of Phthalic Acid Esters of Cellulose and of Polyvinyl Alcohol CARL J. M.&LM, LEO B. GENUNG, AND WILLIAM KUCHMY Cellulose Acetate Development Division, Eastman Kodak Co., Rochester, N. Y. Phthalic acid esters of cellulose and of polyvinyl alcohol have found several comniercial uses. Methods for determining their compositions have been investigated. Phthalyl content can be measured by titration or by ultraviolet absorption at 275 m p . Some of the customary methods for determining saponification value, ethoxyl, and hydroxyl are applicable to mixed esters and ether esters. Free phthalic acid, if present, can be removed in some casea by ether extraction or by a reprecipitation technique and then titrated. The free acid content can also be calculated from several combinations of data. Results in per cent can be converted to degreas of substitution by means of equations or nomograph. Representative data show the applicability of these methods to cellulose acetate phthalates, ethylcellulose phthalates, and polyvinyl phthalates. Using this information methods may be selected for production control and for analysis of the various products.
violet procedure phthalic acid is shown as one equivalent of phthalyl. I t is, therefore, desirable to determine the free acid content to assure accurate analyses. If salts are introduced during manufacture, some of the phthalyl carboxyls may not. titrate. This condition can be detected by comparing phthalyl values found by titration and tiy ultraviolet absorption. with a correction, if necessary, for free acid. The ash content is also an indication of this condition. Ether extraction in a Soxhlet extractor reiiiovm free acid from ether-insoluble esters. Procedures used for determination of acetyl in cellulose acetate ( I O , 18) give a irieasurr of the total ester and acid content of these materials. IIydroxyl contents Can be determined by an acetylation method (17) by making a correction for the titratable acidity. The Ziesel method for ethyoxyl, as adapted to ethylcellulose (2, 2.5). is directly applicable to ethylcellulose phthalates. Various conhinations of these procedures can be used to analyze mixed esters and ether esters, to calculate free acid contents, and also to indicate the accuracy of the methods of analysis. PROCEDURES
OLYIIYDRIC alcohols react with dicarboxylic acid anhy, in the presence of pyridine to form esters in which P d rides one of the carboxyls is combined and the other is free (16). Phthalic acid esters of cellulose or its derivatives were first prepared by Schulze (26) and by 3Ialm and Waring ( B f ) , and have been described further by Malm and Fordj,ce (1.5). Thme esters are soluble in organic solvents, and if the degree of substitution is sufficiently high, their ammonium arid alkali metal salts are soluble in water. Several applications make use of these solubilities and the fiimforming properties of dibasic acid esters of cellulose and polyvinyl alcohol. I n the pharmaceutical field enteric coatings of cellulose acetate phthalate have been patented by Hiatt (IW), Fox and Opferman (9), and Bogin (3) and further described by Malm, Emerson, and Hiatt (14). Photographic film backings have been patented by Malm (13), Staud (29), Fordyce ( 6 ) ; Nadeau (221, and Simmons (28). Talbot (32) has described the use of ethylcellulose phthalate a3 an antiabrasion coating for photographic film. Uses of cellulose acetate phthalate for impregnation of textiles (SO) and ns a protective surface coating for polystyrene ( 5 ) have been proposed. JVater-base paint compositions containing the ammonium salt of cellulose acetate phthalate are described by Fordyce ( 7 ) . Polyvinyl phthalates have heen proposed for use in enteric coatings (2O), photographic eniulsions (1I ) , and antihalation backings of photographic film (23, 24). The phthalyl content of phthalic acid esters of cellulose and polyvinyl alcohol can be determined by titration in a suitahlr solvent, using aqueous alkali and phenolphthalein indicator. Spectrophotometric methods (4,27) making use of absorption i n the ultraviolet, are also proving to be useful. Both of these measure combined and free phthalic acid. Although a small amount of uncombined phthalic acid is not ordinarily objectionable as far as the uses of the product are concerned, it does cause high phthalyl results upon titration; each mole of acid is titrated and calculated as trro equivalents of phthalyl. In the ultra-
245
