ANALYTICAL EDITION
January, 1945
vary, depending on the conditions employed, but *der the same conditions there is good correlation between the relative amounts of sugar obtained from different woods by the two methods. LITERATURE CITED
(1) Kiesel, A., and Semiganovaki, N., Em., 60B,333-8 (1927). (2) Klsson. P..Cellulosechem.. 4, 81 (1923). (3j Luers, H., A n g w . Chem., 45, 369-76 (1932). (4) Monier-Williams, C.W., J. Chem. Soc., 119,803-5 (1921). (5) Ritter, 0.J., Mitchell, R. L., and Seborg, R. M.,J . Am. C h m . SOC.,55, 2989-91 (1933).
(6)
37
Ritter, G. J., Seborg, R. M., and Mitchell, R. L., IND. ENO.
CREAf., ANAL.ED.,4, 202-4 (1932). (7) Saeman, J. F., Harris, E. E., and Kline, A. A., Forest Products Lab. Report, R1459 (1944). (8) ShafTer,P. A., and Somogyi, M., J . Biol. Chem., 100,695 (1933). (9) WillstLtter, R., and Zechmeister, L., Em., 46,2401 (1913). (10) Wohl, A., and Krull, H., Cellulosechem., 2, 1-7 (1921).
PRESENTEO before the Division of Cellulose Chemistry a t the 108th hfeeting of the A M ~ R I C ACHEMICAL N SOCIETY, New York, N. Y. Based on studiea of the U. S. Forest Products Laboratory in cooperation with the Office of Production Research and Development, War Production Board.
Rapid Volumetric Method for Aluminum L. J. SNYDER,
Ethyl Corporation, Baton Rouge, La.
A rapid and accurate procedure is described for the determination of aluminum in the presence of 1 to 10% of impurities such as calcium, copper, chromium, iron, magnesium, manganese, and zinc. Experimental data are presented showing the accuracy and the effects of impurities commonly associated with aluminum. Five to 10 minutes' time is required per analysis.
HERETOFORE,
it has been common practice to determine aluminum by precipitation as the hydrous oxide followed by ignition to constant weight. This procedure is time-consuming and of such doubtful accuracy that materials high in aluminum content are analyzed by determining the other components and calculating aluminum by difference. However, when the impurities are present to the extent of only a few per cent and an w a y of the impurities is not required, a direct, rapid, and accurate volumetric method is desirable. The use of potassium fluoride in the acidimetric determination of aluminum has been studied by a number of investigators (1-6). The method described by Viebock and Brecher (6) appears to have the widest application. Their procedure consists of neutralizing the aluminum solution with sodium hydroxide in the presence of sodium potassium tartrate to the phenolphthalein end point, adding neutral potassium fluoride, and titrating the liberated potassium hydroxide 3a. a measure of the aluminum present. This reaction does not proceed stoichiometrically, the end point appearing about 10% before the equivalence point. Aluminum hydroxide, formed by the neutralization of a solution of an aluminum salt to the phenolphthalein end point, reacta with potassium fluoride to form the neutral salt, AlFI.3KF, and liberates 3 moles of potwium hydroxide permoleof aluminum present. This reaction is slow even with heating (6),but in the presence of sodium potassium tartrate it is practically instantaneous (e). The acid used to titrate the liberated base is a direct measure of the aluminum present. The following are the reactions for the neutralization of the aluminum ion and the liberation of the base by the potassium fluoride:
+
AI+*+ 30H- +Al(OH), Al(OH)* 6KF +ALFI.3KF 3KOH
+
+
EXPERIMENTAL
C.P.
Table A1 Added Qrom 0.1282 0.1028
0.0789 0.0513 0,0266 0.1236 0.1236 0.1236 0.1238 0.1236 0.1236
1.
Analysis of Known Aluminum Solutions Difference AI Found Qram 0.1282 0.1025 0.0768 0.0613 0.0266 0.1237 0.1237 0.1235 0.1238 0.1235 0.1236
hlean of differenae
The purpose of this paper is to show that the reaction between aluminum hydroxide and potassium fluoride in the presence of sodium potassium tartrate, with certain modifications of previous procedures, can be used for the quantitative determination of aluminum in the presence of small quantities of some of the usually interfering ions.
REAGENTS.All chemicals used were of Baker's with no further purification.
