Determination of Sodium in the Presence of Other Metals E. C. ELLIOTT, Sinclair Refining Company, East Chicago, Ind.
T"'
add 100 ml. of freshly filtered reagent, and mix the two solutions. Stir the solutions for 30 minutes and filter in a Gooch crucible, using gentle suction. Wash the precipitate with 10 ml. of the reagent and then with 20 to 30 ml. of alcohol previously saturated with the precipitate. Dry the crucible in an oven at 105' C. for 30 minutes and weigh as NaC2H302:h~g(C2H?02)2.3(U02)(C~H~02)2,6~/2H20. The weight of precipitate times 0.0153 equals the weight of the sodium. In no case should more than 10 mg. of sodium be determined unless 10 ml. more of reagent are used for each additional milligram of sodium. In these experiments volumes of standard sodium chloride solution equivalent to 1, 5, or 10 mg. of sodium were added to known amounts of the salts to be tested.
paper concerns experimental m-ork undertaken t,o ascertain whether sodium could be quantitatively determined by the magnesium uranyl acetate method in the presence of the following metals: beryllium, cerium, columbium, lanthanum, neodymium, tantalum, thallium, thorium, vanadium, and zirconium. Because of the necessity of removing silica from the samples to be analyzed, fluorine and sulfuric acid were also investigated. The effect of many other metals has been reporbed by Barber and Kolt'hoff (1, Z), Kolthoff ( 7 ) ,Bridges and Lee ( 3 ) ,and others. Some of the work was done TTith the zinc uranyl acetate reagent. Their work was checked by this laboratory using the magnesium uranyl acetate reagent in place of the zinc uranyl acetate reagent in the presence of aluminum, calcium, chromium, iron, and titanium; the former gave as accurate sodium determinations in the presence of these metals as the latter. A11 the sodium determinations were made with a slight modification of the method of Caley and Faulk ( 5 ) revised by Caley and Sickman (6). The reagent (in two solutions) is prepared as follows :
TABLE 11. DETERMINATION O F SODITj&f I N A P;TIMoN Y Antimony
Treatment
Me. 63 63 250
Antimony not removed Antimony removed with HIS Antimony not removed
TABLE
Solution A Uranyl acetate 90 grams Acetic acid 60 grams Water t o make 1000 ml.
Solution B Magnesium acetate 600 grams Acetic acid 60 grams Water t o make 1000 ml.
a
Metal hfg.
Be
Be504
19.2
+ HC1
Ce
CeOP
La
Lat(S0,)s
+ HCI
244 139
90
Nd
Kd20~
Ti
TIC1
170
Th
Th(N0~)4
121
V
Zr a
VSOISO~
ZrSO4 Technical grade.
35
142
Na Sa Added Found MQ. >My. None None 1.0 1.02 5.0 4.82 Xone 2.99 1.0 4.02 5.0 8.19 None None 1.0 0 97 5.02 5.0 None None 1.0 0.98 5.0 5.03 None None 1.0 1.00 5.0 4.s1 Sone 0.03 1 0 1.02 5 0 4.95 None 0.18 1.0 1.17 5.0 5.14 Sone 0.01 1.0 1.02 5.0 5.10
Error 310.
R
1102 4.82
+0:02 -0.18
+z.o
1:03 5.20
f0.03 +O 20
+'3.0 +4 0
0:97 5.02
--003 +O 02
f0.4
-3.6
-i.O
-0:oz +0.03
-2.0 +O 6
1:oo 4.51
o:o0 -0.19
0.0 -3.8
0'99 4.92
-0.01 -0.08
-1.6
0199 4.96
-0:Ol
-0.04
-1.0 -0.8
1:o1 5.09
+0:01 +0.09
f'i.0 +1.8
..
MQ.
280 280 140 18.3 500 500 1840 200 200 18 3 9.15 1 53
None None 10 0 i o 05 Xone 10 0 10 0 None None i o 05 5 0" 1 00
2.22 0.61 11.25 10 4 None 10.22 10.49 0.03 0.04 9.60 5.05 1 05
Error My.
70
+i:as
++ii. s 4.0
+o:22 49
+'i.2
.. +0.4
+o
..
-0 4 + 0 05 +o 0.5
+4
9
-4
0
..
+10 + L O
Sodium present as S a F .
H&Oa before sodium determination.
Interfering Elements
..
019s 5.03
X Q .
Table I shows the results of experimental work on elements which did not interfere with the sodium precipitation. As some of the salts used contained small amounts of sodium, blank experiments were made and a corresponding correction was made in the determined weight of each precipitate. This procedure seemed justified by the consistency of the values obtained. Caley (4) reports that not less than 0.20 mg. of sodium can be determined, and this figure was taken by this laboratory as the maximum allomable error. The results of the experiments in Table I are all mithin this limit of error.
S a Found Kain Blank Mg.
FLEORIDE
Na Found
MQ.
b Fluorine removed with
TABLE I. DETERMIK.4TION O F SODIU&f Form
Sodium
Determined MQ. 1.3 1.4 Heavy gelatinous precipitate formed upon addition of reagent
INTERFERESCE O F
Na Added
Compound
Heat separatelx to 70" C. to obtain clear solutions. Mix and cool to between 20" and 30" C., and allow to stand overnight before using. Because of traces of sodium in the reagents, the solution \Till be saturated with the sodium magnesium uranyl acetate and the excess will be precipitated. Filter the reagent immediately before using at a temperature not to exceed 30" C. and carry out all subsequent work with the reagent at this temperature. Caley and Faulk recommend 20" C. for all n-ork, but consistent results were obtained anyyhere between 20" and 30' C., providing the precipitation and filtration were carried out a t the same temperature at which the fresh reagent was filtered. Reduce the volume of the neutral or slightly acid solution containing the sodium as the chloride or sulfate to 3 to 5 ml., rapidly
3Ietal
111.