Phthalyl Content by Titration. The phthalyl content, or apparent phthalj-I conterit i n the case of samples containing other acidity, may lie determined by dissolving accurately weighed, dry, 1-gram sarnplm in 50 ml. of a suitable water-iniscible solvent and titrating the sample and a blank wit,h standard 0.1 .Y alkali to a phenolphthalein en(! point. -1mixture of acetone, ethyl alcohol (3.1 denatured), and water ( 2 : 2 :1 h y volunie) hm lxen found to be a satisfactory solvent for many samples. The composition of this mixture may be varied as required. -4nothf.r niisture, 3.4 alcohol-methylene chloride ( 3 to 2 by volume) is a quick-acnting and good solvent for many of tliesic: type> oi esters. I t is sometimes necessary to add more alcohol during the titration to maintain homogeneous solution. Mixtures of acetone and w t e r have Ileen used for various esters, and 3-1alcohol is a good solvent for many ethylcellulose phthalates. Pyridine is another excellent solvent and has often heen used. The solution is diluted with an equal volume of water just before titrating. ITowever, when pyridine solutions of cellulose acetate phthalate are allowed to stand overnight, significant amounts of combinrd phthalyl hydrolyze. These solutions must, therefore. he titrated as soon a\ possible. The calculation is as follows:
% ' apparent pht,hal>-l = (ml. of NaOH for sample - ml. of S a O H for blank) X normality of NaOH X 14.9 sample weight
(1)
Phthalyl Content by Ultraviolet Absorption. The absorbance a t 275 mp of a phthalate ester dissolved in a suitable solvent under standardized conditions is a direct measure of phthalyl as well as free phthalic acid content. The procedure used was as follows : -4 0.1-gram sample of the ester was dissolved in a nikture of 3.1 alcohol and methylene chloride (3 to 2 by volume) in a 100ml. volumetric flask. This mixture has a density of 1.0 a t 25" C. and is a good solvent for many phthalate esters. T h e
246
ANALYTICAL CHEMISTRY
absorbance a t 275 mM was then measured a t 25" C. in a 1-em. silica cell using a Beckman Model DU spectrophotometer, following general instructions and taking the normal precautions specified by the manufacturer. If the observed absorbance was greater than 1.5, a dilution mas made, a second reading taken, and a correction for the dilution applied. However, in order to keep the absorbance values between 0.2 and 0.8 and, thereby, utilize the maximum sensitivity of the instrument, the following dilutions are recommended, depending on the phthalyl content of the sample: %
Phthalyl 0-10 5-20 10-40
>30
Volume to Dilute, AI1 L s e as is
Apprournate Dilution Factor 1 2
50
25
% apparent phthalyl
=
-
(2x1. of HCI for blank ml. of HCI for sample) normality of HCI X 7.45 sample weight
4
10
10
Accurately R eighed, dry, I-gram samples were treated with 50-ml. portions of 0.1 N alcoholic potassium hydroxide solution (3A alcohol-distilled water 90 to 10 by volume) in 250-ml. Erlenmeyer flasks. The samples and blanks containing exactly the same amount of alkali were allowed to stand for 3 days a t room temperature. Then, 50 ml. of 3A alcohol and 15 drops of mbromocresol purple indicator (0.1 gram of m-cresolsulfonphthalein in 100 ml. of 20% 3A alcohol by volume plus 1.60 ml. of 0.1 N sodium hydroxide; pH of indicator = 6.8) were added to each, and they were back-titrated with 0.1 N hydrochloric acid to a yellow end point.
x (3)
The volume of the stock solution specified is diluted to 100 nil., using the same solvent mixture, and the indicated dilution factor is used in the calculation, Equation 2.
Free Phthalic Acid. EXTRACTION METHOD. Free acid in ether-insoluble samples can be determined using a Soxhlet extractor.
The dilutions were made by volume at 25" C. but were checked I , \ neight to determine the exact correction factor. The solvent mixture should have a low absorbance blank. Before the 3A alcohol and methylene chloride are used, their absorbances should be checked. Various samples of 3.4 alcohol shox ed absorbances of 0.050 to 0 200 while methylene chloride varied from 0.050 to 0.250.
A 10-gram sample was placed in the thimble of an all-glass Soxhlet extraction apparatus and was extracted with diethyl ether for several hours or overnight. The ether solution was then evaporated to dryness and the residue dissolved in 50 ml. of distilled water by heating on a steam bath. This solution was titrated with standard 0.1 A' sodium hydroxide to a phenolphthalein end point. Blank determinations were made, starting with the evaporation of an equal volume of ether.