Barium hydroxide, saturated solution. Hydrochloric acid, 0.3 N solution standardized with reference to a.known aluminum chloride solution, Standard aluminum chloride solution, 0.3 N . Weigh accurately 2.5 to 2.7 grams of pure aluminum wire (Baker's c.P., 99.9970) and dissolve in 200 ml. of approximately 1 N carbonatefree sodium hydroxide, make the solution just acidic with hydrochloric acid (1 to l), and boil slowly to expel carbon dioxide before diluting to 1 liter. Use aliquots of theatandard solution to standardize the hydrochloric acid by the procedure described below. Sodium potassium tartrate, 30% solution. Potassium fluoride (KF.2H20),30% solution, neutral to phenolphthalein and stored in a wax-lined bottle. PROCEDURE. T o the aluminum chloride solution free of ammonium ion and other interfering substances and containing 25 to 130 mg. of aluminum, add standard barium hydroxide solution until the free acid is neutralized as indicated by a slight precipitate of aluminum hydroxide. Introduce 30 ml. of the sodium potassium tartrate solution and continue the titration to the phenolphthalein end point. Add 30 ml. of neutral (to phenolphthalein) potassium fluoride solution and titrate the liberated base with 0.3 N hydrochloric acid to the disappearance of the red color. The end point is considered permanent when the color does not return after 30 seconds. The standard hydrochloric acid used to titrate the liberated base is equivalent to the aluminum present. TESTS ON KNOWNSOLUTIONS. The reliability of the method was determined by analyzing known aluminum chloride samples, prepared by dissolving pure aluminum wire in 1 N sodium hydroxide (carbonate-free). The sodium aluminate was then made slightly acidic with dilute hydrochloric acid and the solution diluted to R known volume. Aliquots were withdrawn and the aluminum was determined by the procedure described. The accuracy was found to be t0.1 mg. Data showing the analysis of known solutions are presented in Table I.
quality
Gram 0.0000
-0.0001 0.0001
- 0.0001 0.000
$0.0001 +0.0001 +0.0002 -0.0001
-0.0001 -0.0001
0.0001
ANALYSISOF ALUMINUM I N PRESENCE OF INTERFERINQ SUBThe effect of impurities was determined by adding the ions in three concentrations to solutions containing known amounts of aluminum and then analyzing the solutions for nluminum (mole ratios of aluminum to impurity were 2, 10, and 100). At an aluminum-impurity ratio of 100, only phosphate, sulfate, and silicate interfered, while calcium, chloride, carbonate, chromium, copper, iron, magnesium, manganese, and zinc did not STANCES.
INDUSTRIAL AND ENGINEERING CHEMISTRY
38
interfere. Sulfate caused high results, of the order of I%, and silicates caused very high results. At a mole ratio of 2, all the ions tested interfered except sodium, potsssium, carbonate, and chloride (Table 11).
Table II. Impurity Added NaCP KCI~
CaCla CUCII MgClt
Fe (NO,) I
Effect of Interfering Substances
Mole Ratio AI-Irnpurit; 2
2 2 2 10 100 2 10 100 2 10 100
2
10 100
2 10
CrClr
100 2 in -100 2
NaaPOk
100 2
NarSO,
100 2 10 100
MnClr
NarSiOa
10
10
2 10
100
AI Preaent Gram
AI Found Qram
0.1501 0.1501 0.1455 0.1501
0.1503 0,1502 0.1454 0.1486 0.1500 0.1501 0.1488 0.1503 0.1502 0.1467 0.1489 0.1489 0.1463 0.1488 0.1495 0.1553 0.1269 0.1236
Difference Gram
+0.0002
+o. 0001
- 0.0001 -0.0018 -0.0001 0.1501 0.0000 0.1501 -0.0013 0.1601 +0.0002 0.1501 +0.0001 0.1489 -0,0022 0.1489 0,0000 0.1489 0.0000 0.1497 -0.0034 0.1497 0 IO009 0.1497 - 0.0002 0.1489 0.0064 0.1236 +O. 0033 0.1236 0.0000 End poin.t obscured by color of A h 0.1101
0. 501 0.t489 0.1489 0.1489 0.1501 0.1501 0.1bO1
0.1489 0,1489 0.1489 0.1489 0.1489 0.1489
0.1500 0.1358 0.1447 0.1487 0.1015 0.1406 0.1496 0.1804 0.1604 0.1503 0.1919 0.1641 0.1639
+o. 0001
-0.0131 -0.0042 -6.0002 0.0486
-
-0.009b -0.0008
+O.OOld
,0016 ++o0.0013 +0.0570 +0.0152 +0.0160
Aseumed that no interference would occur in l o a concentration if high concentration did not interfere. a
The neutralization of the aluminum chloride to the phenolphthalein end point must be done with barium hydroxide solution. Ammonium hydroxide or ammonium salts b d e r the solution to the extent that the end point cannot be quantitatively distinguished. If sodium or potassium hydroxide solution is used, the end point is not 80 sharp as when barium hydroxide is used and lower results are obtained, as shown in Table 111. These results were found to support the statement made by Viebock and Brecher ( 6 ) , that the end point appeared about 10% before the equivalence point. The reason for.obtaining low results with sodium hydroxide is not evident and was not investigated.
