INDUSTRIAL AND ENGINEERING CHEMISTRY
460
~
Table 1. Amine Salt CaHirNHz.HC1 CioHziNHz.HC1 CizHa6NHa.HCI CisHaaNHz.HC1 CisHarNHz.HC1 CisHuNHz.HC1 NazSOI
+
~~
Analyses of Amine !%Its Amine Found Mole
Error
14.47 19.75 20.77 21.86 19.69 13.27 15.81 17.16 15.77 17.72 17.17 18.77
0.002800 0.00480 0.00337 0.001144 0.001144 0.003125 0.003628 0.001055 0.001174 0.001175 0.000869 0.000868
-0.07 0.00 0.00 0.00 0.00 -0.16 -0.06 -0.28 -0.09 -0.00 -0.12 -0.23
21.42 20.21
0.000453 0.000580
-0.22
Amine Calculated Mole
HCI“ NaOHb M1. M1.
0.002802 0.00480 0,00337 0.001144 0,001 144 0.003130 0.003630 0.001058 0.001175 0.001175 0.000870 0.000870
47.58 75.40 61.38 38.88 36.22 49.60 58.07 32.20 30.71 34.18 30.21 32.16
0,000454 0. 000580
30.97 30.84
%
(CaHi7)rNH.HCI 0.001272 38.18 20.16 0.001270 (CizHdzNH.HC1 0.000770 30.72 25.40 None CiaHaiNHz.HC1 0.000522 primary 0,00022 BeeCI~HIT)ZNH.C1 NazS04 ondary 34.82 23.99 0.000522 a 0.0935 N standardized with diphenylguanidiine (4). 0.1140 N prepared carbonate-free ( 9 ) .
L
+
+
+
-0.00
-0.13
....
0.00
*
Table II. Distillation of Amines Amine
Sample
Mole
Gram
Octylamine Dodecylamine
0.1-0.5 0.1-0.6
0.00084.004 0.00054.003
Octadecylamine Alkali on1 5arSOr Alkali Dioct lamine Dido&eoylamine alkali only and alkali I‘$azSOr)
0.1-0.2 0.1-0.2 0.1-0.3
0.0003-0.0006
+
+
Time qf Distillation Min. 5-15 10-30
Hour8
None in over 1 hour
0.0003-0.0006 0.0004-0.001
1-1.5 0.75-1 0.75-1.5
use smaller samples for the sake of rapidity a t the expense of greater accuracy. For the analysis of hexadecyl- and octadecylamines 30 to 50 grams of sodium sulfate may be added to the samples. This raises the boiling point of the system and materially reduces the time required to distill the primary amines. It can be seen from Table I that the addition of sodium sulfate does not increase the volatility of the corresponding secondary amines to the extent that they interfere with the analysis of the primary amines. Table I1 shows the time required for distillation of practical amounts of representative aliphatic amines. APPLICABILITY.This procedure permits quantitative determination of aliphatic primary amines, containing from 12 t o 18 carbon atoms, inclusive, in the presence of their correspond-
Vol. 16, No. 7
ing secondary amines. Salts of mixtures can also be analyzed by this method. When secondary amines are not present, the method may be employed for the determination of primary aliphatic amines and their salts up t o and including octadecylamine. The quantitative removal of the amine component allows determination of the anion in the residual solution by the usual procedures. Since the presence of the amine frequently complicates the accurate determination of the anion, this procedure removes one of the difficulties encountered in the analysis of high molecular weight amine salts. When the method is applied to the analysis of a mixture of primary and secondary amines, it is entirely dependent upon the differences in their respective vapor pressures a t the boiling point of water. The limitations as to chain length have been designated above. INTERFERINQ SUBSTANCES.The most frequently encountered interfering substance in commercial amines is ammonia, which can be removed readily by a preliminary heating of the alkaline sample under reduced pressure. At 50” C. and 20-mm. pressure, the amount of the higher primary amines lost by vaporization is within the experimental accuracy of the analysis (2). Other volatile organic bases are not generally encountered in such mixtures. None of the other common impurities of the commercial aliphatic amines interferes with the method: nitrile distills simultaneously with the amine, but does not react with the acid in the rereiver; free fatty acid is converted t o soap and prevented from hydrolyzing by the excess alkali; and amide does not effect the analysis. ACCURACY.Table I shows the accuracy which can be obtained by this method if reasonable care is exercised. Smaller samples than those suggested can be analyzed accurately by using more dilute standard acid and alkali solutions. Since the composition of commercial amines cannot be determined accurately by any other method, no direct check can be made upon the accuracy of the analyses of these compounds by this method. However, the results agree within 1 to 2% with separations of these compounds by distillation. LITERATURE CITED
(1) Hoerr and Ralston, J . Am. C h a . Soc., 65,976 (1943). (2) Ralston, Selby, Pool, and Potts, IND. ENQ. CHEM.,32, 1093 (1940). (3) Treadwell and Hall, “Analytical Chemistry”. 9th ed., Vol. 11, New York, John Wiley & Sons, 1942. (4) Young, Can. J. Rsaearch, 17, 192 (1939).
