Titrimetric Determination of Thorium JAMES J. FRITZ AND JOHN J. FORD Institute for Atomic Research, Iowa State College, Ames, Zowa
Comparatively few volumetric methods for thorium are available; those involving precipitation of thorium with oxalate, iodate, or molybdate are indirect and subject to coprecipitation errors. A proposed method consists of direct titration of thorium with the disodium salt of ethylene-diaminetetraacetic acid, a soluble complex being formed. An internal indicator gives a sharp, reversible color change at the end point. Stoichiometric results are obtained in the pH range 2.3 to 3.4. Using Alizarin Red S indicator, no interference is caused by lithium, sodium, potassium, magnesium, calcium, stron-
B
tium, barium, lanthanum, chromium, manganese, cobalt, silver, zinc, cadmium, or uranyl ions. The following ions interfere, either by forming stable Versene complexes or by reacting with the indicator: titanyl, zirconyl, cerium(III), vanadyl, iron(III), nickel, copper(II), tin(I1, IV), lead, and bismuth. Levine and Grimaldi have shown that extraction with mesityl oxide separates thorium from all metals except zirconium, uranium, and vanadium. When this extraction is followed by a Versene titration, the result is a rapid and widely applirable analytical method for thorium.
PROCEDURE
ECAUSE of its coioiless nature and single valence state,
few volumetric methods are available for the determination of thorium. Most of the propoqed methods involve precipitation and filtration of a thorium salt, followed by determination of either the acid radical of the salt 01 the eycess precipitating reagent. Examples are the iodometric determination of iodate in thorium iodate (9, I O ) , titiation of thorium oxalate with permanganate (5),and oxidimetric determination of thorium molybdate after reduction of the molybdenum to the trivalent state (1). Blaedel and Malmstadt ( 2 ) developed a method for thorium, using a high frequency titrator to determine the end point. The solution to be analyzed is added to a known excess of oxalate and the remaining oxalate is titi ated with a standard thorium nitrate solution, Amperonietric titration of thorium with molybdate has been reported by Smales and Airey (11) and by Gordon and Stine (6). Deshmukh and Sxvamy reported a titrimetric method involving selenious acid ( 3 ) . The present method consists of a direct titration of thorium with a standard solution of disodium ethylenediaminetetraacetate (Versene), a soluble complex being formed. [Apparently a similar method for thorium is being studied independently bv ter Haar of Eindhoven, Holland. Brief mention of this was made in the oral “discussion” of another paper ( 7 ) . This information reached the authors after this paper had been submitted for publication.] An internal indicator gives a sharp, revcrsible color change a t the end point. The titration is carried out in acid solution where comparatively few cations form Versene complexes of sufficient strength to cause interference. Thorium can be conveniently separated from most interfering cations and anions by a preliminary extraction with mesityl oxide (8). When this separation is followed b y a T’ersene titration, the result is a rapid and widely applicable analytical method for thorium. REAGENTS AND SOLUTIONS
Alizarin Red S. Dissolve 0.05 gram of sodium alizarin sulfonate in 100 ml. of water. Chrome Azurol S. Disolve 0.05 gram of Chrome Azurol S in 100 ml. of water. Thorium Nitrate. Prepare a 0.025 M solution by dissolving about 13.8 grams of rea ent grade thorium nitrate tetrahydrate in 1 liter of 0.02 to 0.03 nitric acid. Standardize the solution gravimetrically by precipitation as thorium oxalate followed by ignition to the oxide. Versene. Prepare a 0.025 IM solution by dissolving 9.3 grams of reagent grade disodium dihydrogen Versenate (the disodium salt of ethylenediaminetetraacetic acid) in water and dilutin to 1 liter. Standardize the solution against primary standard cafcium carbonate using the procedure described by Goetz, Loomis, and Diehl ( 4 ) .