A stock solution of phthalic acid was prepared and used as a known with each set of samples. The solution contained 0.841 gram of phthalic acid (99.8% pure) diluted to, 500 ml. (5092.02 grams) with 3 8 alcohol-methylene chloride mixture a t 25 C. Ten milliliters of the stock solution was diluted to 100 ml., using the same solvent used in the analysis of the phthalate. The data for the solution meie as folloms: Phthalic acid = 03*' loo = 0.0167% 502.02 X 10 149 Phthalyl = 0.0167 X - = 0.0150% 166 rlbsorbance reading = 1.28 0.06 Solvent blank = 1.22 0.0150 x 1000 = 12.3 Factor = 1.22 The phthalyl or apparent phthalyl content of the sample was then calculated:
yophthalyl
= [absorbance of sample
-
absorbance of blank] X 12.3 X dilution factor
(2)
This equation applies to a 0.1-gram sample of ethylcellulose phthalate in 100 ml. of the specified solvent with the indicated correction for further dilution, if necessary. Other experiments showed that the factor in Equation 2 for cellulose acetate phthalates and polyvinyl phthalates was 12.0 rather than 12.3 Saponification Value. .4 saponification value, often calculated as per cent apparent phthalyl (equivalent weight 74.5) or as apparent acetyl (equivalent weight 43), can be determined by methods used for the analysis of cellulose acetate. It measures both carboxyls of combined and any free phthalic acid present plus other acid and ester groups. EBERSTADT METHOD. This heterogeneoqs saponification method, which has been described adequately ( I , 10, SI), is directly applicable to many phthalates of cellulose and its derivatives. SOLUTION METHOD. The saponification-in-solution method of Malm, Genung, Killiams, and Pile (19) is applicable when a homogeneous method is desired, particularly for cellulose acetate phthalates. The SOLUTION METHODFOR ETHYLCELLULOSE PHTHALATES. following procedure uses alcoholic alkali, which is a solvent for the ester and also for the ethylcellulose remaining after saponification.
% free phthalic acid
=
(nil. of NaOH for sample - ml. of NaOH for blank) normality of NaOH X 8.3 sample n eight
--
x (4)
REPRECIPITATIOK NETHOD.A 3.00-gram sample was dissolved in 100 ml. of solvent and trarfsferred to a 500-ml. separatory funnel with a blank carried in parallel. [The mixture of 3A alcohol and methylene chloride (3 to 2 by volume) was used for cellulose acetate phthalates and polyvinyl phthalates, and 3A alcohol only, for ethylcellulose phthalates.] Then 150 ml. of distilled water was added with shaking to precipitate the ester. After addition of 100 ml. of solvent naphtha 1 (or hexane) the mixture was shaken again and the layers were allowed to separate. The ester precipitate rose into the naphtha layer, and the water layer could be drawn off. The naphtha layer was washed once with 100 ml. of distilled water, and the combined water layers were titrated n-ith 0.1 iV sodium hydroxide to a phenolphthalein end point. Equation 4 was used for the calculation. Table I shows results obtained when samples were treated repeatedly by this procedure to check its effectiveness. A single treatment gives a result sufficiently accurate for most purposes and in a comparatively short elapsed time. Some useful variations can be made in the above procedure. When salts of the combined phthalic acid are present, the addition of water sometimes forms an emulsion which prevents completion of the determination. I n such cases, and in others where a measurement by ultraviolet absorption is preferred over a titration, the procedure was as follows: A 3.00-gram sample was dissolved in 100 ml. of solvent (3A alcohol for ethylcellulose phthalate or a 50:40: 10 mixture of 3A alcohol, methanol, and distilled water for polyvinyl phthalate) with warming if necessary. Then 150 ml. of 0.03 N hydrochloric acid \vas added, followed by vigorous shaking. A 5- to IO-ml. portion -'as filtered off and its absorbance a t 275 mp measured with a Beckman DU spectrophotometer, using a 1-em. silica cell. A blank and a known were run, and the calculation was made similar to Equation 2, except that a factor of 13.7 (for
Table I.
Effectiveness of Reprecipitation &lethod Reprecipitation and Titration 2 3 Blank 1
Cellulose acetate phthalate 0.1042 A' NaOH. ml.
Free phthalic acid, To Polyrinyl phthalate 0.1042 S S a O H . nil. Free phthalic acid, %
0.12
9.50
0 12
12.12
..
..