Rapid Determination of Zinc in Magnesium
Alloys
SIDNEY WEINBERG AND T H O M A S F. BOYD, Industrial Test Laboratory, United States Navy Yard, Philadelphia, Pa.
M
ETHODS ordinarily used in the quantitative analysis of zinc in magnesium alloys employ preliminary separations and subsequent determination of the zinc with potassium ferrocyanide (2, 9 ) or as zinc oxide adapted from an aluminum alloy method (1, 8). Methods employing electroplating in a sodium hydroxide medium after preliminary separations (6, 9 ) , and an iodometfic titration procedure have appeared (4). A rapid and direct means of obtaining results with no preliminary separations was attempted in this laboratory and the electrodeposition of the zinc in ammoniacal solutions containing ammonium chloride (6, 7 , 10) was found to give results well in agreement with the classical methods. Weak organic acid solutions were also used for the deposition of the zinc by the authors, but the method is more time-consuming and offers no advantage over the electroplating in ammonium hydroxide.
Sufficient ammonium chloride is introduced to prevent the precipitation of magnesium hydroxide, and tartaric acid also is added to keep the aluminum present in solution. The total time needed for a single determination is about 25 minutes. Sources of error due to manipulation are removed, as the determination ie completed in one beaker. PROCEDURE
Dissolve a 0.5-gram sample in a 200-ml. electrofytic beaker by adding 25 ml. of dilute sulfuric acid (1 to 14). Add 2 ml. of tartaric acid solution (25%) and 17 to 20 grams of ammonium chloride, dilute to 100 ml., and stir to dissolve the salts. Make the solution barely ammoniacal to rosolic acid (or any neutral indicator) and add 4.0 ml. of ammonium hydroxide (0.90) with stirring. Electrolyze for 20 minutes at 2 am eres, with a entle stream of air a ’tating the solution. (Turn tge current on &fore the beaker is pgced in contact with the electrodes to avoid any
ANALYTICAL EDITION
July, 1944
Table I. Comparison of Electrol tic Method with Standard MethoJ ~
Zinc Zinc Zinc (Gravi(Eleo- (Ferp- metnctrolyaia) cyaruds) ally) Deviation
Alloy
Alloying Elements
265 266 265 265 265 260 260 57-8
3% Zn. 6% Al. 0.2% Mn
................... ................... ................... ................... 2% Zn, 9% AI, 0.2% Mn ...................
1% Zn, 6% Al. 0.6% Mn
Table II. Zinc
Sample
%
Av.
3.23
%
%
3.07 3.04 3.20 3.02 3.30 2.34 2.30 1.05
3.05 3:22
..
2144 2.32
..
%
%
3100
10.02 +0.04 -0.02
3:Ol 3.18 2.30 2.32 1.10
Precision of Electrolytic Method Deviation Sample Zinc % %
Av. 2.48
0.02
+0.01
+0.12
.... ....
-0.05
Deviation
%
0.03
possible solvent action of ammonium hydroxide on the copper plate or nickel of the cathodes.) Wash by rapidly lowering the electrolyte and replacing with a beaker of distilled water. Repeat the operation using a second beaker of water. Immerse the cathode in neutral alcohol and place in an oven at 100’ C. until the alcohol has completely evaporated. Remove immediately from the oven to prevent oxidation of the zinc plate, cool to room tem erature, and weigh. Tge zinc-plated electrode is best kept in a desiccator if it is not to be weighed as soon as cool. Platinum electrodes which have a bri h t unoxidized copper plate or nickel electrodes are used as cet%odes and are tared before using. A platinum wire spiral serves as the anode. Gauze cathodes 4.4 cm. (1.75 inch) in diameter by 5 cm. (2 inches) in length are used. The nickel electrode may be stripped of zinc by immersing in dilute sulfuric acid (1 to 15). If appreciable amounts of lead, tin, cadmium, copper, or silver are present, a hydro en sulfide separation is necessary. I n this letter case use small #asks for dissolving the Sam le, dilute to approximately 70 ml., and pass a rapid stream of gydrogen sulfide through the solution for 10 minutes. Warm on a stream bath for a few minutes to coagulate the precipitated sulfides. Cool in a stream of running water and filter through a close paper, catching the filtrate in a 200-ml. electrolytic beaker; wash with acidified hydro en sulfide water. Boil the filtrate until free of hydrogen s d d e and cool to room temperature. Now proceed aa in the routine method.
