Sensitivity of the Carbonate Test for Lithium EARLE R. CALEY AND A. L. BAKER, JR., Princeton University, Princeton, N. J.
T
HOUGH lithium carbonate is mentioned as a characteristic slightly soluble lithium compound in comprehensive works on qualitative analysis, no previous investigation appears to have been made of the sensitivity of a qualitative test based upon the precipitation of lithium as the carbonate. From the solubility of lithium carbonate in water at room temperature (1.31 grams per 100 ml. of solution a t 20" C. according to Mellor, I ) , one would expect that such a precipitation test would be very insensitive when performed by adding a sodium carbonate solution to a solution of a lithium salt a t ordinary temperatures. Experiment showed this to be the case. For example, the addition of 1 ml. of sodium carbonate reagent of any concentration to 1 ml. of lithium chloride solution containing 10 mg. of lithium was found to cause no precipitation a t room temperatures. On the other hand, on heating such a mixed solution to 100" C. an abundant precipitation of lithium carbonate resulted, as might be expected from the marked decrease in the solubility of lithium carbonate with rise in temperature (1). By performing the test a t this elevated temperature considerably less than 10 mg. of lithium can be detected, as is shown in Table I. OF LITHIUMCARBONATE BY SODIUM TABLEI. PRECIPITATION CARBONATE SOLUTIONS (From pure lithium chloride solutions at looo C.) Lithium Present
Me.
Lithium Solution ME.
Reagent Solution
M1.
Ap earance or Nonappearance of h e c i itate with Reagent of gtated Normality N 2N 3N
a high concentration of sodium ion has very little adverse effect on the precipitation of lithium carbonate, whereas potassium ion in high concentration has a slightly more noticeable effect. However, neither sodium nor potassium interferes very much with this test for lithium. For the detection of lithium in the presence of other alkalies the following procedure is satisfactory: Reduce the solution to be tested to a volume of about 1 ml., transfer to a small test tube, and add 1 ml. of 2 N sodium carbonate solution. After mixing the solutions well, stopper the test tube loosely and place it in boiling water for about 10 minutes. The presence of lithium is shown by the appearance of a white crystalline precipitate which usually adheres to the side of the test tube. Care must be taken not to prolong the test to such an extent that salts separate from the solution from evaporation. TABLE111. PRECIPITATION OF LITHIUM CARBONATE WITH 2 N SODIUM OR POTASSIUM CARBON.4TE (From lithiurn chloride solutions containing added sodium or Dotassium chloride) Li Na K Reaction with Given Reagent Present Added Added NazCOa KzCOs
MQ. 4 4 4 4 4
4 3 3 3 3 3
10 10 10
3
Table I shows clearly the need for closely restricting the volumes of the reacting solutions. A certain optimum concentration of sodium carbonate is required for best results. That potassium carbonate is a slightly less sensitive reagent than sodium carbonate in solutions of equivalent concentration is shown by Table 11. OF LITHIUM CARBONATE BY 2 N TABLE 11. PRECIPITATION POTASSIUM CARBONATE
1 1
50
100 0
0 0
MV. 0 0
'
0 26 50 100 0 0 0 25 50
100
+ T +++ ++ + ++-
+ ?: t+ -
Interfering Substances Ammonium salts increase the solubility of lithium carbonate to a marked extent and should therefore be removed from the solution before applying the test. Of course nearly all other cations give a precipitate with carbonate ion and thus interfere with the immediate application of the test. However, by taking advantage of the fact that lithium carbonate is the only metal carbonate which exhibits marked retrograde solubility with rise in temperature, the test may be applied after removal of other metal ions as carbonates by precipitation in cold solution. It is advisable to precipitate the interfering cations in dilute solution a t around 0" C. with just a sufficient amount of dilute sodium carbonate solution. After removal of the precipitated carbonates by filtration, the solution is then concentrated by vacuum evaporation a t room temperature or below. A second filtration to remove separated salts may be necessary before the lithium test can be applied to the small volume of liquid that must be used.
(From pure lithium chloride solutions at 100' C.) Appearance or Lithium Lithium Reagent Nonappearance Present Solution Solution of PreciDitate MQ. M1. M1.
1
25 50 100 0 0 0 25
Though 3 mg. is the smallest amount of lithium that can be detected by this particular procedure, correspondingly smaller amounts can be detected by reduction of the volumes of the reacting solutions. However, in working with very small volumes a special technique must be employed to avoid error caused by evaporation. A convenient microchemical modification of the test is the following: By means of a capillary pipet place one or two drops of the unknown solution in the bottom of a short length of 6-mm. glass tubing which has been sealed at one end. In a similar way introduce one or two drops of 2 N sodium carbonate solution and mix the solutions by means of a fine platinum wire. Seal off the open end of the tube as close to the liquid as possible. Place the prepared capsule in an ordinary test tube containing distilled water, heat t o boiling, and maintain a t the boiling point for at least 5 minutes. In the presence of a few tenths of a milligram of lithium a white crystalline precipitate will separate on the walls of the capsule.
10 10 10 10
1 1 1
Me.
