Process of determining the zinc content of phosphating baths

摘要

Described is a process for determining the zinc content of an acid aqueous phosphating bath. A bath sample is titrated at a pH value between 1.2 and 1.7 in the presence of ethylenediaminetetraacetic acid, hexamethylenetetramine and alkalihexacyanoferrate (III) with an alkalihexacyanoferrate(II) until the potential at an oxidation-reduction electrode immersed into the bath sample reaches an inflection point. Any content of nitrite and/or iron(II) ions must be removed by an addition of nitrite-destroying substances and by an oxidation to iron(III) ions at a pH value above 8. The process is particularly suitable for determining the zinc concentration of phosphating baths which contain 0.4 to 2.0 g/l Zn. The presence in phosphating baths of additional cations such as manganese and/or calcium and/or nickel and/or magnesium does not interfere with the process. The analysis can be automatic and may directly be used to control a metering device for supplying make-up concentrates to the phosphating bath.