461
The zinc deposit is bright, metallic, and adherent, but is easily oxidized during electrodeposition if the copper-plated electrode is oxidized or during the drying period if too prolonged. Nickel electrodes give slightly more consistent results and avoid the necessity of obtaining good copper deposits on the platinum electrodes. The current is best turned on before the electrodes are in contact with the electrolytic solution, t o avoid any tendency of the ammoniacal solution to attack the copper plate or nickel which will be deposited again but not always bright and free of oxides. Elements interfering with the direct deposition of zinc are seldom encountered in appreciable amounts. Interference of copper is prevented by selective solution, so that the copper remains undissolved. The residue need not be filtered for routine work unless it is large. Silver, lead, tin, and cadmium are not present ordinarily. The first three elements are almost completely removed from solution through displacement by undissolved magnesium when they are present in amounts as small as 0.5 mg. The insoluble residue is removed by filtration. However, when any or all of these elements are present in larger quantities or as alloying elements, a hydrogen sulfide separation is necessary. The addition of hydrogen sulfide to several commercial alloys.tested gave no visible precipitate. Iron also will interfere if present, but, fortunately, it is present only as a trace in magnesium alloys. SUMMARY
The rapid routine determination of zinc in magnesium alloys is accomplished by the direct electrolysis of the samples, after selective solution, in ammoniacal medium. The elements interfering with the deposition are seldom encountered in magnesium alloys and if present a t all are ordinarily in trace quantities and need no separation. Significant amounts of elements which are electrodeposited in acid solution must be separated. A hydrogen sulfide precipitation is proposed. Deposition is rapid and quantitative in 20 minutes and the entire determination need require but 25 minutes. The electrolytic method compares favorably with the accepted procedure.
Table 111. Precision Employing Pure S I C Amount Added
Zn Ma. 10.0
10.0 10.0 10.0
10.0
A1
MQ. 6i:O 65.0 65.0
..
Mn MQ.
Mg
Pb
Mo.
Mo.
...
...
2.5 2.5 2.5
...
900 YUU
...
Found
MO.
... ...
10.1
2.0
;:
... ...
9.9
E.!
RESULTS
The purity of the zinc deposit and the completeness of the electrodeposition were determined on the pure salt solutions. The zinc plate sheared the presence of a trace (less than 0.1 mg.) of manganese and a trace of iron. The electrolyzed solution was examined for zinc colorimetrically by means’of dithizone and 0.1 mg. or less was found to be present. Lead, tin, silver, and cadmium if present also deposit, causing high results and must therefore be removed prior t o electrolysis. Regularly encountered types of magnesium alloys were analyzed by the three procedures discussed; the comparative results are shown in Table I. The reliability of the method is shown by results obtained by two different analysts in Table 11. DISCUSSION
The accuracy of the direct electrodeposition of zinc in magnesium alloys compares favorably with that of the zinc oxide or ferrocyanide procedures and the electrolytic method is much faster. The deposits are sufficiently pure and quantitative for routine analytical purposes, as shown by tests of the zinc plate and the electrolyzed solution which was composed of pure salts added in known amounts.
ACKNOWLEDGMENT
The authors wish t o thank R. D. Spiers for his assistance and suggestions. The opinions as expressed by the authors are not to be construed as representing the Navy Department. LITERATURE CITED (1) Aluminum Co. of America, New Kensington, Pa., “Aluminum Alloys”, pp. 42-4, 1941. (2) Aluminum Co. of America, New Kensington, Pa., “Magnesium
Alloys, Methods of Analysis”, July 26,1935. (3) Am. SOC.Testing Materials, Philadelphia, Pa., “Methods of Chemical Analysis of Metals”, 1943: “Chemical Analyaia of Magnesium and Its Alloys”, p. 151; “Chemical Analysis of Aluminum and Its Alloys”, p. 137. (4) Casto and Boyle, IND.
[email protected].,ANAL.ED., 15,623(1943). (5) Classen and Hall, “Quantitative Analysis by Electrolysis”, p. 164,New York, John Wiley & Sons, 1913. (6) Cohen, A., Helzr. Chim. Acta, 26,75-88 (1943). (7) Ingham, L. H., J. Am. Chem.Soc., 26,1280(1904). (8) Laws, E. Q.,Analyst, 66,64-7 (1941). (9) Osborn, G.H., Ibid., 66,412-14 (1941). (10) Smith, E. F.,“Electro-Analysis”, pp. 122-3, Philadelphia, P. < Blakiston’s Son and Co., 1912.
