Azolitmin , indicator papers

Litmus (azolitmin) is the classical indicator for acid-base titrations, although by now it has been supplanted by much better indicators. Neither litmus nor azolitmin should be used in colorimetric determinations of pH because of their salt and protein errors. Litmus is of value only in the form of indicator paper (cf. Chapter Eleven). 

Indicator papers frequently aid in the identification of chemicals. Strong mineral acids are acid towards methyl violet or thymol blue papers, moderately strong acids react acid towards Congo paper, and very weak acids are acid towards htmus and azolitmin paper. Strong bases show an alkaline reaction with turmeric or tropeolin 0 paper, medium strong bases with phenol-phthalein, and very weak bases with litmus or azolitmin paper. These papers are not recommended for use in quantitative analysis. ... 

The capillary phenomena in filter paper have been the subject of numerous investigations. It is possible to tell when the sensitivity limit of an indicator paper is being approached simply from the appearance of the reactions taking place. A drop of 0.001 N HCl placed upon congo paper will, of course, diffuse radially. The center of this region shows the red alkaline color of the indicator. This is surrounded by a circle showing the acid reaction, which is followed in turn by a water circle. Other indicator papers such as dimethyl yellow, azolitmin, litmus papers, etc., behave in similar fashion. 

The sensitivity of litmus and azolitmin papers increases as the indicator concentration diminishes. The usual paper is prepared from 1% solutions. 

When determining sensitivity with alkali, the water used for diluting should be completely free from carbon dioxide. Otherwise too low a sensitivity is found. It is obvious from the above data that azolitmin paper prepared from 0.1 % indicator solution is the most sensitive reagent paper for strong acids and bases. The presence of 10 N hydrochloric acid and sodium hydroxide can be demonstrated readily with this paper. It is best also for weaker acids and bases. 

---