Determination of the pH of Textile Materials HELMUT R. R. WAKEHAM AND EVALD L. SKAU Southern Regional Research Laboratory, U. S. Department of Agriculture, New Orleans, La.
In all previously published methods for the determination of the pH of leather, paper, and textiles, a single-extraction operation has been used on the assumption that the amount of water per unit weight of sample is relatively insignificant. The results of the present investigation, however, show that for textiles the pH of the extract solution is a function of the quantity of water used in the extraction process. A n extrapolation method based on this fact is described for the determination of the pH of a fabric. The pH of a textile is defined as the pH of the water present in the cloth under A. S. T. M. standard conditions of temperature and humidity.
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TUDIES by Kolsky and Jones (7) and by Crist (4) have called attention to the importance of hydrogen-ion concentration in textiles as a factor affecting color stability, tensile strength, and other fabric properties. Their work has emphasized the need for a general understanding of what is meant by the pH of a cloth and the desirability of a uniform and reliable method of measurement. The U. S. Army Quartermaster Corps has included a pH requirement in a recent tentative specification for barracks bag material (9), proposing a method somewhat similar to the A. 5. T. M. standard method for determining the pH of scoured wool and of cotton tape (1). Both these methods are based on a single-extraction operation, using 75 to 250 ml. of water or buffer solution and 2 to 10 grams of sample, and measurement of the pH of the extract solution, either colorimetrically or potentiometrically. The value thus obtained is taken as the pH of the textile material. In general, these methods are similar to those now in use for the determination of the pH of paper and leather (2, 8, 14, 16). The differences in procedures proposed by the various investigators indicate the need for an adequate definition of pH for a substance of low moisture content such aa cloth. The purpose of the present investigation is to set up such a definition and to examine the single-extraction method in the light of that definition.
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Definition Applying the equation pH = minus log [H+], in which [H+] is the molar hydrogen-ion concentration in a water solution, the definition of the pH of a fabric must be based on the moisture content of the fabric. In extraction, the water in contact with the textile will dissolve out the minute amounts of acids, bases, or buffer salts contained therein. The pH of the extract solution will be determined by the concentration of these soluble substances and, therefore, by the amount of water used for the extraction. The smaller the amount of water used, the more accurately the pH obtained represents the pH of the original air-dried fabric. The nature of this effect of concentration on pH can be shown by simple measurements of acid, base, and buffer salt solutions.
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One-tenth mole portions of reagent-grade ammonium chloride, ammonium acetate, sodium acetate, otassium carbonate, and oxalic acid were weighed out and dissoged in five 10-ml. portions of distilled water. Using a Beckman Model G pH meter with the standard 6.25-cm. (2.5-inch) electrodes furnished with the
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50 100 150 MILLILITERS OF SOLUTION PER MILLIMOLE OF SOLUTE
FIGURE 1. EFFECI-OF CONCENTRATION ON pH OF SOLUTIONS 616
ANALYTICAL EDITION
October 15, 1943
617
in making determinations in neutral or nearly neutral mixtures some time is required for the absorbed gases or minute amounts of acids or bases from previous measurements on the electrodes to be Peplaced by the new solution, the meter isdicator must be allowed to come to equilibrium and the electrodes should be carefully rinsed with distilled water after each measurement. When the pH has been measured, the solution is returned to the beaker and another milliliter of water stirred into it. This mixture is allowed to stand for 15 minutes, then is stirred and squeezed to yield a more dilute solution for another pH measurement. I n this manner pH measurements are obtained for watercloth mixtures containing 3, 4, 5, 7, 10, 15, 20, 30, 50, and 100 ml. of water. The pH values of these dilutions are plotted against the volume of water added and extrapolated back to the zero ordinate-that is, to the air-dried condition-to give the pH of the fabric as defined earlier. I n actual practice it has been found practical to make measurements of dilutions with 3, 5, 10, 20, and 30 ml. of water only, since these values are usually sufficient for the extrapolation.
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FIGA.
E.
C. D. E.
F. Q.
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ML OF WATER ADDED TO 2 - G R A M SAMPLE CURVESON VARIOUSLY TREATED 2. EXPERIMENTAL CLOTHSAMPLES
Cotton twill without further treatment: Cotton twill treated with dilute potassium carbonate solution and airdried. Cotton twjll treated wjth dilute sodium acetate ,solution and air-dried. Cotton twill treated with dilute oxalic aord solution and alr-drled. Cotton twjll dyed with a sulfur color, wsshed with hot water, and airdried wthout further aftertreatment. Cotton twill dyed with a sulfur color and aftertreated with a bath oontaining acetic acid, copper sulfate, and sodium dichromate, then warhed and air-dried. 8.5-ouncejute burlap, 12 X 12 yarn count, used in the Southern Regional Research Laboratory for studies in mildew-proofing.
