Thresholds Shown Calibration Lines
by the Iodine-Starch and Iodide-Iodate-Starch
SIR: The color given with excess of starch and iodide in moderately acid solution has been used by numerous for the determination of small es of iodine ( 1 3 ) . Although a t one time it was believed that the iodine-starch reaction obeys Beer’s law (4, 5 ) it has been shown in recent years that the calibration line relating concentration of iodine and absorbance through the origin bration line for the photocolorimetric determination of iodide by reaction with excess of iodate and starch also s h o w a threshold (3]6). The iodine-starch threshold cannot be overcome by increasing the iodide or starch content of the solution, or by altering the acid concentration; similarly the iodide-iodate-st,arch w s a constant intera irrespective of the excess of iodate or of starch. Xo satisfactory explanation for the existence of a threshold has hitherto been given. Lambert and Zitomer ( 7 ) ascribed the threshold to loss of iodine by deposition in the interior of the starch helices, but this view is open to the objection that the threshold would then vary with the starch content of the reaction mixture. whereas if starch is present in excess. the concentration of
the latter has no effect on the calibration line. Drey (3) put forward the alternative explanation that the iodide-iodatestarch threbhold (and presumably the iodine-starch threshold) is caused by hydrolysis of iodine to hypoiodous acid 12
+ H20 = HOI + H + + I -
and that a minimum concentration of hypoiodous acid is necessary before reaction of iodine with starch can occur; he was, however, unable to provide experimental evidence in support of this hypothesis. Such proof has now been provided hy Mokhnach and Rusakova (S), who found that the ultra-violet region of the iodine-starch spectrum contains an ahsorption band a t about 360 mp which is characteristic of hypoiodite ion ( 1 , 9). llokhnach and Rusakova concluded that, the addition of starch to an aqueous solution of iodine causes the following reactions to take place
+ HzO = 01- + 2 H + + I I* + I Is- + starch starch-iodine complex. I p
= 13=
This sequence of reactions would thus account for the observed thresholds in
the iodine-starch and iodide-iodatestarch calibration lines. LITERATURE CITED
(1) Chia, Y.-t., Thesis, University of California, CCRL-8311, p . 21, 1958. ( 2 ) Crouch, W.H., AXAL.CHEM.34, 1698 (1962). 11. E. A , , J . Pharm. Pharmacol. plement), 210 T (1963). ( 4 ) Gross, R. G., Wood, L. K., hlcHargue, J. S., ANAL. CHEM.2 0 , 900 (1945). ( 5 ) Houston, F. G., I b i d . , 22, 493 (1950). ( 6 ) Lambert, J . L.,Ibid., 23, 1251 (1951). ( 7 ) Lambert, J. L., Zitomer, F., Ibid., 35, 405 (1963). ( 8 ) Mokhnach, 1.. O., Rusakova, ?;. M., Doklady d k a d . >Yauk S.S.S.R. 145, p. 1290; Consultants Bureau transl. p. 7,53 (1962). (9) Ibid.,143, p . 122; Consultants Bureau transl. p . 160 (1962). (10) Sovick, R . P., Biochem. J . 83, 236 (19621. 11) Sugawara, K., Koyama, T., Terada, K., Bull. Chem. Soc. Japan 28, 494 (1955).
12) Su-oboda, P. A . T., Lea, C. H., Chem. & Ind. ( L o n d o n ) 1958, 1090. 13) Zak, B. In “Chemical Analysis,” B. L. Clarke, P. J. Elving, I. hl. Kolthoff, eds., J 7 d . 8: “Colorimetric Ijetermination of Xonmetals,” eds. I). F. Boltz, ed. p. 215,, Interscience, Kew York, 1958 R . E. A . DREY Control Laboratories Burroughs Wellcome & Co. Dartford Kent, England
Determination of Phenoxyacetic Acids with J and Phenyl J Acids SIR: Only one direct colorimetric method for the determination of 2,4-dichlorophenoxyacetic acid (2,4-D) is reported in the literature ( 2 ) . When chromotropic acid (1&dihydroxynaphthalene-3,6-disiilfonic acid) is heated \vith 2,4-D in concentrated sulfuric acid, a \vine-puriile color is produced. This reaction involves the splitting of formaldehyde from phenoxyacetic acid with concentrated sulfuric acid which subsequently reacts with chrornotropic acid to give the characteristic colored product ( I ) , Recently Kame1 and Kizinger (3) reported that formaldehyde reacts with J acid (6-amino-l-naphthol-3sulfonic acid) and phenyl J acid (6anilino-1-naphthol-3-sulfonic acid) in concentrated sulfuric acid to give characteristic colored products. This reaction was adapted by Sawicki, Thomas, and Sylvester for the colorimetric determination of formaldehyde and formaldehyde-releasing compounds ( 4 ) . In 2200
ANALYTICAL CHEMISTRY
benzene (m.11. 138-139’ (3.). Purified samples of 2-chloro-, 4-chloro-, 2,4,5trichloro-, pentachloro-, and 2-methyl4chlorophenoxyacetic acids and 2methyl-4-chlorophenoxybutyric acid EXPERIMENTAL were supplied by Chipman Chemical Apparatus. Spectra n ere recorded Co., Bound Brook, N. J. .\I1 otmher on a I3eckman 1113 spectrophotometer compounds (East’man Kodak n’hite Label grade) were used without n i t h I-cm. cell-. A I3auqch and Lomb further purification. Standard soluSpectronic 20 spectrophotometer n i t h tions of the phenoxy acids were pre1-inch cuvettes \ \ a i ubed in all pared in et,hanol. quantitative anal) seq. Reagents. J acid (6-amino-1 -naphGeneral Procedure. appro thol-3-sulfonic acid, I< 8: K Laborapriate volume of the phenoxyacetic acid standard solution is placed in a t o m - , Jamaica, iY.Y ) and phen>l 50-ml. beaker and the ethanol is J acid (6-anilino-l-naphthol-3-~~.1lfonic evaporated just to dryness on a steam acid, K 8: I< Laboratories) nere embath. Three milliliters of the allployed ab 0 370 (1%. v.) in concenpropriate reagent are mixed comtrated sulfuric acid J acid reagent ita3 pletely with the residue. The beaker filtered, prior to uqe, through a \intered gla- crucible to remove inis then covered with a watch glass and placed in an oven set a t 165” soluble impurities These t x o reagents are stable for about 6 months 2” C. for exactly 5 minutes. T h e if kept a t 4” C solution is cooled to room temperaPhenoxy Alkyl Acids. 2,4-Dichloroture. J Acid Procedure. A yellow color phenox? acetic acid (Eastman Kodak is produced for a positive test whereas Chemical..) n a s r e c q ~tallized from the present paper, J and phenyl J acids were used for the colorimetric determination of phenoxyacetic acids.
*