%
For moderately small amounts of thorium use a IO-ml. buret. Take a sample containing 6 to 50 mg. of thorium and adjust the volume to about 25 ml. Add 4 drops of .4lizarin Red S indicator and adjust the acidity with dilute ammonia until a definite pink color appears (about pH 2.5). Then titrate with standard 0.028 M Versene until the color begins to fade rapidly. Adjust the pH to 3.0 using a pH meter and continue the titration. The end point has been reached when the last trace of pink disappears, giving a clear yellow color. llagnetic stirring adds to the convenience of the titration. For larger amounts of thorium, take a sample containing 0.12 to 0.24 gram of thorium in a volume of about 100 nil. Add 6 drops of indicator and titrate as described above, using a 50-ml. buret. The procedure for extraction with mesityl oxide is given by Levine and Grimaldi (8). Collect the aqueous layers stripped from the mesityl oxide after the extraction in a 50-ml. volumetric flask and dilute to volume. Withdraw a 5- or IO-ml. aliquot and titrate with Versene, using the procedure described above. Calculations Mg. of T h = 232.1 V M where V = volume of Versenate solution and X = molarity of Versenate solution. INDICATOR
Several substances form colored complexes with thorium and can be used as indicators in the Versene titration. Among these are Alizarin Red S, Chrome ilzurol S, carminic acid, Thorin, and ammonium purpurate. Of these, hlizarin Red S is recommended because it gives sharper end points and fewer cations interfere. EFFECT OF PH
Alizarin Red S is an acid-base indicator (transition range 3.8 to 5.0). -4s the color of its basic form and that of the thorium complex are almost identical, Alizarin Red S can function as an indicator for thorium onlv in solutions having a pH of 3.8 or less. The intensity of the thorium-Alizarin Red S color decreases rapidly with increasing acidity, starting at about p H 2.1. This sets a lower pH limit for the titration somewhere in the neighborhood of pH 2.1. A study was made of accuracy as a function of pH. The results (shown in Figure 1) indicate that the titration is stoichiometric in the pH range 2.3 to 3.4 (thorium hydroxide begins t o precipitate around pH 3.5). The sharpest color change was found around pH 2.8. In the above titrations a 1 to 1 thorium-Versene complex is formed. Two moles of hydrogen ions per mole of thorium are
1640
V O L U M E 25, N O . 11, N O V E M B E R 1 9 5 3
1641 though the others still interfere. Potassium, strontium, chromium, silver, and uranium do not interfere. .Ilthough chromiuni(111) forms a very stable Versene complex (K = it does not interfere because of the verv slow rate of complex formation. Approximately 0.01 M concentrations of aluminum do not interfere, provided the pH a t the end point is around 2.4 or 2.5. If very large amounts of aluminum are present, some preliminary separation is required. If uranium is present, some dilution niay be required before titration. Zirconium and most of the other interfering cations cause a delayed and indistinct end point but do not interfere quantitatively. .4 method is now being developed, however, which should estend the S'ersene titration to the quantitative determination of zirconium.
?
~
Table 11.
Added Impuritj , hIg. None
3Ig
34 3 68 5 11 4
Effect of pH on Accuracy
Figure 1.
released during the titration. This causes some difficulty, as the final p H must be in the range 2.3 to 3.4. Use of a buffer (such as sodium chloroacetate-chloroacetic acid or sodium bicarbonatesulfanilic acid) is obvious, but the authors have found that buffers often decrease the sharpness of the end point and cause erratic results. By keeping the thorium within a given concentration range and by adjusting the pH near the end point, the above procedure gives accurate results without the use of a buffer.
114.0 142.7 34.3
Table I.
Interference Study"
Netal Sa Li + Ba++ .\f g++ Ca 3ln++ La+++ +
+
co
;+
E*
+
Versene Complex, Log Form, K
KO Interference
1.66 2.79 7.76 8.69 10.59 13.47 15.4 16.10 16.48 16.58
Interfering Ions 18.2 Pb++ 18.38 cut'++ 18.45 Ni 25. Fe++' Conditions. 29 mg. of thorium titrated in presence of 5 to 15 mg. of foreign cations Indicator. Alizarin Red S. 0 Some additional interfering and noninterfering ions are discussed in the text.
Zn++ 5 .\In++ 5 AI+++ 2
Tahle 111.
~
~-
Ver3ene, 311. 4 '(9 97 1 99
Theoret. Versene, 311 4 98 9 96 1 99
+ o 01 +o 01 + o on
19.42 24.35 4.96
19.44 24.32 4.96
-0.02 +0.03 2zo.00
4 98 5 00 .5 16
4 96 98 I 16
r O 02
+
Dlff , ,111
=to 02
1 0 00
~. __ Titration of Thorium Solutions Recovered by Extraction
T-erqene, 311 2.49 1,99 2.48
Theoret. Versene, SIl. 2.49 4.97 2.49
5.00
z.49 0.01 4.96
2.49 4.97 4.97
in.nn
5.00 5.00
2.33 2.51
2 49 2.49
+0.04 +n.o2
Alipuot. 1\11
