The amino acids cysteine and cystine contain sulphydril (-SH) and disulphide (-S-S-) groups respectively. Methionine also contains a sulphur atom. Methods for their demonstration often depend on this.
Although sulphuric acid is used in some staining techniques, it is not as commonly used as other acids. This is most likely due to the care with which concentrated sulphuric acid needs to be handled.
Sulphonic groups are important in dyes and are one of the commonly found auxochromes. Often the sulphonic group serves to convert a basic dyes to an acid dye equivalent, acid fuchsin being a case in point.
Some mucosubstances have sulphur containing groups. These mucosubstances are collectively named sulphated acid mucins and are strongly metachromatic with dyes such as the azures and their relatives. Dermatan sulphate and chondroitin sulphate are examples. Some mucosubstances which are not normally metachromatic can be made to be so by sulphation. This is a procedure in which sulphur containing groups are attached to compounds in order to change their staining characteristics, and thus identify them.