2.7 3.5
0.50 0.1
1.30
0.3
0.22
0.03 1.30
0.3
cellulose acetate phthalates, can 1)e calculittcd from phthalyl and apparent acetyl data if they contain no free ncid: 86
60
55
-
- 60
P ht halyl Groups per
c6
- 55
ANALYTICAL CHEMISTRY weight per cent hydroxyl number of acetyl groups per anhydroglucose unit number of phthalyl groups per anhydroglucose unit number of hydroxyl groups per anhydroglucose unit 3.86~ 1.02 ph
-u
1.11 ph 102.4 1.02 ph
-u
102.4
-
3
-
N o
N,
= number
A:.
=
of ethoxyl groups per anhydroglucose unit
.
5.T8e 1.596ph
-e
1.75 ph 1.596ph
-e
16o.i 160.7 3
-
- Ne - - V p h
-Nph
1700 N
1700 N h 162 f 42 N o f 148 N p h
162
The nomograph, Figure 1, is convenient for these conversions. A ruler laid across the per cent acetyl value on the left scale and the per cent phthalyl value on the right scale indicates the numbers of acetyl, hydroxyl, and phthalyl values a t the intersections with the corresponding center scales.
h
+ 28 N , + 148
Nph
The nomograph, Figure 2, serves a similar purpose. polyvinyl Phthalate Let
Ethylcellulose Phthalates = weight per cent ethoxyl Let e
N p h
=
number of phthalyl groups per polyvinyl alcohol unit
Ph
E
14,900 l v p h 44 148 N p h
+
(20)
Table 11. Phthalyl and Free Acid Contents of Known Samples
Sample
Ethylcellulose Grams
P h thal io Acid Added, Gram
1 1.0349 None 2 1.0000 0.0516 3 0,8908 0.1065 4 0.9903 0.1564 a Absorbance of blank = 0.143.
+
Sample Solvent, Grams
DATA Absorbance, Sample Blank0
101.03 100.97 100.96 101.09
0.768 0.937 1.037 1.297
Apparent PhthalYl, % BY Bs: ultratitration, violet equation 1 18.6 18.7 22.4 26.8 26.0 35.7 28.4 40.5
Free Phthalic Acid,
%
Found 0.1 4.9 10.8 13.4
Taken None 4.9 10.7 13.6
Table 111. Data on Cellulose Acetate Phthalates, Ethylcellulose Phthalates, and Polyvinyl Phthalates Acid-Free 5ample~ %sed 1 2 1 Cellulose Acetate Phthalates 39.7 39.5 36.2 35.7 1 30.1 33.2 2 29.2 33.4 33.6
E uation
Apparent acetyl, Eberstadt method Apparent phthalyl by titration Apparent phthalyl by ultraviolet Free acid B y ether extraction By repreci itation Calculatecffrom phthalyle Phthalyl combined Acetyl combined Hydroxyl Observed Calculated Degree of substitution (per C$ Phthalyl Acetyl Ash Apparent phthalyl by titration Apparent phthalyl by ultraviolet Apparent phthalyl by saponification Ethoxyl Free acid b y reprecipitation method Free acid calculated Phthalyls by titration and ultraviolet Phthalyls b y titration and saponification Phthalyl combined De ree of substitution (per Cd hhalyl Ethoxyl Apparent phthalyl by titration Apparent phthalyl by ultraviolet Apparent phthalyl by saponification Free acid By reprecipitation Calculated from phthalyls Calculated from phthalyl and saponification Phthalyl combined Degree of substitution a
Acid-free basis.
38.5 35.4 34.4
38.2 37.9 34.4 4.6 ... ... 3.9 29.7 (31.2): 18.7 (19.6)
6 8 9
0.0 0.. . 1 0.1 1.0 30.0 18.9
0.0 0.~ .1 0.1 -0.2 33.3 20.5
0.9 . .... 2.3 34.0 19.4
2.0 1.8 ~1.1 32.0 19.0
11 15
5.4 5.4
3.2 2.9
... ...
... ...
13 12
0.63 1.38
0.78 1.55
0.70 1.44
4
4
0.77 1.65 0.02 Ethylcellulose Phthalates 1 18.6 5.1 2 18.6 5.0 3 ... 4.6 37.5 47.5 4 0.0 0.1
...
...
4.0 4.2
% phthalyl
0.0
0.1
1.8
...
0.6 5.1
1.3 14.2
0.35 0.085 2.32 2.6 Polyvinyl Phthalates 1 72.6 69.6 2 72.6 69.6 70.0 72.4
17 16
4 6 5 8 21
0.3 0.1
0.8 0.0
0.2 -0.4 72.1 68.2 0.75 0.62 0.08
..
