Transformation interval

Martens R.M., Rosenhauer M., Buttner H., and von Gehlen K. (1987) Heat capacity and kinetic parameters in the glass transformation interval of diopside, anorthite and albite glass. Chem. Geol. 62, 49-70. 

The commercial product may be purified by recrystallization from methyl or ethyl alcohol. The stock solution should contain 0.1% of the indicator in a 60% alcohol solution. As is the case with all single-color indicators, the transformation interval will depend upon the concentration of the indicator (cf. section eleven, below). The interval lies between pH 8.0 and 9.8 (colorless to violet-red) when 1-2 drops of the 0.1% indicator solution are added per 10 c.c. of solution under investigation. The color fades in strongly alkaline solutions. 

The transformation interval of o-cresolphthalein is between 8.2 and 9.8 (colorless to red). The colors of the pyrogallolphthalein in alkaline media are not very stable. 

Azo derivatives of phenolphthalein. H. Eichler has described the preparation and use of several azo derivatives of phenolphthalein without, however, mentioning their transformation intervals. It is also uncertain whether these new indicators are any better than the classical indicators. These compounds are ... 

Db. Eichler has kindly sent the following samples to the author who has established their transformation intervals ... 

The indicator dissolves in water forming an orange-red solution. It is readily soluble in ether, methyl alcohol, and ethyl alcohol, much less soluble in benzene, toluene, and xylene, and insoluble in ligroin. Its transformation interval is from pH 6.0 to 7.6 (yellow to blue). 

The transformation interval of bromphenol blue is pH 3.0-4.6, yellow to purple. The indicator is not well suited for colorimetric pH determinations because of the disturbing effect of dichromatism. Tetrabromophenoltetrabromosulfonephthalein is more suitable for this purpose. 

C25H28O7, melts at 184°. A stock solution should contain 0.1% of the indicator in alcohol. Its transformation interval is pH 2.6-4.0, and the compound is very useful both for colorimetric and titrimetric purposes. 

The transformation intervals of the most typical of the above compounds are as follows ... 

The transformation intervals of the compounds derived by varying Ri and R2 in the above six types of azo indicators are listed in the following table. 

A. Thiel and W. Springemann have studied the light sensitivity of the various azo indicators dissolved in organic solvents. In this connection, Thiel and his collaborators prepared a number of additional azo derivatives. The author has determined the transformation intervals of certain of these compounds prepared by Thiel. His findings are summarized below. 

The commercial preparation may be recrystallized from water. Stock solutions should contain 0.1% of the indicator in water, and two drops of the indicator solution are required per 10 c.c. The transformation interval is between pH 1.3 and 3.0, the color changing from red to yellow-orange. This indicator is very useful, possessing a small salt error. 

As was shown in the very beginning of this chapter, a single indicator always possesses a transformation interval rather than a transformation point. By combining two suitable indicators it is often possible to prepare mixtures which change color rather sharply at a given pH. This can be done in the following manner ... 

Quinine was studied most thoroughly by J, Eisenbrand (l.c.). It has two transformation intervals pH 5.8 (forget-me-not blue fluorescence) to 6.5 (bluish violet fluorescence), and pH 9.0 (violet fluorescence) to 10.0 (colorless). 

This expression corresponds to the one derived for the acid indicators. The entire discussion concerning the transformation interval applies equally well to the basic indicators. 

The concentration of indicator will influence also the extent of the transformation range. Suppose we have two single-color indicators possessing colored forms perceptible at the same concentration (i.e. the same [I min.3) having equal dissociation constants. If we always use saturated solutions of the indicator (i.e. CHI] = L), then it follows from equation (9) that the transformation interval begins at a hydrogen ion concentration given by... 

It is not possible to determine the sensitivity of neutral indicators (which have a transformation interval in water in the neighborhood of pH = 7) in the above manner, because traces of impurities in water have too great an influence upon the... 

F. M. Cray and G. M. Westrip have determined the influence of acetone upon the transformation interval of a number of indicators. They worked with acetone containing 10% by volume of water. They prepared various buffer mixtures, the pH s of which were measured potentiometrically by means of the quinhydrone electrode. The following table shows how large an effect acetone has upon the magnitude of the dissociation constant. 

Transformation Interval and pKi of Several Indicators in Acetone Containing 10% of Water... 

Thymol blue is the indicator which fulfills best all requirements. A mixture of methyl red and bromphenol blue may be used in the region between the transformation intervals of thymol blue. It is thus possible to cover the range between 1.0-10.0 using only two indicator solutions. Methyl red, however, is less satisfactory for this purpose than the indicators of Clark and Lubs, so that it is probably better to replace it by bromcresol green. 

We have already stated that, in buffer solutions, indicator papers will show approximately the same transformation intervals as do the corresponding indicator solutions. The hydrogen exponent can be estimated rather closely if a sufficient number of comparison solutions are at hand. Hemple found that the use of laemoid paper was practical between pH s 3.8-6.0. A drop of the solution under investigation was placed on the paper, and the color compared with papers containing drops of a series of buffer mixtures. The accuracy of the measurement was about 0.2-0.5 pH unit. 

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