Table Ill. Neutralization of Aluminum Chloride with Barium Hydroxide and Sodium Hydroxide
Neutraliretion with BB(OH)I Neutralization with NaOH (carbonate-free)
A1 Added Gram
AI Found
0.1244 0 1244 0.1244 0,1244 0.1244 0.1244
0.1244 0.1243 0.1244 0.1159 0.1103 0.1143
Gram
Difference Gram 0.0000 -0.0001
.
Vol. 17, No. 1
tion of the potassium fluoride solution, the change in pH per drop of standard base was so small that a satisfactory neutralization could not be made. However, when using phenolphthalein a sharp change waa obtained with the addition of only one drop of acid. I p the titration of the liberated hydroxide with standard acid, the end point fades the first time it appears and then becomea permanent with the addition of a few drops of acid. This is probably due to the fact that the aluminum potassium fluoride precipitates from a basic solution and adsorbs some of the base which is desorbed near the end point. This effect may be minimized by adding an excess of standard acid, shaking well, and back-titrating with standard barium hydroxide. A serious error is possible if the sodium potassium tartrate ia added to a strongly acid solution of the aluminum salt. The tartrate forms highly insoluble potassium acid tartrate which precipitates immediately. The precipitate is acid in nature and redissolves slowly, resulting in a fading end point to phenolphthalein. The precipitation of potassium acid tartrate is avoided by the addition of sodium potassium tartrate to the solution only after most of the free acid' has been neutralized. This procedure will always give a clear, pink permanent phenolphthalein end point. If a precipitate of potassium acid tartrate forma, it may be dissolved by making the solution alkaline with a 1- to 2-ml. excess of barium hydroxide, heating to 60" to 70" C. until all the precipitate dissolves, and adjusting the solution to the phenolphthalein end point before adding the fluoride. Theoretically, 6 moles of potassium fluoride are necessary to react with 1 mole of aluminum. This quantity is not enough, 88 the reaction does not go to completion and low results are obtamed. The use of a t least twice the theoretical amount of fluoride is recommended.
Table
IV. Anrlysis of Known Solutions with Hydrochloric A c i d Standardized with Reference to Potassium Biphthrlate
A1 Added Gam
AI Found Gram
0,1282 0.1020 0.0709 0.0518 0.0256
0.1272 0.1017 0.0762 0.0509 0.0254
Difference Gram -0.0010 0.0009 0.0007
-0.0004
-0.0002
% 0.8 0.8
0.9 0.8 0.8
The analysis may be applied to the simultaneous dekrmination of free acid and an aluminum salt by adding neutral sodium potassium tartrate, and neutralizing the free acid and aluminum salt with barium hydroxide to the phenolphthalein end point, which is a measure of the total acid plus aluminum salt. The subsequent titration of the liberated potassium hydroxide, after the addition of potassium fluoride, with hydrpchloric acid is a measure of the aluminum and the difference between the equivalents of acid and base used is a measure of the free acid.
o.Ooo0
-0.0085 0.0081 -0.0101
ACKNOWLEDGMENT
The author wishes to thank 6.F. Kirby and W. H. Thomas for their helpful comments in connection with this paper. LITERATURE CITED
The hydrochloric acid must be standardized against a known aluminum eample to obtain quantitative results. When the hydrochloric acid is compared with standard alkali previously standardized with potwium biphthalate, the Bluminum analysis waa found to be consistently low, aa shown in Table IV. The BBme result may be obtained by increasing by 0.8% the normality obtained by standardizing against primary standards. Phenolphthalein waa found to be a suitable indicator. When either phenol red or m-cresol purple waa used in the neutraliza-
(1) Craig, T.J. I., J. SOC.Chem. Id., 30, 185 (1911). (2) Malaprade, L., Congr.. chim. id., Compt. rend. 18mr congr., Nancy, p. 116,Sept.-Oct., 1938. (3) Scott, W.W., J. IND. ENO.CHXM., 7,1059 (1915). (4) Titus, A. C., and Cannon, M. C., IND. ENO.CHBY.,ANAL.ED., 11, 137 (1939). (6) Travers, A., Compt. rend., 185, 893, 1043
