Colorimetric Determination of Chlorine with p-Aminodimethylaniline D. H. BYERS WITH M. G. MELLON Purdue University, Lafayette, Ind.
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HE American Public Health Association recommends the o-tolidine method (2) for the determination of residual chlorine. A more recent colorimetric method is based upon the use of p-aminodimethylaniline as the color-forming reagent. Important papers dealing with the latter procedure are those of Kolthoff (7), Alfthan and Jarvis ( I ) , and Haase and Gad (6). Since a spectrophotometric study had revealed interesting facts concerning the o-tolidine method (4), it seemed worth while to investigate the p-aminodimethylaniline method in a similar manner. This paper presents a summary of what were considered the most significant results obtained. After a preliminary investigation of the earlier proposals for applying the method, the work was finally confined to the more promising modification of Haase and Gad.
COLORIMETRIC STAKDARDS.As it is necessary, in using this method with the standard series or comparator technique, to empIoy permanent standards, the acidified methyl red solution previously recommended ( I , 6) was studied. A 0.00161 per cent solution was finally selected. One milliliter of it, when diluted to 100 ml. in a Nessler tube, is colorimetrically equivalent to 0.10 p. p. m. of chlorine, treated with an excess of p-aminodimethylaniline, in a sample of 100 ml. The selection of this concentration was based upon the purity and brightness values calculated for I. C. I. illuminant C from the curves in Figure 1 (6, IO). Visually the match between the unknowns and standards is satisfactory, although the respective transmission curves do not check closely. Subsequent work in this laboratory indicates (8) the possibility of using as a standard a solution of potassium permanganate containing excess periodate. However, the methyl red
Experimental Work APPARATUS AND REAGENTS.The general technique followed, including the preparation and handling of standard solutions of chlorine, was reported reviously (4). Most of the color measurements were made wit: a recording spectrophotometer, set for a spectral band width of 10 mp, and the pH values were determined with a glass electrode. The reagent was re ared by dissolving 0.10 gram of p-aminodimethylaniline hyct)roc?doride (E. K. No. 492) in 10 ml. of water, to which were added first 25 ml. of 85 per cent orthophosphoric acid and then 15 ml. of water containing 1 gram of iron-free sodium dihydrogen phosphate dodecahydrate. This reagent showed no deterioration in 6 weeks. It was used by adding 0.40 ml. t o 100 ml. of sample containing chlorine. Standard comparison solutions were made by diluting t o 100 ml. the required volumes of an acidified solution of methyl red prepared accordin t o the directions of Alfthan and Jarvis (1) except for making &e concentration 0.00161 per cent.
THECOLORREACTION.When the reagent is added to a dilute solution of chlorine, a purple hue develops. Presumably the color may be attributed to a meriquinone (3, 9, l l ) , known as Wurster’s red, formed by the oxidizing action of chlorine on p-aminodimethylaniline. Excess chlorine decreases the color, probably through the formation of some quinone, Optimum color development for concentrations of chlorine up to 1.6 p. p. m. were obtained with 0.40 ml. of the reagent. The color developed immediately with chlorine in solution as such, but with chloramine 6 to 7 minutes were required for full color development. AIthough the colored system is not as stable as one would wish, making comparisons within 5 minutes after development of the color keeps the error from this source within the limit of visual matching errors. Beer’s law was found not to hold for concentrations greater than 0.65 p. p. m. Previous workers specified a pH range of 2.6 to 3.4 ( I , 7 ) . The optimum range found is 2.6 to 3.4 for concentrations up to 0.6 p. p. m. and 3.2 to 4.5 for higher concentrations. A change in hue from purple to yellow occurs in the range p H 8 to 9. Figure 1 (solid curves) shows spectral transmission curves, for a cell thickness of 5 cm., for concentrations from 0.05 to 1.00 p. p. m. As the curves for 0.40, 0.60, and 1.00 p. p. m. were calculated from measurements for 1-cm. cells, they are probably not as reliable as the others. The small band at 530 mp and the general symmetry of the curves are rather exceptional. The meriquinone system has a true purple hue. 202
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FIQURE1. SPECTRAL TRANSMISSION CURVES Solutions of chlorine plus p-aminodimethylaniline (solid curves) and for oorresponding methyl red standards (broken curves)
APRIL 15, 1939
ANALYTICAL EDITION
standards were stable for several weeks and conform closely to Beer’s law over the range used. INTERFERING IONS.Since the color reaction rests on the oxidizing capacity of chlorine, interference may be expected with certain substances, such as the ferric and nitrite ions, just as with the o-tolidine method. Iron increases the color intensity. For free chlorine the error for 0.1 p. p. m. of iron is equivalent to about 0.01 p. p. m. of chlorine. This error is doubled for 1.0 p. p. m. of iron. As the iron interference, a t least a t the beginning, is a function of time, larger errors may be expected with the slower acting chloramine. In this case, in the normal course of a determination, 0.1 p. p. m. of ferric iron will interfere to the extent of 0.02 p. p. m. of chlorine. Kitrites decrease the color intensity, the error being approximately of the same magnitude as that for iron. Chloramine gives a larger error than free chlorine.
