Spectrophotometric Determination of Thiocyanate in Human Saliva

Abstract

The equilibrium constant between iron(III) ion and thiocyanate ion to form a thiocyanatoiron(III) ion can be conveniently measured with visible spectrophotometry because the FeSCN+2 solutions are deep blood-red. Hence this reaction is often used when teaching chemical equilibrium to students of general chemistry. The same reaction can be exploited in the qualitative and quantitative analysis of SCN- ions in solution. The experiment can be easily made more attractive to students when the thiocyanate ion concentration measured is from human saliva. Here is described how qualitative and quantitative analysis of human saliva thiocyanate ion can be performed as a part of the laboratory exercise for the determination of chemical equilibrium between Fe+3 and SCN- ions. For qualitative analysis a few drops of saliva (each student is using his or her own saliva) is treated with a drop of acidic Fe(NO3)3 solution. The deep blood-red color of FeSCN+2 complex is clearly demonstrated. Then each student measures his or her saliva thiocyanate ion concentration with visible spectrophotometry.