16.5 14.9 15.3 39.9 1.3
5 8
18.6
0.68' 1.48
...
6
0.13
Ash
Acid-Containing Samples 2 3
Table I1 prwent8 data on determination of a p p a r e n t phthalyl by ultraviolet absorption and on the use of the results, together with a p parent phthalyls by titration, to determine free-acid contents. The sample used was an ethylcellulose phthalate which was known to contain practically no free acid. Solutions containing varying amount8 of added acid were prepared as listed in Table 11, then 4.92 grams of each were diluted to 100 ml. (99.7 grams) giving a dilution factor of 20.2. The absorbance of each was measured, corrected for the blank, and calculated to phthalyl content:
+
0.26 2.33 73.0 69.0 70.2
75.0 69.8 71.8
73.9 70.4 70.4
4.4 4.4
3.2 5.8
4.2 3.9
3.1 3.5 3.9 66.4 (69.5P 69.3 (71.6)" 66.7(69.6)" 0.59 0.65 0.58 (0.66)O (0.73)" (0.67)s 0.18 0.22
.........
=
corrected absorbance X 12.3 X 20.2 X 0.1 X (wt. of sample solvent) sample weight X 99.7 The agreement observed in free-acid results indicates that the method has practical utility. Analytical data on cellulose acetate phthalates, ethylcellulose phthalates, and polyv i n y l p h t h a l a t e s a r e presented in Table 111. Calculations have been made using some of the data, with the methods indicated by the equation used. The samplea selected include both acid-
V O L U M E 25, NO. 2, F E B R U A R Y 1 9 5 3 free batches and others containing acid in order to test the utility of the methods. LITERATURE CITED
(1) Am. Sac. Testing Materials, Designation D 871-48. (2) Am. SOC.Testing Materials, Designation D 914-47T, “Tentative Methods of Testing Ethylcellulose,” Sections 13-16. (3) Bogin. H. H., U. S.Patent 2,491,475 (Dec. 20, 1949). (4) Eastman Kodak Co., unpublished data. (5) Fiedler, S. O., Bjorksten, J., and Yaeger, L. L., U. 8. Patent 2,578,683 (Dec. 18, 1951). (6) Fordyce, C. R., Ibid., 2,000,587 (May 7, 1935). (7) I b i d . , 2,338,580 (Jan. 4, 1944). (8) Fordyce, C. R., Genung, L. B., and Pile, M. A., IND.ENG. CHEM., ANAL.ED., 18,547-50 (1946). (9) Fox, S. H., and Opferman, L. P., U. S. Patent 2,390,088 (Dec. 4,1945). (10) Genung, L. B., and Mallatt, R. C., IND.ENQ.CHEM., ANAL.ED., 13.369-74 11941) (11) Hart; J. A. H., and Lee, E. W.,Brit. Patent 585,760 (accepted Feb. 24,1947). (12) Hiatt, G. D., U. S. Patent 2,196,768 (April 3, 1940). (13) Malm. C. J.. Ibid., 1.884.035 (Oct. 25. 1932). (14) Malm, C. J., Emerson, J., and Hiatt, G. D., J . Am. Pharm. A s SOC., XL,520-5 (1951). (15) Malm, C. J., and Fordyce, C . R., I n d . E’ng. Chem., 32, 405-8 (1940). (16) Malm, C. J., and Fordyce, C. R., U.S. Patent 2,023,485 (Dee. 10, 1935).
249
(17) SIaim. C. J., Genung, L. B., and Williams, R. F., IND.ENG. ( ’ H E M . , -4x.k~. ED.,14, 935-40 (1942). (18) Malm, C . .J., Genung, L. B., Williams, R. F., and Pile, hl. A , , Ibid.. IS, 501-4 (1944). (19) Ihid., 5 3 7 pxticularly groupII, p. 502. (20) S h i ~ i i(’. , J., and Hiatt, G. D., U. S. Patent 2,455,790 (Dec. 7, 1948). (21) Slalm, C. J., and Waring, C. E.,
(22) (73) (24) (25) (26) (27) (28) (29) (30) (31) (32)
Ibid., 2,093,462, 2,093,464 (Sept. 21, 1937); Brit. Patent 410,118 (May 10, 1934). Nadeau, G. F., U. S. Patents, 2,211,346, 2,211,347 (Aug. 13, 1940), 2,289,799 (July 14, 1942), 2,326,056 (Aug. 3, 1943), 2,326,067, 2,333,809 (Nov.9, 1943), 2,376,175 (May 15, 1915). Xadeau, G. F., and Starck, C. B., Ibid., 2,131,747 (Oct. 4, 1938). Salo, ?J., Ibid., 2,276,685 (March 10, 1942). Samsel, E. P., and McHard, J. A , , ISD. ESG. CHEM.,ANAL.ED., 14,75&4 (1942). Schulre, F., U. S.patent 2,069,974 (Feb. 9, 1937). Shrew, 0. D., and Heether, M. R., ANAL.CHEY.,23, 441-5 (1951). Simmons, N. L., U. S. Patent 2,327,828 (Aug. 24, 1943). Staud, C. J., Ibid., 1,954,337 (April 10, 1934), 2,271,234 (Jan. 27, 1942). Stone, H. G., and Malm, C . J., Ibid.,2,108,455 (Feb. 15, 1938). Subcommittee on Acyl Analysis, ANAL. CHEY., 24, 4OC-3 (1952). Talbot, R. H., U. S.Patent 2,331,746 (Oct. 12, 1943).
