Colour maps of acid–base titrations with colour indicators: how to choose the appropriate indicator and how to estimate the systematic titration errors

Abstract

To recognise the end point of acid-base titrations, colour indicators are still frequently used. Colour indicators can cause systematic errors, and the theoretical evaluation of these errors is given in many textbooks. Random errors caused by the transition range of an indicator can also be estimated with the help of pH-logc i diagrams. However, very often, undergraduate students have problems to correctly interpret the colours of the different species of an indicator (its acidic and alkaline forms) and colour changes during titration. Frequent questions are of the type: ''Is it still blue, or is it already red?'' The aim of the present text is to show how carefully a suitable colour indicator has to be chosen, and what colour changes happen at the inflection point of a titration. Typical acid-base titrations like the alkalimetry of hydrochloric acid, sulphuric acid, oxalic acid, and the acidimetry of borax at two concentration levels using eleven common colour indicators are compared with potentiometric end point determination. Photographs were taken of solutions containing the indicators during the course of titration, i.e., at different pH values. These photographs were used to prepare a colour map for each indicator. Such colour maps are very helpful to find the most suitable indicator for a certain reaction, and also to estimate the maximum accuracy.

Prepared Disappearing Ink and Deciphering of Documents

“…However, in cases where the pH cannot be set to an optimum value (pH = 14), although it is over 10.5, the ink turns blue and becomes invisible as soon as it is written on the paper. The transition of this indicator is difficult to see, since carbon dioxide from ambient air affects the results .…”

Covalent Conjugation of Amphotericin B to Hyaluronic Acid: An Injectable Water-Soluble Conjugate with Reduced Toxicity and Anti-Leishmanial Potential

“…The solutions were maintained under magnetic stirring for 3 h at room temperature, and the hydrochloric acid released in the reaction was then titrated with 0.1 M NaOH standard solution. The titration end point was reached when the solution turned from red to light yellow-orange (the transition interval of methyl orange is 3.0 (red) ≤ pH ≤ 4.4 (yellow), and within the transition interval, the mixed color is orange). Estimation of DO % (eq ) was calculated based on the volumes of sodium hydroxide used in the titration: where V NaOH is the volume of NaOH consumed at the equivalence point, C NaOH is the molar concentration of the NaOH solution, which in turn was determined with a titration against KHP using 0.5% w/v phenolphthalein as the indicator, m is the dry weight of OxHA used, and M is the molecular weight of a single d -glucuronic acid in the disaccharide unit of HA since only this monomer can suffer oxidation (Mw of d -glucuronic acid minus a water molecule that is lost during the condensation reaction to form the glycosidic bond between the d -glucuronic acid and N -acetyl- d -glucosamine monomers: 194.139 g/mol – 18 g/mol = 176.139 g/mol).…”

“…The solutions were maintained under magnetic stirring for 3 h at room temperature, and the hydrochloric acid released in the reaction was then titrated with 0.1 M NaOH standard solution. The titration end point was reached when the solution turned from red to light yellow-orange (the transition interval of methyl orange is 3.0 (red) ≤ pH ≤ 4.4 (yellow), and within the transition interval, the mixed color is orange 38 ). Estimation of DO % (eq 1) was calculated based on the volumes of sodium hydroxide used in the titration:…”

Synthesis and Characterization of New Azo Compounds Linked to 1, 8-Naphthalimide and Studying Their Ability as Acid-Base Indicators

“…This property is very useful to detect the pH-level and was used extensively by researchers in various fields; it is employed in biological research to determine the acidity of the living media [4] and in analytical chemistry to detect the end point in acid-base titration. [5] Furthermore, azo compounds were studied extensively because of their excellent thermal and optical properties in applications such as optical recording medium, inkjet printing [6], oil-soluble light fast dyes [7], organic photoconductors [8] and molecular memory storage [9].…”