omogeneows Iace ment F. H. FlRSCHlNG Departmenl of Chemistry, University of Georgia, Athens, Ga,
b Barium is precipitated as the sulfate from a solution in which the multivalent cations are complexed with (ethylenedinitri1o)tetraacetic acid or 1,2-diaminocyclohexane N,N,N',N' tetraacetic acid. Magnesium ions are slowly added to the stirred solution and gradually replace barium ions from the complex. This slow release of barium ions produces a precipitation of barium sulfate from homogeneous solution. When the alkaline earth ions are present in equal molar concentrations, less than 4 0 of the strontium and less than 0.1 0 of the calcium are coprecipitated with the barium.
-
-
B
HE precipitation of barium sulfate
from homogeneous solution has been widely studied (3). Cations that form insoluble sulfates usually interfere. Recently, Afanasieva (1) used EDTA to separate barium from strontium and calcium, using the volatilization of ammonia to bring about the gradual release of barium from the chelate. Data on the gravimetric results were not presented. The method described here uses either (ethy1enedinitrilo)tetraacetic acid (EDTA) or 1 ,MiaminocyclohexaneN,N,N',N'-tetraacetic acid (DCYTA) to complex the multivalent cations (B). Sulfate ions can then be added without causing precipitation. When magnesium ions are slowly introduced into the stirred solution, a precipitate of magnesium sulfate will not form, for it is soluble. Instead, the barium ions are gradually replaced from the barium complex by the magnesium ions, which form a slightly more stable complex. The concentration of barium ions slowly increases until the solubility of barium sulfate is exceeded. Then precipitation begins and continues as barium ions are homogeneously released by the slow addition of magnesium ions to the solution.
stable EDTA complex than barium must be complexed.) Add 4 grams of ammonium chloride and adjust the p H to about 8 or 9 with ammonium hydroxide. Then add 1 mmole of ammonium sulfate and adjust the volume to about 200 ml. Place the beaker in a heating bath until the temperature is about 70' to 80' C., then slowly add ,0.02M magnesium chloride solution by dropwise
Table 1.
Precipitation of Barium Sulfate under Varying Conditions Using Barium-140 as Tracer Barium taken, 0.51 mmole or 70 mg. Magnesium chloride added, 0.8 mmole
Cation, Mmole
Chelating Agent,
Sulfate, Mmolea EDTA
Mmoles
...
1.5 1.5
0.6 0.6 0.6 0.6
... ...
0 . 5 Si+$ 0.5 Sr+z
3.0 6.0 1.5 1.5 1.5 1.5 1.5
1.1 1.1 1.1 1.1 1.1
0.5 Ca+z 0.5 Cats 0.5 Pb+Z
1946
a
ANALYTICAL CHEMISYRY
Ba Found, Grav., Mg.
Ba in Filtrate, Radiometric, Mg.
70.0 69.0 70.3 70.0 69.8 69.9 69.7 69.9 69.9
0.1
. ..
0.3 0.2 1.6
0.2 0.1 0.4
0.4 1.8 0.1
...
0.1
DCYTA ...
1.1 1.1 1.1 1.1 1.1
s;
0 . 5 +a 0.5Sr+I 0 . 5 Pb+l
Table II.
1.0 1.0 1.0
...
70.8
1.0 1.0
71.1
71.1
...
...
Interference Study Using Strontium-90 and Calcium-45 as Tracers
Cation Added, Mmole
Chelating Agent, Mmoles
Sulfate, Mmoles
0.1 Sr+a 0 . 1 Sr+*
0.4 0.4 0.8
1.0 1.0 1 .O
0 . 5 Sr+a 0.5 0.5 0.25 Ca+* 0.25 Cate 0.25 0.5 Caf2
0.6
1.3
3 .O 6.0 1.0 1 .0 1.0 1.0 1.0
0.5
1.1 1.1 1.1
1.0 1.o 1 .O
1 . 0 Cafa
1.1 1.1 0.6 0.6
0.8
Ba Taken, Mg. EDTA 33.0 33.0 33.0 70.0 70.0 33.0 33.0 33.0 33.0 33.0
Ba Found, Grav., Mg. 32.6 33.0 33.7
72.1
70.3 32.7 33.4 33.4 32.8 33.4
Cation in Fpt., Radiometric, Mg.
0.3 Srt* 0 . 4 Sr+l 0.9 Sr+z 1 . 6 Sr+z 1 . 6 Sr+* 0.06 Ca+l 0.05 Ca+l 0.04 Ca+l 0.02 Ca+l 0.03 Ca+2
DCYTA
PROCEDURE
Place a sample containing about 0.25 mmole of barium in a 250-ml. Teflon beaker. Dissolve the sample in distilled water and add a slight excess of EDTA. (All the barium and all the cations in the Bolution that form a more
addition from a buret (about one drop every 10 seconds), while the solution is stirred. The amount of magnesium chloride added must be sufficient to exhaust all the excess EDTA and also to replace all the barium (see Tables I, 11, and 111). After the magnesium chloride has been added, set the beaker aside to cool and then filter through a tared Selas crucible. Use 0.001M (Na)$Or
0.5 Sr+2
ea+*
70.0 70.0
71.7 70.8 71 .O
1.9 Sr+a 2.3 Sr+2
70.0 0.04 0.Pmmole magnesium chloride added when 33-mg. barium taken, and 0.8-mmole magnesium chloride added when 70-mg. barium taken. 0.5
as a wash solution.
