as high as 1 nig. were noniiiterfrring. Naterials capable of reacting with hyrlt ogen peroxide to form peracids will ( a w e high readings. Salts of copper, ii:olybdenuni, iron, manganese, and the like are examples, but the interferences due to the presence of these ions may be rliminated by addition of a small amount of a complexing agent such as sodium hexanietaphosphate (Calgon). Aldehydes which form peracids with alkaline peroxide also produce color. Because traces of aldehydes are usually present in acetone, the acetone (even c.P.) used in this test niust be treated with potassium permanganate prior to use. Reducing agents of various kinds such as thiosulfate compete with the
oiganic base for the perphosphonic acid and cause low readings. I n the absence of inteiferences, concentrations of sarin and TEPP can be deteiniined to nithin 5% of their true values. For estimation of concentrations of saiin in air, air samples may be dran-n through a bubbler containing distilled diethyl phthalate. The removal of all phthalic anhydride from the diethyl phthalate is essential. Sufficient acetone is then added to the diethyl phthalate aliquot to effect a single phase when the aqueous perborate solution is added. LITERATURE CITED
(1) Epstein,
J., Rosenblatt,
D.
H.,
Deniek, 11. M., J . Org. Cheiu 21, 796 (1956). (2) Feigl, F., “Qualitative .knalysis by Spot Tests,” pp. 345-6, Elsevier, Xew York, 1947. (3) Feigl, F., “Specific and Special Reactions,” pp. 127-8, Elsevier, ?;em York, 1940. (4) Wheeler, C. L. (translator), “A N e w Reaction for the Detection for the Metalloid (Kon-1letal) Labile Halogen Linkage,” PB 119887, U. S. Department of Commerce, August 1944.
RECEITEDfor review July 21, 1956. Accepted Sovember 1, 1956. Pittsburgh Conference on Analytical Chemistry and ,\pplied Spectroscopy, Pittsburgh, Pa., February 1956
Colorimetric Determination of Sulfate with Barium Chloranilate R. J. BERTOLACINI and J. E. BARNEY, II Research Department, Standard Oil Co. (Indiana), Whiting, Ind.
F A new method for the colorimetric determination of sulfate is based on the reaction of solid barium chloranilate with sulfate ion at pH 4 in 50% ethyl alcohol solution to liberate highly colored acid-chloranilate ion. The broad absorption peak of the ion at 530 mM is used for the determination. Ethyl alcohol decreases the solubilities of barium sulfate and barium chloranilate and increases the sensitivity of the method to about 2 p.p.m. of sulfate. Precision and acInterfering curacy are about 1%. cations are removed with ion exchange resins. Phosphate, oxalate, bicarbonate, chloride, and nitrate do not interfere. The method has successfully been applied to the determination of sulfate in water and of sulfur in petroleum products.
C
methods for determining sulfate have received much less attention than gravimetric and titrimetric methods, principally because sulfate ion forms few colored systems. Nearly all colorimetric methods have measured the excess of some compound or ion that reacts with sulfate to yield a n insoluble compound. Older colorimetric methods involve four approaches: the diazotization of benzidine sulfate (8, Q), the reaction between phosphotungstomolybdic acid and the benzidine not precipitated by sulfate OLORIMETRIC
(15). the determination of a riietallic ion precipitated in a complev compound with the sulfate (4,14. 80),and the piecipitation of barium sulfate with excess barium ion folloned by the precipitation of the excess barium n-ith excess chromate and ieaction of thc chromate ion n-ith either nietabisulfite (18) or diphenylcarbazide (5. 13. 16). Of t n o more recent methods. one (12) depends on the ielease by sulfate of amaranth dye from a mixture with thorium boiate; the other ( 7 ) depends on the reaction of excess 4-amino-4’-chloiodiphenyln i t h sulfate and measuiement of the excess by ultraviolet spectrophotonietiy. All these methods suffer froni lack of sensitivity, interference by othei anions, or inconvenience. -4n interesting reaction discovered by Coutinho and Alnieida ( 2 ) suggested a neir colorimetric method for determining sulfate. They found that the silver salt of chloranilic acid (2.5-dichloro3,6-dihydroxy-p-quinone),added to a solution containing chloride ion, forms silver chloride and releases reddish purple acid-chloranilate ion (19). The principle of this method should have wide applicability in determining other anions. I n the reaction
T- + MA (solid) + -4-+ 1 I Y (solid) where Y is the anion to be determined and A is a colored anion of a n organic
acid, 111-niust be so niuch less soluble than hIA that the reaction is quantitative. -11-4 must be only sparingly soluble so that blanks will not be too high. 9rapid, sensitive colorimetric method for determining sulfate has been de~ i s e dto cover the range of 2 to 400 p.p.m. The reaction of slightly soluble baiium chloianilate with sulfate ion in acid solution is used to gire barium sulfate and the acid-chloranilate ion: Sod--
+ BaC&!1204
+ Hf
-+
+ BaSO?
