CCLV.—The colour changes of methyl-orange and methyl-red in acid solution

Abstract

IT is well known t.hat the colour of an indicator in solution depends, within certain limits, on the concentration of the hydrogen ion (or hydroxyl ion) in the solution. These limits, which vary greatly with the nature of the indicator? have been determined for a large number of indicators by Fels (Zeitsch. Elektrochem., 1904, 10, 208) and Salessky (ibid., p. 205), and a knowledge of them enables us t o choose, for any given volumetric operation, the indicator that will give the best results. Conversely, by testing a solution with a large number of indicators, we can arrive at an estimate of the concentration of hydrogen ions in it. Friedenthal (Zeitsch. EZeEtrochem., 1904, 10, 114) and Salm (Zeitsch. physikal. Chem., 1906, 57, 471) have measured in this way the degree of dissociation of weak acids and weak bases, and have obtained results agreeing often to within a few per cent. of those obtained by conductivity measurements; but it can hardly be seriously proposed to supersede the latter, more accurate, although more laborious, method by the former, except when the acid or base under investigation is extremely weak. On the other hand, the determination of the degree of hydrolysis of salts in aqueous solution is often both laborious and at the same time subject t o large percentage errors; a simple colorimetric method would therefore be of considerable importance, provided that it could be made as accurate as, or more so than, the methods usually employed. Friedenthal's method cannot be considered accurate enough for this purpose; the difficulty of judging colours without employing a suitable apparatus is very great, and, moreover, it is impossible t o keep weakly acidic standard solutions of indicators (for comparison) unchanged for any length of time.It is far better to determine once and for all the relation between depth of colour and the concentration of the hydrogen ion.

Color Standards for the Colorimetric Measurement of H-Ion Concentration

“…In fact, the mass action equation requires that, if one is able to determine both limits, equally distant from the half-transfor-mation point (at pair 4), then pairs 3, 4, and 5 will be substantially correct as calculated by the plan in question, but pairs 2 and 6 will be in error by nearly 0.10 pH, and pairs 1 and 7 by about 0.25. The mass action equation is, however, in accord over the useful interval of pH with the measurements of Tizard (1910) for methyl red, of Barnett and Chapman (1918) for phenol red,2 and of the present writer for all indicators studied by Medalia, except the acid range of thymol blue, which was not studied.…”

Electrochemical Carbon Nanotube Filter for Adsorption, Desorption, and Oxidation of Aqueous Dyes and Anions

“…The images in Figure S4 (Supporting Information) yield insight into near surface charging effects that may account for the electrostatic desorption process. Methyl orange is a pH indicator that gradually turns from yellow to red as the pH is decreased below 4.5 . Upon application of 3 V to an adsorbed MO monolayer, the effluent solution in Figure S4 (Supporting Information) takes on a pink to red color, indicating a decrease in pH below 4.5 and thus significant production of protons at the MWNT anode surface.…”