Nelson McKaig, Jr. St. Petersburg Junior College
st. Petersburg, Florida
Glass Tubes Coated with Methyl Violet as an indicator
The classical, or Fresenius, system of qualitative analysis separates the cations of Groups I1 and I11 by difference in the solubility of their sulfides in mildly acid solution (usually about 0.3 M is suggested). Laboratory textbooks differ in their instructions for obtaining solutions which are 0.3 M in acidity. Among a dozen recently published laboratory manuals, eight
used arbitrary procedures involving neutralization and subsequent acidification, with or without buffering, and with instructions for controlling the volume of the solution. Three others used strips of methyl violet paper as an indicator. One used "wide range" and "short range" indicator papers as alternatives to a modified methyl violet paper for determining the point
Volume 41, Number 8, August 1964
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of 0.3 M acidity.' The acidity of the solution is not critical, but it must he between 1.0 and 0.1 M to insure an acceptable separation of Groups I1 and 111. Methyl violet paper has been used in qualitative analysis instruction a t this school for a number of years. Despite its disadvantages, it is more reliable in the hands of the average freshman than an arbitrary dilution techcique. However, the commercially p r o pared strips are usually intensely colored, which makes it difficult to observe the blue-green shade corresponding to 0.3 A4 acidity. Homemade strips, less intensely colored, are better for observing the color but are less uniform and fade more rapidly. The addition of hemotoxylin to the methyl violet, recommended by Icing1 (p. 618) seems to offer little improvement although it does change the colors which are obtained. Table 1. Phosphate Buffer Solutions Used to Match the Color of Methyl Violet Solutions of Different Dearees of Aciditv
0.2 M NaH2P04 (ml) 50.00 50.00
t%.: 6.6 7.0
0.2 M NaOH (ml) 17.74 29.54
Dilute to (ml) 200 200
Methyl violet equivalent (moles acid/l) 1.00
n .sn
Better results have been obtained by using small sections of glass tubing coated on the inside with methyl violet. These can be prepared easily and cheaply in quantities sufficient for large classes. Standard 4-ft lengths of 3-mm od tubing are filled, using suction, with a 0.5% solution of methyl violet in acetone and are immediately allowed to drain. They are then dried by gently aspirating air through them for a few seconds while the tube is rotated in a nearly horizontal position. This rotation produced an even deposit of indicator on the inside of the tube. More than 100 tubes can be coated in a n hour. When dry, the tubes are cut into 2-cm lengths for student use, thus providing about 60 pieces from each 4-ft length of tubing. To determine the acidity of a solution, a drop of liquid is transferred to the end of the coated tube with a stirring rod. Capillarity will cause the drop to wet the inside of the tube for a distance of about 1 cm. The color of the indicator in the wetted portion immediately changes to show the acidity of the solution. The color produced is clear and of a desirable intensity for comparison with a set of standards. Since methyl violet fades rapidly in acid s ~ l u t i o n , ~ it cannot be used for preparing a set of reference standards. However, a solution of methyl violet at 0.3 M acidity almost exactly matches the color of a hromotbymol hlue solution of pH 7.2. Similarly, methyl violet a t 1.0 M acidity matches bromothymol hlue a t pH 6.6; methyl violet a t 0.5 M acidity matches bromothymol blue a t pH 7.0, and methyl violet at 0.1 &I' acidity matches bromothymol blue a t pH 7.6. Buffer solutions of these p H values can be prepared by mixing known volumes of NaHpPOa and NaOH in
' KING,EDWARD, J., "Qualitative Analysis and Electrolytic Solutions," Harcourt Brace and Co., New Yark, 1959, p. 421. a See accompanying note, MCKAIG,N., THIS JOURNAL, 41, 441 (1964).
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ratios which have been worked out by Clark,J and diluting the mixtures to a known volume. These ratios, with their methyl violet equivalents, for four concentrations of acid useful in the Group I1 procedure, are shown in Table 1. To develop the color, 5 parts of buffer solution are mixed with 1 part of 0.04% bromothymol blue indicator solution. This makes a suitable concentration for filling the reference tubes described below. Sets of reference tubes containing the stable bromothymol blue colors can be prepared from 5-mm od tubing. Fifteen-cm lengths are heated in the center and drawn to a capillary. The capillary is filed off at the point where the taper widens. The open ends of the 5-mm tubing are then sealed in a flame. The resulting 7.5-cm tubes are filled by submerging them, capillary end up, in a large test tube containing buffered hromothymol blue solution of the desired pH. This assembly is heated in a beaker of boiling water to displace the air in the small tubes. The tubes are then allowed to cool while submerged. After cooling, the filled tubes are sealed shut in a flame.
Figure 1 . Set of color r t m d a r d r , mounted on o 5 - X 8-in. c a r d , and rmoll tuber used for testing at upper right. An unmounted color standard t u b e is shown at the lower right.
The sets of reference tubes, with colors corresponding to methyl violet solutions of 1.0, 0.5, 0.3 and 0.1 M acidity, can he mounted on a white 5-X 8-in. card for use by the students. Such a card is shown in Figure 1. The bottom of each tube is wrapped in a single thickness of 1-in. wide masking tape, then placed on the card in such a position that the bottom 1.5 in. of the card may be folded up to cover the bottom of the tube. The tubes are fastened in this position with masking tape, and the fold on the card is stapled shut. The cards may be arranged along the reagent bottle racks over the student working spaces in the laboratory a t the rate of 1 card for each 2 to 4 student spaces. a CLARK,W. MANSFIELD, "The Determination of Hydrogen Ions," 3rd ed., The Williams and Wilkins Ca., Baltimore, 1928, p. 200.