HCsCI2O4-
The amount of acid chloranilate liberated is propoitional t o the sulfate ion concentration. The reaction is carried out in 50% aqueous ethyl alcohol solution, buffered a t a n apparent p H of 4. Ethyl alcohol increases the sensitivity of the method by decreasing the solubilities of barium sulfate and barium chloranilate; the solution is buffered because the absorption of the chloranilic acid solution depends on pH. As little as 2 p.p.m. of sulfate can be determined in the presence of 100 p.p.ni. of comnion anions. The only precaution necessary is t o remove cations. REAGENTS
Barium chloride, reagent grade. Chloranilic acid (2,5-dichloro-3,6-dihydroxy-p-quinone). Eastman, practical grade. VOL. 29, NO. 2, FEBRUARY 1957
281
Barium Chloranilate. Barium chloranilate is prepared by mixing 1 liter of 0.1% aqueous chloranilic acid with 1 liter of 5% aqueous barium chloride and permitting the mixture to stand overnight a t room temperature. The aged precipitate is washed with water until the supernatant liquid is free of chloride ion. Water is removed by centrifuging the precipitate three times with ethyl alcohol and once with diethyl ether and drying it 1 hour a t 60" C. in a vacuum oven. Buffer, pH 4.0. A 0.05M solution of reagent grade potassium acid phthalate. Ion exchange resin. Dowex 50 X 8, 20-50 mesh, hydrogen form. Potassium sulfate, reagent grade, for preparation of standards. PROCEDURE
.
An aqueous solution containing sulfate ion is passed through a column 1.5 cm. in diameter and 15 cm. long containing Dowex resin. The effluent is adjusted to p H 4 with dilute hydrochloric acid or ammonium hydroxide and pH paper. To an aliquot containing up to 40 mg. of sulfate in less than 40 ml. in a 100-ml. volumetric flask are added 10 ml. of the buffer and 50 ml. of 95% ethyl alcohol. The mixture is diluted to volume with distilled water, approximately 0.3 gram of barium chloranilate is added, and the flask is shaken for 10 minutes. The excess barium chloranilate and the precipitated barium sulfate are removed by centrifuging or filtering. The absorbance of the filtrate is measured with a colorimeter or spectrophotometer a t 530 mp us. a blank prepared in the same manner. The sulfate concentration is then obtained from a calibration curve prepared from standard potassium sulfate solutions. RESULTS
The method was tested with two standard solutions containing 20 and 200 y of sulfate per milliliter as potassium sulfate. -411 measurements were made with a Beckman Model B spectrophotometer a t a sensitivity setting of 2 in 1-cm. cells. Sulfate results, expressed in micrograms per milliliter, were : 20,20,20,20,20, 19, 19,20, 20, 20
Av. 19.8
Colorimetric
Turbidimetric
25,6,25.6 28.0 50. 22.8
25.8 28.8 51. 22.8
The tan material appeared to be amorphous; the purple, crystalline. When the tan solid reacted with sulfate ion, it formed a colloidal suspension that could not be removed by centrifuging or filterirfg. The purple crystalline material yielded easily filterable solutions upon reaction with sulfate. The solvent defines the solubilities of barium sulfate and barium chloranilate and hence affects the sensitivity of the method. If the reaction is carried out in aqueous solution, the solubilities of barium sulfate (9.6 X 10-6 mole per liter) (17) and barium chloranilate (2.2 X mole per liter) set a lower limit for the method of about 30 y per ml. of sulfate in the final solution. (The solubility of barium chloranilate in distilled water and in 50% ethyl alcohol was determined by equilibrating the aged precipitate with the solvent for 3 days with stirring a t 20" C. and determining the barium concentration in the supernatant liquid by emission spectroscopy.) I n 1 to 1 ethyl alcohol-water, however, the solubilities of barium sulfate and barium chloranilate are only 2.5 X lo-' mole per liter ($2) and 5.2 X 10+ mole per liter, respectively, and the sensitivity of the method is fixed by the absorbance
Agreement between the two methods was excellent. Sulfur in petroleum is conveniently determined by oxidizing the sulfur to sulfate and determining the latter. A motor oil, a motor-oil additive, and two fuels were analyzed by the colorimetric and gravimetric ( I O ) methods. I n each case, the sample was oxidized in the high-temperature induction furnace, and the evolved oxides of sulfur were absorbed in ammoniacal 3% hydrogen peroxide. The solutions were boiled to remove the ammonium hydroxide and to destroy the excess hydrogen peroxide before being subjected t o colorimetry. Results, as per cent of sulfur, were:
Experimental motor-oil additive
Colorimetric
Gravimetric
8.74.8.72
8 68
Co-iiercial motor oil 1.50, 1.50 Diesel fuel 1.09,1.09 Residual fuel oil 3.23, 3.30
1.50 1.00 3.20
The authors' past experience indicates thatthe gravimetric method gives low results when applied to organic samples of this type.