1 2 1 2 1 2
Table I11 shows the influence of various concentrations of sodium or potassium ion on the sensitivity of this test. I n each of these experiments the volume of the test solution was 1ml., and 1ml. of reagent was used. It will be seen that 101
102
INDUSTRIAL AND ENGINEERING CHEMISTRY
Conclusions Though the carbonate reaction for lithium is too insensitive a t room temperature to be of much practical use, the increase in sensitivity when the test is performed a t 100" C. is such that the reaction becomes as sensitive and useful as some other qualitative reactions. About 3 mg. is the least amount that can be detected by a macromethod, but by the application of microchemical technique a few tenths of a milligram can be detected. The other alkalies do not interfere with the test. Ammonium salts prevent precipitation and must be removed. Nearly all other interfering cations may be conveniently removed by precipitation as carbonates in cold solution, the test then being applied to the filtrate after concentrating it to the proper volume. The carbonate test for lithium is the
VOL. 11, NO. 2
most nearly specific of the known precipitation reactions for lithium, though it is not nearly so sensitve as tests based upon precipitation as aluminate, arsenate, fluoride, phosphate, stearate, or triple uranyl acetate. I n spite of its comparatively low sensitivity it may be useful for establishing the presence of lithium as an essential constituent of an unknown material when a satisfactory decision as to the approximate amount present cannot be obtained by the usual flame or spectroscopic tests.
Literature Cited (1) Mellor, "Comprehensive Treatise on Inorganic and Theoretical Chemistry," Vol. 11, p. 755, London, Longmans, Green and Co., 1922. RECEIVED September 8, 1938.
Determination of Carbonyl Compounds by Means of 2,4=Dinitrophenylhydrazine Water-Insoluble Carbonyl Compounds H. A. IDDLES, A. W. LOW, B. D. ROSEN, AND R. T. HART, University of New Hampshire, Durham, N. H.
T
HE use of 2,4-dinitrophenylhydrazinein the qualitative identification of carbonyl compounds has been developed extensively by Allen (I), Brady (2), and Campbell (S), and its use as a quantitative reagent has been reported frequently for individual carbonyl compounds (4, 6, 7) and for a group of water-soluble carbonyl compounds by Iddles and Jackson (6)* Since many carbonyl compounds are insoluble or only slightly soluble in water, it seemed desirable to extend the earlier study (6) to include other carbonyl compounds which may be dissolved in alcohol. Consequently the present report is concerned with a study of the best conditions for the quantitative precipitation of certain alcohol-soluble carbonyl compounds as their 2,4-dinitrophenylhydrazones. Experimenta1 I n adapting the previous work in aqueous solutions to carbonyl compounds soluble in alcohol, preliminary trials were run on a representative alcohol-soluble ketone, acetophenone, to determine (1) the effect of temperature on the completeness of reaction and (2) the final dilution necessary to ensure a quantitative precipitation of the hydrazone. In the determination of carbonyls reported from this laboratory, the temperature was held a t 0" C. This led to the suggestion by Perkins and Edwards (6) that some occlusion of the reagent might occur when the reagent was saturated Eat room temperature but was used in a reaction which was cooled down to 0" C. To test this point three parallel trials were made as shown in Table I, in one of which the reagent was saturated at 0" C. and the run made a t the same temperature, and in the others the reagent was saturated at room temperature and the runs were made at 0" C. and at room temperature. The close agreement of the results shown in Table I indicates that the reagent was used up by the reaction sufficiently to compensate for any decrease in its solubility when a lower temperature was employed. In another test 50 ml. of precipitating reagent (saturated a t room temperature) and 10 ml. of water, the minimum volume of carbonyl solution previously used, were mixed and cooled to 0" C. No precipitate
was produced, showing that the dilution effect was sufficient to prevent precipitation of the reagent and any resulting occlusion. From these data it was concluded that determinations could be made a t 0' C. or a t room temperature. However, room temperature was selected for all later runs, as it offered the advantages of better particle size of precipitates, greater ease of filtration, and sufficiently low solubility of the hydrazones formed. I n determining the effect of dilution upon the precipitation of the hydrazones, trials were made in which 0, 50, and 100 ml. of 2 N hydrochloric acid were added after precipitation.
TABLE 1 . DETERMINATION OF ACETOPHENONE Volume of 2 4-Dinitro- Volume bhenylhyof drazine Sample M1. MI. Reagent saturated at Oo C. De- 30 10 termination made at 0' C. 30 10 30 10 30 10
Precipitation
Sample
Uram/ml.
%
0.006072
99.1 99.8 99.9 99.3
0.00647
Reagent saturated at room temperature. D e t e r mi n a t i on made at 0' C.
30 30 30 30
10 10 10 10
0.006072
0,00647
99.9 99.7 99.9 99.1
Reagent saturated at room temperature. D e t e r m i n a t i o n made at room temperature
30 30
10 10
0.00647
99.4 99.75
OF DILUTION IN THE DETERMINATION OF TABLE11. EFFECT ACETOPHENONE
Volume of 2 4-Dinitroihenylhydrazine MI. 30 30 30 30 30 30
Volume of Carbonyls
MI. 10 10 10 10 10 10
0.0647 gram per 10 ml.
Volume of 2 N Hydroohlorio Acid MI.
... ...
60 50
100 100
Total Volume MI. 40 40 90 90 140 140
Precipitation
% 99.0 98.6 99.1 98.1 98.8 98.6