Ignite in a muffle furnace a t about 800' C. Add &SO4 and reignite. Weigh as BaSO4. RESULTS AND DISCUSSION
The theory, application, and rediocounting techniques used in this method are similar to those used in the precipitation of barium chromate from homogeneous solution using complexation and replacement (2). Magnesium ion forms soluble sulfate and has a stability constant with chelating agents that is just slightly greater than barium, but still less than calcium, and about equal to strontium. Other cations, such as copper, nickel, or zinc, forming more stable chelates than calcium and strontium, would replace these ions from their chelates as well as barium. The use of magnesium ion aids a separation of strontium and calcium and is the only logical choice for a replacement ion. Results (Table I) show that barium can be determined reasonably well under varying conditions. The presence of equal molar concentrations of calcium, strontium, and lead does not seriously interfere in the determination. A compensation of errors is indicated when equal molar quantities of strontium are present.
Table 111.
Effect of Concentration Barium taken, 1.23 mmoles Sulfate taken, 2.5 mmoles 36 ml. of 0.05M magnesium chloride added
Barium Found, Mg. 167.8 168.5 168.4 171.4 171.5 172.1
Original Vol ., MI. 250 250 250 120 120 120
Final Vol.,
M1.
285 285 285 155 155 155
Table I1 shows that less than 4% of the strontium is coprecipitated when barium and strontium are taken in equal molar concentration. Calcium is coprecipitated to less than 0.1% when equal, double, and quadruple molar concentrations are present. Table HI shows the effect of concentration on coprecipitation. A high concentration causes an increase in coprecipitation. The use of barium-140 (Table I) indicates that the amount of barium in the filtrate would not account for so large an error. Magnesium is probably the chief source of coprecipitation error. The effect of this coprecipitation error has been decreased by using dilute solutions in the recommended procedure. However, the
fluctuation of about 1 mg. that occurs in some analyses is no doubt due to varying coprecipitation. Teflon beakers are used t o prevent the tenacious film of barium sulfate that often forms on the surface of glass equipment. The precipitation was made from hot solution to produce easily filtered crystals. The presence of ammonium ion also aided in the production of an easily handled precipitate (1). The addition of magnesium chloride a t one time results in a colloidal precipitate, even though the solution can be thoroughly mixed before a visible precipitation occurs. An attempt to apply this method t o the determination of sulfate met with discouraging results. The precipitate tended t o be colloidal and the gravimetric results erratic. LITERATURE CITED
(1) Manasieva, L. I., Zhur. Anal. K h h . 14, 294 (1959). (2) Firsching, F. N., Talanta 2,326 (1959). (3) Gordon, L.,,Sdutsky, M. L., Willard,
ET. N.,"Precipitation from HomogeneOUB
Solution," p. 69, Wiley, New York,
1959.
RECEIVEDfor review May 2, 1961. Accepted September 11, 1961. ACS Regional Meeting, Richmond, Va., November 1959.
Determination of Pow Distillation and NearHANS BRANDENBERGER' and HEINZ BADER1 Nest/&Applied Research Laboratory, Vevey, Switzerland
bAzeotropic distillation with dioxane is used for removal of powder moisture, followed by near-infrared spectrophotometry to determine the water content in the dioxane distillate. The method has been developed for instant coffee powders; but it has a large range of application, especially in the food industry and other fields where moisture values are still determined by a drying procedure. One determination requires about 20 rninUtes. The estimated reproducibility is A0.0270 of water content in the powder. HE moisture content of foodstuffs such as instant powders, dried vegetables, or dry mixes is often of great importance, especially because of its influence on keeping quality. I n most cases, drying methods are still used to
determine the moisture content; drying temperature and drying time are selected according to the requirements of the drying apparatus and the product under investigation. Such moisture values have a relative rather than absolute character; the results of one method cannot be compared to those of another. Under mild conditions, the evaporation of water is seldom complete; higher temperatures or longer heating periods bring about losses of volatiles and/or decomposition of the sample, resulting in additional weight losses. Often, a sample dried a t above 100' C. shows a continuous weight loss over the entire heating period. To eliminate these inconveniences, we have worked out a method which excludes most of the error possibilities encountered in the drying procedures by combining azeotropic distillation of the powder humidity by means of
dioxane (2) with spectrophotometric determination of the water content in the dioxane distillate a t a specific absorption band for water in the nearinfrared region ( I , s). PROCEDURE
A sufficient number of 50-ml. flasks with ground-glass joints, 10-ml. measuring cylinders with ground-glass stoppers, and 10-ml. bottles with poiyethylene covers are dried for 2 hours at 120" C., then cooled and stored in a desiccator. To remove traces of moisture, a Widmer column as illustrated in Figure 1 is dried by distillation of pure 1,4dioxane (Merck) before starting a series of determinations. For this purl
Present address, Clhemical Laboratory,
Institute of Legal Medicine, University of Zurich, Zurich, Switzerland VOL. 33, NO. 1 3 , DECEMBER 1961
8947
