ANALYTICAL CHEMISTRY
218 3,ci-ciiiiitrobenzoate (1.2 mg. j; fmction 300-320 nil. !vas chiefly isopropyl 3,%1iinitrobcrizoate (0.1 mg.). The position of niethyl 3,5-dinitrobenxoate is different froin that in FiLmres 1 and 2, as a somewhat different solwrit pystpni \vCi5 r i w i in the tievelopment. The method of separation described here has certain advaiitages over the paper-chromatographic method of Rice, Keller. and Kirchner (6)in that complex mixtures can be separated i n :I single system, the compounds can be recovered for further stud!,, and somewhat miailer quantities of the cornpounds 3re used. LITERATLRE CITED 1 I Ilenatock 1I
J
C'lirriL
II., and lliilliket~,S, l'., "Identification uf 1'ut.e Organic Compounds," 1,. 644. S e w Y c J ~ .John ~ , Wilev L Sons.. 1946. ,'I I Lipsoomb. IT. >-.*and Baker. I{. FI., .I. A m . Chem. S'oc., 64, 1751 (1942). i 4 ) Marvel. C:. P.,and Rands, R. I).. .Jr.. Ibid., 72, 2642 (1950). ( 5 ) Rice, R. G., Keller, J. G., and Kirchner. J. G., ~ ~ N A LCHEM., . 23, 194 (1961). ( 6 1 Trueblood, K. S . , and Maimberg, E. JT., J . A m . Chem. SOC.,72, 411' (1950). : 7 ) \ \ l i t e . J . \\-., J r . , Food Reseurch, 15, 08 (1960). $ 8 ) i\-hite, J , Ti.. ,Jr , and Dryden E. C . , - ~ X A L , CHEJII.,20, 563 (1948I , 12) Huiitress. E.
RECEIVED Julr 19, 1951. .lpproved as Journal Paper S o . 868. S e w York State l g r i c u l t u r a l Experiiiient Station, Geneva, S T., on July 12, 1951.
S O L 1933, 21b
Estimation of Thorium by Selenious Acid THE use of selenious acid to precipitate thorium from neutral or buffered solutions was first reported by Kota ( 2 ) ; the thorium selenite was deterniined gravimetrically. Avsilnble information in the literature about the operative conditions and the accuracy and reproducibility of the above method is, however, meager. Series of esperiments were, therefore, carried out t o investigate the adaptability of selenious acid for quantitative estimation of thorium. The present paper reports a simple iodometric method for its determination in thorium selenite. EXPERIMENTAL
h i e selenious acid, d t a i n e d by recrystallizing a sample of Slerck's reagent grade several times from absolute alcohol, was dissolved in water and allowed t o stand for about 4 hours. The clear aqueous solution was used t o precipitate thorium. Preliminary experiments showed that the precipitation of thorium from its aqueous solution by selenious acid is not quantitative, presumably because of the partial dissolution or hydrolysis of thorium selenite. To a n aliquot quantity of pure thorium nitrate (or chloride) solution with a p H adjusted between 5.5 and 6.0, an equal volume of alcohol was therefore added and thorium selenite was precipitated completely by adding a slow stream of an aqueous 20% solution of selenious acid with constant stirring. The precipitate was filtered through Whatman KO.41 paper and washed with 25 t o 30 ml. of 2% selenious acid solution followed by absolute alcohol till free from excess of the precipitant. It was then dissolved in a minimum quantity of concentmted hydrochloric acid and diluted t o 250 ml. The total acidity of the made up solution was maintained between 1 and 2 N .
'lable 1 .
K X ~ , t .s
o.
Estinlation of 'I'horiunl b y Selenious .\cid CeOI in CeCij or Ce(l;Oa)z .Added (:raiii
4
5 13
... ...
7
8 9
10 11 12
13 11
0 .'lo48 0,122'
0.0766 0.1476
0.15138 -~
'I'tiOz T a k e n . 'i'liOr Obtained. C;rLll,l Grail1 0 . 06.10 0,0638 0,0796 0.0798 0.1558 0,1556 0.1580 0.1014 0.3150 0.2370 0.4740 0,1014
0.1010 0 1558 0.1664 0,2012 0.2136
0.1580 0.1010 0.3155 0.2350 0,4736 0 1011 0.1008 0,155G
Difference, Graiu - 0,0002 - 0.0002 - 0.0002
0.1663 0.2011 0.2133
_ _ _ _ _ _ _ _ _ ~ ~
-o.0004 - 0.0005 - 0.0020 - 0,0004
-0.0002 -0.0002 -0.0002
staiidard thiosulfate and back-titrating it against iodine solution of h o \ v n strength ( 3 ) . .4s the addition of a large excess of thiosulfate leads t o high results in the iodometric estimation of selenium ( I ) , it was necessary to predetermine by trial experiments th(1 appropriatr quantity of thiosulfate reacting with the thorium selenite solution taken for titration. The reaction may be represented as follows:
+ 4Xa?S?O3+ 4HC1+ Sa2SeS10e+ Na&06 + 4SaCI + 3H20 (1)
H2S~r)z
hcLording to Equation 1, 1 mole of selenious acid corresponds to 4 moles of thiosulfate: it follows that a knowledge of the amount of tliiosulfnte reacting with selenious acid available from thorium seleiiitc serves :is a direct measure for the estimation of' thorium. h typical set of results obtained by the alternative method is given iri Table I. In all but one case (experiment 7) the error does not exceed 1%, an accuracy that compares favorably with the classical gravimetric procedures. Selenious acid precipitates quadrivalent cerium from neutral or slightly acid solutions. Prior reduction of cerium to the cerous condition is therefore necessary to prevent its interference in the estimation of thorium. The addition of excess alcohol to a cerous salt solution leads to the precipitation of cerous selenite. A quantitative separation of thorium from R solution containing cerous salts is therefore carried out by effecting the initial precipitation of thorium selenite in a medium containing not more than 50% alcohol and filtering the precipitate quickly through IVhatman No. 41 filter paper. Experiments 10 to 14 (Table I ) refer to such a separation of thoriuni from solutions containing varying quantities of cerous chloride (or nitrate). ACKNOWLEDGMENT
The authors \\-ish to express their sincere thanks to S. S.Joshi for facilities and keen interePt in the a o r k and to the Xational Institute of Sciences of India for :in sward of a research fellowship to one of them (G.S.D.1.
-0.0001
i o . 0002 -0.0003
__
To 10 nil. of this solution about 2 grunis of potassium iodide were added and the liberated iodine was titrated against standard thiosulfate ( 4 ) . Alternatively, the estimation was carried out b ~ adding . t o il known volume of thorium selenite solution ii slight excess of
LITERATURE CITED A L ED., , 9,431 (1937). ( 2 ) Kota. Chem. L i s t g , 27,.79, 100, 150, 194 (1933). J . , 18, 705 (1896). (3) yorris and F ~ ~ , (4) yogel,"Text Book of Quantitative ~ ~ A ~ n a ~l y spp, ~ , 362~~~ 3 . London, Longmans, Green & Co.. 1945.
( 1 ) Coleinanri and McCrosky, TND. ESG. CHEM.,. ~
aem,
RECEIVED.\larch 15, 1981.
~
~
