Gaseous catalysts for end-point indication in titrimetric analysis in the microgram range

The method of end-point detection described is very sensitive. In titrations with sodium sulphide, the first drop of titrant in excess cause evolution of hydrogen sulphide which acts as a catalyst for the indicating iodine—azide reaction. The gas formed at the end-point is transferred by a nitrogen stream to the indicator vessel. Microgram amounts of metal ions (Sb, Ni, Fe, Hg⁺ Cd, Pb⁺ Cu, Ag⁺ Cu) and permanganate can be titrated in acidic solution with standard sulphide solution. In a variation of this method, acid-base titrations are possible.