Textile pH used in this sense should not be confused with total acidity, which may be obtained by titration of acid or base present. Goode and Cox (6)have described a micro glass electrode that measured textile pH values for very low moisture concentrations. Their method appears to measure the pH of cloth as defined above more closely than any other published procedure, but it fails, nevertheless, to determine the true pH of the cloth under actual conditions of use.
Method of Measurement Measurements of the pH of textiles are being made in the Southern Regional Research Laboratory by the extrapolation method described below. A %gram sample of the fabric to be tested is weighed and cut into smdl pieces, which are placed in a small beaker and wet with 3 ml. of boiled distilled water. The fabric pieces are macerated with a spatula until the mass is uniformly wet. [Von Bergen and Crowley (IS) of the Forstmann Woolen Co. have found from a series of experiments performed on wool, that for successful application of the method, the fabric must be wettable by the water.] The bcaker is covered with a watch glass and allowed to stand for about 30 minutes, after which the mixturc is again stirred. The cloth is then pressed against the side of the beaker until about 1 ml. of water has been squeezed out. This is poured into the 5-ml. cup of the Beckman pH meter, and the pH is measured on the standard 2.5-inch electrodes of the meter. Since
Precautions against the usual errors in pH measurement are taken throughout each experiment. The pH of the water should be frequently checked to guard against contamination from acid or &dine vapors in the air of the room. The pH meter should be calibrated periodically with standard buffer solutions and the accuracy of potentiometric measurement determined by checking against a standard cell. The typical curves shown in Figure 2 were obtained in the above manner from a number of cloth samples. All the cotton samples were from 36-inch, 96 by 64, 2.85 cotton twill that had been prepared for dyeing by a regular caustic boil and bleaching process. Samples were given the several treatments indicated in Figure 2. Examination of the curves will show that only in some case8 does the pH of the extract liquid equal the extrapolated pH value. The assumption that the pH of the goods will be the same as the pH of an extract solution appears not to be valid for certain textile samples and thus cannot be relied upon as the basis of any general method of measurement. Curves C and E, for example, show nearly identical extrapolation values but widely different pH values at the 100-ml. concentrations. Attempts to measure relative hydrogen-ion concentrations by comparing extract solutions obtained with some standard quantity of water would, therefore, lead to erroneous results. From the curves in Figures 1 and 2 it is apparent that aa the solutions become more dilute the pH approaches that of pure water. The curve for sample E in the region below 30 ml. is an exception to this generality; the pH here rises farther above 7 with greater dilution. This abnormality seems to be typical of a number of sulfur-dyed cotton fabrics measured in this laboratory. No explanation for this behavior is offered.
TABLE I. REPRODUCIBILITY OF MEASUREMENTS ON SAMPLES OF UNTREATED SULFUR-DYED COTTON,E Sample
pH at 5 MI.
Deviation from Mean
pH at 10 M1.
Deviation from Mean
1 2 3 4 5 6 7 8 9 10
8.21 8.28 8.23 8.25 8.38 8.19 8.29 8.32 8.33 8.23
0.06 0.01 0.04 0.02 0.11 0.08 0.02 0.05 0.06 0.04
8.52 8.49 8.43 8.43 8.50 8.53 8.53 8.61 8.57 8.48
0.01 0.02 0.08 0.08 0.01 0.02 0.02 0.10 0.06 0.01
Mean Average deviation from mean
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8.27
8.51
0.05
0.04
In order to test the reproducibility of measurement with a particular sample, 10 samples of the untreated sulfur-dyed cotton ( E ) were treated with 5 ml. and then with 10 ml. of water, pH readings being taken at each dilution. The results of these
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measurements, along with deviations from the mean and the
average deviation from the mean value, are shown in Table I. The value obtained at the 5-ml. point for a single observation is, therefore, subject to an average error of 0.05, and this error would increase somewhat for smaller amounts of water. For a given set of observations the average error of measurement involved in the extrapolation would be of the order of 0.1 pH unit. Greater accuracy may be obtained by averaging several measurementa on similar samples. Some inaccuracy in extrapolation is encountered when the curvature of the plot at higher concentrations is fairly pronounced. As was pointed out in connection with Figure 1, however, the extreme value of pH attainable by extrapolation would be that of a saturated solution. Time is not a critical factor in measurements of the pH, since equilibrium between the water and the fabric is usually attained rather rapidly. All extractions were made at room temperature because a hot extraction may result in decomposition of materials in the cloth and thus lead to abnormal values.