03
I
DISCUSSION
Factors that affect the accuracy, precision, and sensitivity of the method are: the method of preparation of the barium chloranilate, the solvent, the wave length of maximum absorption of the acid chloranilate ion, pH, color stability, and interfering cations and anions. When solutions of barium chloride and chloranilic acid were mixed, a finely divided tan precipitate was formed. Upon aging overnight in the mother liquor, it turned deep purple.
1
04 5 0
550
500 WAVE
Figure 2. Absorption curves of acid chloranilate ion 1. 2.
ANALYTICAL CHEMISTRY
)Lo
Figure 1.
I
600
LENGTH, ln,P
In water In 50% ethyl alcohol
200, 200, 200, 200, 200, 204, 201, 200, 204, 200 Av. 201
The standard deviation and relative error of the method for the standards were 1%. Three municipal waters and a lake water were analyzed for sulfate by the colorimetric method and by a conventional turbidimetric method (df). Sulfate results, expressed in parts per million, were: 282
Source of Water Hammond, Ind. Whiting, Ind. Chesterton, Ind. Lake Michigan
/*I
so4=
Effect of solvent on calibration curves
of acid-chloranilate ion. Figure 1 shows the effect of the solvent on sensitivity. The curve for ethyl alcohol-water was obtained by the recommended procedure; t h a t for water alone was obtained similarly, except that 50 ml. of distilled water were used instead of 50 ml. of ethyl alcohol. The systems obey Beer’s law up t o a t least 400 y per ml. of sulfate. Measurements were made with a Beckman Model B spectrophotometer in 1cm. cells. K i t h 5-em. cells, the usable sensitivity is 1 y per ml. of sulfate in the final solution, or 2 p.p.m. in the original sample. Wave length of maximum absorption for acid-chloranilate ion in aqueous chloranilic acid solution and for that produced according to the colorimetric method is shown in Figure 2. Both curyes were obtained with a Beckman Model DU spectrophotometer and 1-cm. cells. Curve 1 represents the absorption of 0.017, aqueous chloranilic acid; the p H of this solution was 2.2. Curve 2 \\-as obtained by reaction of 0.3 gram of barium chloranilate with 100 y pctr nil. of sulfate at p H 4 in 50% ethyl alcohol. The broad absorption peak observed a t 530 mp has also been reported as occurring a t 535 m p (11) and at 550 mp (23). The pII of the solution governs the absorbance of chloranilic acid solutions at a particular n a v e length; chloranilic acid is yellow, acid chloranilate ion is dark purple, and chloranilate ion is light purple (19). However, the sohibility of barium chloranilate increases with decreasing pH, and blanks are too high a t p H 2 . At p H 4 the sensitivity is adequate, adjustment of p H is simple, and blanks are small. Color stability was reached 15 minutes after the addition of 0.3 gram of barium chloranilate to 100 ml. of an aqueous
solution containing 250 y per ml. of sulfate. The absorbance increased an additional 5% in 24 hours. Cations interfere in the colorimetric method by forming insoluble chloranilates (6, 6, W ) ; anions may interfere by forming insoluble barium compounds. In the determination of 250 y per ml. of sulfate, the per cent error introduced by 250 y per ml. of six cations was : cut+ K+
92 0.8
Mg;+
J
Na 4”
0.8 0
+
Zn++
96
illuminum, calcium, ferric. and plumbous ions completely precipitated the acid-chloranilate ion. However, any interfering cations are readily removed by ion exchange (3). Anions tested for possible interference were chloride, nitrate, bicarbonate, phosphate, and oxalate; a t the 100-p.p.m. level, none reacted with a suspension of barium chloranilate in 50% ethyl alcohol a t p H 4. CONCLUSION
The analysis for sulfate can be completed in 20 minutes. The method is being adapted to the determination of sulfur in the products of lamp combustion (1). It is useful for the determination of sulfur wherever i t can be converted to sulfate. LITERATURE CITED
(1) Am. Soc. Testing Materials, “ A S . T.hl. Standards on Petroleum Products and Lubricants,” Philadelphia, Pa., 1955, Method D 1266-55T. (2) Coutinho, A. B., Almeida, 14. D., Anais. assoc. quim. Brasil, 10, 83 (1951).
