The determination of thiols with diphenylpicrylhydrazyl as a spectrophotometric reagent.

Diphenylpicrylhydrazyl (DPPH), a stable, intensely purple free radical, is used as a reagent in the quantitative determination of various aromatic and aliphatic thiols by indirect spectrophotometric analysis. Plots of degree of reaction vs. time show that thiophenol and its derivatives react more quickly than aliphatic thiols with DPPH. Calibration plots are linear over the concentration range 0.05-3.00 x 10(-5)M thiol. The average relative error is in the range 1-2% and the absolute standard deviations range up to 0.50 x 10(-6)M.

Spectrofluorometric determination of acetylsalicylic acid, salicylamide, and salicyclic acid as an impurity in pharmaceutical preparations.

Spectrofluorometry, either direct or in combination with a separation technique, provides a sensitive and accurate method for the determination of certain extent fluorescent analgesic drugs and the determination of impurities in many combination preparations. A critical examination of the UV spectra of common analgesics and related compounds indicates that the fluorescence inner filter effect should be negligible below 10(-5) M and that selective excitation and emission wavelengths should minimize interference from other fluorescent species. Fluorometric procedures are presented for the determination of salicylamide, acetylsalicylic acid, and salicylic acid, as an impurity, in preparations containing salicylamide, acetylsalicylic acid, acetaminophen, caffeine, and phenacetin as major constituents. Inner filtering is the limiting factor only for the direct and indirect determination of salicylamide and the direct determination of acetylsalicylic acid. Results of fluorometric determinations compare favorably with other reference methods. Salicylic acid is determined in the 10(-7) M concentration range after separation from salicylamide, acetaminophen, and caffeine.

p-Aminophenol fluorescence and determination in the presence of acetaminophen.

The spectrophotometric and fluorometric properties of the aminophenols and of several compounds related to p-aminophenol were examined. A direct spectrofluorometric method for p-aminophenol determination at trace levels in methanol was developed and evaluated for the effect of inner filtering by acetaminophen. The method was applied to the determination of p-aminophenol as an impurity in acetaminophen and acetaminophen-containing tablets.

Tetracyanoethylene pi-complex chemistry. Indirect spectrophotometric determination of Diels-Alder-active 1,3-dienes.

An indirect spectrophotometric method, based on the rapid Diels-Alder reaction between cisoid 1,3-dienes and tetracyanoethylene (TCNE) and the destruction of an aromatic-TCNE pi-complex, was developed to determine eleven 1,3-dienes in the 0.05-1.00 x 10(-3)M range. These dienes were: cyclopentadiene; 1,3-cyclohexadiene; trans-1,3-pentadiene; 2,4-dimethyl-1,3-pentadiene; trans-2-methyl-1,3-pentadiene; 2-methyl-1,3-butadiene; 9-methylanthracene; 9,10-dimethylanthracene; 1,6-diphenyl-1,3,5-hexatriene; 2,3-dimethyl-1,3-butadiene; and 1,4-diphenyl-1,3-butadiene. Three 1,3-dienes were determined in the 0.05-1 x 10(-4)M range: cyclopentadiene, trans-2-methyl-1,3-pentadiene, and anthracene. The limit of detection for cyclopentadiene in carbon tetrachloride solutions is 0.11 microg/ml. Fourteen 1,3-dienes were found to form stable pi-complexes and could not be determined by the proposed method. For these 1,3-dienes, the spectra of some of the complexes are reported; in addition, relative equilibrium constants for the pi-complexes of 2,5-dimethyl-2,4-hexadiene, cis-1,3-pentadiene, 4-methyl-1,3-pentadiene, and 1,3-cyclo-octadiene were estimated. An explanation of the transient colour in the 1,3-diene-TCNE Diels-Alder reaction is suggested.

Colorimetric detennination of pyrogallol and 2,6-dimethoxyphenol with diphenylpicrylhydrazyl.

A colorimetric study was made of the reactions of the free radical 2,2-diphenyl-1-picrylhydrazyl (DPPH) and the phenols pyrogallol and 2,6-dimethoxyphenol. Both of these phenols were colorimetrically determined by use of DPPH, in the presence of various interferents. It was shown that the determination of pyrogallol and 2,6-dimethoxyphenol was more accurate when absolute ethanol was used than with 95 % ethanol. It was also shown that the best results were obtained when the ratio of phenol to interferent was no greater than 1:1.

Free radical oxidation of dihydric phenols with diphenylpicrylhydrazyl.

A spectrophotometric study has been made of the reactions between the free radical 2,2-dipheny1-1-picrylhydrazyl (DPPH) and the three dihydric phenols. It has been shown that water affects the rate of all three reactions and the stoichiometry of the DPPH-catechol (pyrocatechol) reaction. Because of the differences in the rate of oxidation in absolute ethanol, it is possible to determine selectively either hydroquinone or catechol in the presence of resorcinol.