Universal indicator

In chemistry, referred to as universal indicator a pH indicator that provides a very wide range of pH values ​​by a wide range of color changes visible.

These are not usually made of a single substance, but a mixture of several indicator substances per different color and different cover areas that is tuned so that the product changes color with each pH unit. One possibility is a mixture of different amounts of thymol blue, bromothymol blue, methyl red and phenolphthalein.

The universal indicator is therefore not suitable for titrations where a possible well-defined transition point is required - here used to single substances such as methyl orange or phenolphthalein. They are rather to an unknown pH value in the range from 0 to 14 gross to be determined.

By impregnating filter papers with indicator solutions obtained so-called indicator papers. In the case of universal indicators we speak of universal indicator paper. This is known commercially as strip in coils but also in other blanks available. With a damp universal indicator paper and gases can be detected, such as ammonia in the crossmatch. The following table shows an example for a typical universal indicator, the color occurring on the pH - value depending on:

A mixture of equal parts of methyl red, methyl yellow, thymol blue and bromothymol blue changes color like a traffic light from red to yellow to green.

A natural universal indicator is included in the red cabbage, and thus also for the cooking of cabbage benutztem water. It is to the dye cyanidin ( in red cabbage connected with sugar residues as anthocyanidins ), which under acidic red (hence red cabbage when prepared with vinegar), in a weakly basic medium blue ( blue cabbage in preparation with soda ) and green, and yellow in strongly alkaline is colored. In practice, this indicator is, however, only for rough estimation of the present milieu, since the pH values ​​at which turns the color can vary significantly depending on the condition and concentration of the red cabbage juice. In particular, red cabbage juice should not be used if you want to check whether there is a risk of chemical burns in an unknown substance.