RECEIVED for review September 5 , 1952. Accepted November 13, 1952. Presented before the Division of Cellulose Chemistry a t the 122nd Meeting of the AMERICAN CHEMICAL SOCIETY, Atlantic City, S . J.
Spectrophotometric Determination of Aluminum in Thorium Extraction of Aluminum with 8-Quinolinol in Chloroform D. W. MARGERUM, WILBUR SPRAIN, AND CHARLES V. BANKS Iowa S t a t e College, Ames, Iowa The preparation of highly pure thorium metal has necessitated accurate methods for the determination of trace impurities of common metals such as aluminum. While this may often be accomplished spectrographically, wet chemical methods are advantageous because of their greater accuracy and i n some cases their simplicity. A rapid and sensitive chemical procedure has been devised for the determination of trace quantities of aluminum in thorium. This procedure utilizes a spectrophotometric determination of aluminum following an extraction with a solution of 8-quinolinol in chloro-
T
H E determination of microgram quantities of aluminum following a chloroform-&quinolinol extraction has proved highly satisfactory (1, 3, 5, 8, 11). I n these procedures aluminum is determined spectrophotometrically subsequent to its extraction into chloroform as tris(8-quinolinolo)aluminuminum(III). In the present investigation this method was applied t o the separation and determination of aluminum in thorium metal, for which no previous chemical method has been reported. Because chloroform-&quinolinol solution readily extracts thorium and other metals as well as aluminum, selective masking agents and accurate p H adjustment are essential to the method. Arsonic acids are noted for their selectivity for quadrivalent cations (10). A sulfonated arsonic acid was sought which would form a soluble thorium complex. The preparation of the sodium salt of 4-sulfobenxenearsonic acid had been reported ( 7 ) and it was therefore chosen as a masking agent for thorium. I n this application the thorium masking agent must prevent not only t h e precipitation of thorium hydroxide a t a p H of nearly 5 but
form. Extraction of thorium is prevented by the use of either 4-sulfobenzenearsonic acid or concentrated acetate buffer as the masking agent. From 2 to 120 micrograms of aluminum may be determined to within 3 ~ 0 . 5 microgram. The method can easily be extended for greater amounts of aluminum. Extraction with 8-quinolinol in chloroform from an acetate-acetic acid buffer results in removal of acetic acid into the chloroform phase, causing partial ionization of the 8-quinolinol. The absorption spectrum for a 1% solution of 8-quinolinol in chloroform containing acetic acid is given.
also the extraction of any thorium by the chIoroform-8-quinlinol phase. By itself the sodium salt of 4-sulfobenzenearsonic acid is peculiarly unfitted as a masking agent because its thorium salt has a low solubility and it will not prevent the precipitation of thorium hydroxide above a pH of 3. However, when this compound is used in conjunction with a dilute ammonium acetate-acetic acid buffer it meets all the requirements of a proper masking agent. Although the sodium salt of 4sulfobenzenearsonic acid forms a soluble complex with zirconium a t pH 5, it cannot be used to mask this element for an gquinolinol extraction. Strong emulsions form along with extensive zirconium extraction upon shaking with a chloroform-8-quinolinol solution. Small quantities of other sulfonated arsonic acids such as 2sulfoethanearsonic acid have been prepared in this laboratory. Preliminary tests have indicated that 2-sulfoethanearsonic acid is a better masking agent for thorium than is 4-sulfobenzenearsonic acid.