Summary
A spectrophotometric study of the p-aminodimethylaniline method for the colorimetric determination of residual chlorine has shown the characteristics of the colored system and confirmed reports of others on certain factors affecting the appli-
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cation of the method. A change in concentration of the methyl red solution for standards is recommended. The sensitivity varies from 0.01 p. p. m. at the lower limit to 0.03 a t the higher concentrations. This method seems to have no advantage over the more familiar o-tolidine method unless one prefers matching purple rather than yellow hues.
Literature Cited Alfthan and Jarvis, 3. Am. Water Works Assoc., 20,407 (1928). Am. Public Health Assoc., “Standard Methods of Analysis,” p. 20 (1936).
Clark, Cohen, and Gibbs, U. S. Pub. Health Rpts., Suppl. 54 (1926).
Dragt and Mellon, IND. E m . CHEM.,Anal. Ed., 10, 256 (1938). Haase and Gad, 2. anal. Chem., 107, 1 (1936). Hardy, “Handbook of Colorimetry,” Cambridge, Technology Press, 1936. Kolthoff, Chem. Weekblad, 23, 203 (1926). Mehlig, IND.ENG.CHEW,Anal. Ed. (to be printed). Michaelis, J . Am. Chem. SOC.,53, 2953 (1931). Swank and Mellon, J. Optical SOC.Am., 27, 414 (1937). Willstatter and Piccard, Ber., 41, 1458 (1908). ABSTRACTEDfrom a thesis presented by D. H. Byers t o the Graduate School of Purdue University in partial fulfillment of the requirements for the degree of master of science.
Separation of Wood Extractives into Simpler Components E. F. KURTH, Institute of Paper Chemistry, Appleton, Wis.
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OOD is not uniform-that is, it is not chemically or structurally homogeneous. Its composition varies among species, in individual trees, and within the tree itself. In recent years, methods have been developed whereby wood can be quantitatively separated into its three major components : extractives, holocellulose, and lignin. Means for separating the carbohydrate fraction, holocellulose, into simpler components have been described elsewhere (4). Lignin, although amorphous, is relatively homogeneous when prepared from extractive-free wood taken from a single species. The extractive fraction contains by far the greatest variety of compounds. It is with the isolation, estimation, and characterization of the numerous components of this wood fraction that this paper deals. Extractives are those substances which are removed from plant materials by inert solvents such as ether, alcohol, and water, They are not an organic part of the structural elements or of the wood substance. Materials removed from wood by the use of chemical reagents, such as alkalies, mineral acids, and bleaching agents, lie outside of the above definition, as these reagents attack the wood substance and remove portions of the holocellulose and the lignin. It is extremely important to remove the extractives completely before subsequent analysis of the wood is undertaken. An approach to the separation of the extractives into simpler components may be based on the physical properties of the substances present, such as (1) volatility with steam, (2) solubility in ether, (3) solubility in alcohol, and (4) solubility in water. Included in Group 1 are the volatile oils, acids, and hydrocarbons. Representative of this group are alpha- and betapinene in the southern pines, dehydroperillic acid and cedrol in western red cedar, and n-heptane in Jeffrey and Digger
pines. Group 2 contains the materials in Group 1 that have not been removed previously by steam distillation and, in addition, the fats, fatty acids, resin acids, resenes, sterols, waxes, and nonvolatile hydrocarbons. A11 the above classes of substances are present in the Pinaceae. Group 3 contains materials in Groups 1 and 2 that have not been removed previously and, in addition, the tannins, phlqbaphenes, and the natural pigments. Group 4, after extractions have been successively made with ether and alcohol, includes the soluble carbohydrates, cycloses, and salts. The separation into the above groups is based on the assumption that the wood has been air-dried and ground to pass a 40-mesh screen. The ether-soluble material may be removed by 8 hours’ continuous extraction in a Soxhlet type of extractor. Prolonged treatment with ether is to be discouraged, for in such instances some of the natural pigments related to the phlobatannins will be dissolved. Substitution of petroleum ether or chloroform for the ether leaves these pigments undissolved. Extraction of the tannins, phlobaphenes, and related pigments, when a further investigation is desired, is preferably done with alcohol a t room temperature. With wood containing a large amount of tannin and phlobaphene, it is sometimes necessary to remove the last traces of these substances by extraction with hot alcohol acidified with approximately 3 per cent of acetic acid. The difficulty of removing all the tannin and phlobaphene, particularly from old heartwood, stumpage, and roots, is due to the reaction of these substances with the mineral salts in the soil water to form insoluble salts. Treatment with acidified alcohol decomposes these salts and renders the tannin and phlobaphene soluble. Since the phlobatannins are related to the benzopyran pigments, they are capable of acting as indicators and they give