(3) Fritz, J. S., Yamamura, S. S., AKAL. CHEM.27, 1461 (1955). (4) Iokhel’son. D. B.. Ilkrain. Khein. Zhur. 9,‘25 (1934). ( 5 ) Iwasaki, I., Utsumi, S., Tarutani, T., J . Chem. SOC. Japan, Pure Chem. Sect. 74, 400 (1953). Johnson, IV. C., ed., “Organic Reagents for Metals,” p. 22, Chemical Publishing Co., Kew York, 1955, Jones, A. S., Letham, D. S., Analyst 81, 15 (1956). Kahn, B. S., Leiboff, S. L., J . Biol. Chem. 80.623 (1929). Klein. B.. IND.ENG.’ C m x . ASAL. ED.’16,’536(1944). Kolthoff, I. XI., Sandell, E. B., “Textbook of Quantitative Inorganic Anal\-sisi” 3rd ed., p. 322 f f , .\lacmillan, New York, 1952. ’ Knbo, S., Tsutsunii, C., X e p t . Food Research Znst. (Tokuo) 2 , 145 (1949). La mbert, J. L., Tasuda, S. IC, Grotheei 11, P.. ASAL. CHEM. 27,800 (i955). ’ Lang, K., Biochenz. 2. 213, 469 e,
(1929). \ - - - - ,
hlahr, C., Krauss, K., 2. anal. Chem. 128, 477 (1948). Marenzi, A. D., Banfi, R. F., Anales
farm. y bioquiin. (Buenos Aiies)
8, 62 (1937). Morgulis, S., Hemphill, I f . G., Biochem. Z. 249,409 (1932). Neuman, E. T., J . Am. Chem. SOC. 5 5 , 879 (1932). Rnbia Pacheco, J. de la, Blasco Lopez-Rubio, F , Informi. p i n . anal. (Madrid) 4, 119 (1950). Schwarzenbach, G., Suter, H., Helv. Chim. Acta 24, 617 (1941). Seifter, S., Novis, B., A N A L . CHIXI. 23,188 (1951). Snell, F. D., Snell, C. T., “Colorimetric hlethods of Analysis,” vol. 11, 3rd ed., pp. 767-8, Van h-ostrand, New York, 1949. Suito, E., Takiyama, K., Bull. Chem. SOC.Japan 28, 305 (1955). Tyner, E. H., . ~ N A L . CHEX. 20, 76 (1948). RECEIVEDfor review .iugust 3, 1956. Accepted Kovember 9, 1956.
Rapid Method for Determination of Malic Acid ALAN E. GOODBAN and J. BENJAMIN STARK Western Utilization Research Branch, United States Department of Agriculture, Albany 7 0, Calif.
,Because malic acid is one of the acids involved in the citric acid metabolic cycle, its determination in plant materials is often important. A method is presented which requires about 2 hours for the analysis of several samples, is specific for malic acid within the limits tested, and can be applied to an aqueous extract of plant materials. Recovery from standard malic acid solution was 100.3%, and standard deviation was 2.2%.
A crude separation of malic acid from other materials is effected by ion exchange resins, and malic acid is determined colorimetrically in an eluate after reaction with sulfuric acid and 2,7-naphthalenediol.
M
as well as other acids, can be determined by fractionation of ion exchange resins in the acid form (5, 15, 20) or by partition ALIC ACID,
.
chromatography on silica gel columns (4, 8, I S , 14) and subsequent analysis of the fractions. These methods have the advantage of simultaneous analyses for a number of acids, but some require considerable time for each determination, and others lack the desired accuracy and precision. Chemical methods which have been used are time-consuming or low in specificity (1, 7, 11, 19, 18). Methods utilizing bacterial oxidation (9, 19) and VOL. 29,
NO. 2 , FEBRUARY 1957
283
