include carbonates, silicates, ferric, chromic, and ammonium ions. Boiling the alkaline solution before titration removes ammonium ion; filtering removes the precipitated hydroxides of ferric and chromic ions. The low results obtained when iron and chromium are present may be due to coprecipitation of aluminum hydroxide with ferric or chromic hydroxide near the first end point (pH 9.80). Barium chloride solution added after boiling, just before the initial titration is begun, serves to remove the carbonate. Thus, the most common interfering elements are easily removed.
5.Q 6.0
7.0 I Q
8.0 9.0 10.0 11.0
EXPERIMENTAL
0 5
IO 15 20 25 30 35 40 Milliliters of HCI
Figure 1. Titration curve for aluminum with hydrochloric acid
acid required for the titration after the addition of potassium fluoride was found to provide a direct measure of the aluminum content of the solution. The yellow end point a t 9.35 was much sharper and easier to see than the green end point in solutions containing precipitated cryolite. The probable reactions which occur a t various stages in this determination are shown in Equations 1 and 2.
+
+ H + = AlOt- + H 2 0 (green end point) + 6F- = AIFa--- + 4 0 H - (blue solution)
A102OHA1022H20
+
(1)
(2)
These equations indicate that four hydroxyl ions are released per mole of aluminum. Experiments showed that an average of 3.6 moles of hydroxide are released per mole of aluminum. This loss of alkali is probably caused by a coprecipitation of hydroxide in the precipitated cryolite. The discrepancy may also be attributed in part t o the fact that a t p H 9.80, slightly more acid has been added than is required for the neutralization of excess alkalinity. This end point is near the pH required for precipitation of aluminum hydroxide and the solution may contain small amounts of precipitate, although a precipitate is not visible. Therefore, fewer than the four moles indicated in Equation 2 are released on the addition of fluoride. The change in pH during this determination is shown in Figure 2. The alkaline solution starting near A is titrated t o the green end point, B , a t which point the potassium fluoride solution is added. The hydroxyl ion released by the formation of the cryolite increases the pH to point C. This released hydroxyl ion is then neutralized to the
Reagents. Solution of approximately 0.LV hydrochloric acid, standardized as follows. High purity aluminum is dissolved in concentrated hydrochloric acid and an aliquot containing approximately 5 mg. is taken. The sample is diluted to 100 ml. and 10 drops of mixed indicator is added. The solution,is then made strongly basic with sodium hydroxide pellets or carbonate-free sodium hydroxide solution. The color of the solution a t this stage is deep violet. The strongly alkaline solution is then titrated to the green end point a t pH 9.80 (point B, Figure 2) with dilute hydrochloric acid. A 20-ml. portion of 770 potassium fluoride is added to the reaction mixture, and the resulting solution is titrated to a yellow end point a t pH 9.35 (point D, Figure 2) with the standard hvdrochloric acid. grams of A1 used Grams of aluminum per ml. of HC1 = net ml. of HC1 from B to D Reagent grade sodium hydroxide pellets or a carbonate-free solution of sodium hydroxide. Anhydrous potassium fluoride, 77, solution, with the pH adjusted to 9.8 with sodium hydroxide. The p H adjustment can be made using the mixed indicator. Mixed indicator, prepared by mixing 2 parts of 0.1% thymolphthalein and 1 art of 0.1% Alizarin Yellow in ethyl alcohol. Procedure. g o INTERFERISG SUBSTANCES PRESENT. A sample of solution containing about 5 mg. of aluminum is diluted t o 100 ml. with carbonate-free distilled water. Ten drops of mixed indicator is added, followed by solid sodium hydroxide or carbonate-free sodium hydroxide solution until the solution is strongly alkaline and has a strong violet color. The excess sodium hydroxide is titrated with hydrochloric acid until the solution is green (pH 9.80). The hydrochloric acid used for this titration need not be standardized. Then 20 ml. of 77c potassium fluoride is added to the solution. The resulting solution is then titrated with standardized hydrochloric acid to a yellow end point (pH 9.35.) hIg. of A1 per sample = ml. of standard HC1 X mg. of A1 per ml. of HCl
IN PRESENCE OF INTERFERING SUBSTANCES. When ammonium, chromic, or ferric ion is present, the solutionismadestrongly
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 . 4 5 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 p a r t per hundred 0.05 mg. 0,50p a r t 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. In 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 at 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 up 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-