PRESENCE O F
An attempt was made to determine sodium in the presence of columbium and tantalum. These metals formed heavy gelatinous precipitates upon addition of the acid reagent. For satisfactory sodium determinations these elements should be removed before determining sodium. Several authors have reported that antimony precipitates with the reagent and such was the experience in this laboratory with large amounts (250 mg.) of antimony. On a sample containing smaller amounts of antimony (63 mg.) consistent results were obtained with and without removal of antimony with hydrogen sulfide (Table 11). Some of the samples for analysis of traces of sodium con-
..
-'i.o ..
416
JULY 15, 1940
ANALYTICAL EDITIOS
tained 70 to 90 per cent of silica, 1 to 20 per cent of aluminum and other metallic oxides, and the remainder moisture. Removal of the silica was attempted by adding hydrofluoric acid to the sample and volatilizing fluosilicic acid and the excess hydrofluoric acid on a hot plate. The residue was dissolved in hydrochloric acid and the sodium determined without removal of the metals. The results obtained were very erratic; some duplicate results differed as much as 100 per cent. Experiments with pure aluminum fluoride, aluminum sulfate, sulfuric acid, and sodium fluoride (Table 111) led to the conclusion that a readily soluble fluoride such as sodium fluoride will not interfere with the sodium determinations but varying amounts of aluminum fluoride will be precipitated in the concentrated reagent. This difficulty was overcome by adding 1 ml. of concentrated sulfuric acid with the hydrofluoric acid and evaporating to dryness. This procedure removed all the silica and fluoride and eliminated the former erratic results.
417
Summary The magnesium uranyl acetate method for determining small amounts of sodium can be successfully used in the presence of beryllium, cerium, lanthanum, neodymium, thallium, thorium, vanadium, and zirconium. Silica, if present, can be removed with hydrofluoric acid and sulfuric acid. Columbium and tantalum n ill precipitate in the acid reagent, but no difficulty is encountered if these elements are removed previous to the sodium determination.
Literature Cited (1) Barber and Kolthoff, J . Am. Chem. Soc., 50, 1825 (1928). (2) I b i d , 51, 3233 (1929). (3) Bridges and Lee, IXD.ENG.C H E V . , Anal. Ed., 4, 264 (1932). (4) Caley, J . Am. Chem. S o c , 54, 432 (1932). (5) Caley and Faulk, Ibzd., 51, 1664 (1929). (6) Caley and Sickman, Ibid., 52, 4247 (1930). (7) Kolthoff, Z . a n d . Chem., 70, 397 (1927).
Weighing Flask for Precision Standardization of Strong Alkalies in the Absence of Carbon Dioxide PAUL NAHINSKY AND H. A. LAITINEN Institute of Technology, University of Minnesota, Minneapolis, RIinn.
D
URING a recent investigation involving the testing and use of several alkalimetric and acidimetric primary standards, it became necessary to carry out a large number of highly precise acid-base titrations in the absence of carbon dioxide. None of the weighing flasks described in the literature (1) was suitable for the purpose. This note describes a rapid and highly reproducible method for the standardization of strong alkalies under such conditions.
A set of consecutive results selected from the data for the standardization of nearly saturated solution of barium hydroxide is given in Table I. The conditions under which these titrations were carried out were extremely unfavorable, the humidity being close to, the saturation point and the laboratory temperature about 30" C.
A 125-ml. Pyrex Erlenmeyer flask is fitted with a standard interchangeable two-in-one glass stopper, A , A', and inflow, B , and outflow, B', tubes carrying small ungreased stopcocks. The gas inlet tube is somewhat constricted at the tip to permit a fine stream of bubbles to present a maximum of surface and stirring effect. The complete assembly should not weigh over 60 grams. METHODOF PERFORMING TITRATION.Suppose that it be required to standardize a stock solution of strong alkali, using either constant-boiling hydrochloric acid or an appropriately diluted solution of it. The flask, previously dried with a stream of dustless air and tared with a glass vessel of approximately equal surface, is weighed empty to the nearest hundredth gram (or milligram if desired). A roughly determined amount of the standard acid is introduced and the flask weighed a ain. Two or three drops of an indicator solution are added an% weighed (if an aqueous indicator solution is used it may be weighed with the empty flask, but with an alcoholic indicator solut'ion the above procedure is recommended to prevent loss by evaporation). OF BARIUM HYDROXIDE TABLE I. WEIGHTKORMALITY SOLUTION
I
I1
I11
0.17875 0,17877 0.17876 0 17877
0.17881 0,17881 0.17880 0.17881
0.178888 0.178883 0.178887 0,178886
The flask is then connected directly to the stock buret, which is provided with a long tip carrying a small rubber stopper t o fit stopper A ' , A stream of nitrogen or carbon dioxide-free air is bubbled through the acid for several minutes, the alkali is added with one drop in excess, and the vessel is weighed with stopper A in place. Stoppers A and A' are then removed, the sides are washed down Tvith freshly boiled distilled water, and the backtitration is performed with dilute acid from an ordinary buret. With practice, using two weighing flasks, four such determinations can be made per hour with high precision.
If a solid primary standard, such as benzoic acid or sulfamic acid, is employed, the procedure is similar, except that the standard is weighed into the flask from a weighing bottle and dissolved in an appropriate amount of water. S o danger of overloading the analytical balance exists, since the maximum total weight of the flask and solution never exceeds 150 grams. Literature Cited (1) Bessenberger, F. K., J. Am. Chem. Soc., 39, 1321 (1917).