Discussion Using the method described, a large number of pH measure'ments have been made in this laboratory on cloth samples under prevailing atmospheric conditions. It is apparent from the results of this investigation that the pH of a fabric depends upon ita moisture content, which, in turn, is a function of the relative humidity and temperature of the surrounding atmosphere. I n setting up specifications and testa, therefore, it would seem essential that a precise definition including the cloth conditions be
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adhered to. In order that a definite value may be asaigned to it, the pH of a fabric or textile material might be defined m the p H of the water present under A. S. T. M. standard conditions of 21 C. and 65 per cent relative humidity. This value is the one which would be obtained by the proposed extrapolation method using cloth samples with a moisture content brought to equilibrium in a standard conditioning room.
Literature Cited A. S. T. M. Standards, Part 111, D232-36, p. 448, and D25939T, p. 998 (1939).
A. S. T. M. Standards, Suppl. 111,D548-41, p. 193 (1941). A. S. T. M. Standards on Textile Materials, D629-41T, p. 26 (1941).
Crist, J. L., Am. DyeatufReptr., 31, 133 (1942). Goode, E. A., and Cox, A. B., Proc. SOC.Chem. Ind. Vidork, 37. 1215 (1937).
Goodings,A. C., Am. Dysstuf Reptr., 24,109 (1935). Kolsky, S. I., and Jones, B. M., Ibid.. 20,133 (1931). Launer, H. F., J. Research Natl. Bur. Standards, 22, 653 (1939). U. 5. Army Quartermaster Corps, Tentative Specification for Barracks Bags, 0. D . , P. Q. D. No. 142, par. F-3a, Feb. 27, 1942, as amended by the elimination of the use of buffer solution, Amendment No. 1, Aug. 26, 1942 (Mimeographed). Urquhart, A. R., J. Tezt. Inst., 18, T55 (1927). Urquhart, A. R., Bostock, W., and Eckersall, N., Ibid., 23, T136 (1932).
Urquhart, A. R., and Eckersall, N., Ibid., 21, T499 (1930); 23, T163 (1932).
von Bergen, W., and Crowley, T. N., private communication, 1943.
Wallace, E. L., J . Am. Leather Chem. Aesoc., 30,370 (1935). Wehmer, P. F., Paper Trade J . , 111, No. 1 2 , 3 3 (1940).
Photometric Estimation of Silicon in Magnesium and Magnesium Alloys A. J. BOYLE AND V. V. HUGHEY, Basic Magnesium, Incorporated, Las Vegas, Nevada In a rapid and accurate method for the determination of small amounts of silicon in magnesium metal a n d magnesium alloys, use is m a d e of the molybdenum blue color developed f r o m sodium sulfite reduction of the silicomolybdic acid complex at a pH range of 7.0 to 7.3. Interfering weak bases are complexed by the use of a m m o n i u m tartrate after color development.
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H E necessity for the determination of small amounts of silicon in magnesium metal and its alloys has become increasingly urgent. According to Mellor (8), silicon and silica are converted to silicides in molten magnesium and appear as a eutectic mixture. Occasionally, with larger amounts of silicon, crystals of MgZSi form, according to Beck ( d ) , which may be detected metallographically. Examination of the hydrochloric acid residue from 100 grams of incendiary bomb alloy revealed no free silicon. I n view of these findings, it is assumed that all silicon in magnesium metal is present as silicide which may be readily converted to silica by treatment with nitric acid. This work is intended to describe a rapid and accurate photometric method for the determination of silicon in magnesium and its alloys. It is based on the reduction of the silicomolybdic acid complex H&Si(MoO,)s].HzO (3,6) to molybdenum blue, Mo,Os.H:O (9).
The first attempt in this laboratory to determine the amount of silicon in magnesium metal and its alloys made use of the yellow complex (10, 1 1 ) . Iron, which offered the principal interference in this procedure, was removed as bask formate ( I d ) . Following this practice, the silicon found was consistently low. A procedure for the determination of silica has been reported utilizing the blue color from a sodium sulfite reduction of the silicomolybdic acid. complex (4, 6, 7). In order to prevent the hydrolysis of salts of aluminum, zinc, and iron present in the solution of the alloys, the molybdenum blue was first developed by sulfite a t a pH of 3, using formic acid to attain this acidity. As in the case of acetic acid ( 7 ) continued reduction of the ammonium molybdate reagent yielded utterly unreliable results. At thie pH, microscopic bubbles of sulfur dioxide very definitely interfere with transmission readings. To make possible the comparison of the molybdenum blue on the neutral or alkaline side, ammonium tartrate was employed to complex the aluminum, zinc, and iron present in the alloys. This procedure gave consistent results. The cloudiness due to the initial hydrolysis of the salts of weak bases during the develop ment of molybdenum blue disappeared completely a t room temperature after 50 minutes. Longer periods of standing resulted in extremely small changes in transmission. In the higher concentrations of silicon the development time had greater significance. This was not s d c i e n t , however, to warrant a develop ment period longer than one hour. Reagents and samples were prepared in paraffin-lined f l h and quartz beakers, respectively. Vycor glassware was also found
