Q. Which grade of nitric acid do you recommend for post-treatment of zinc-plated parts? There are three grades listed by my chemical supplier: commercial/technical, L.R. and A.R. M.S.H.
A. For those of our readers not familiar with the various grades of chemicals, they are as follows: A.R. is analytical reagent grade, L.R. is laboratory reagent grade and commercial/technical is what is usually used in most non-pharmaceutical/food commercial processes. The purest grade, A.R., is usually used in the laboratory environment and is specified when performing analytical testing. Reagent grade is usually specified for laboratory processes. Commercial/technical grade is used in many manufacturing processes. The difference in the various grades has to do with the amount of trace impurities present. The amount of impurities allowed in the L.R. and A.R. grades are specified by governing bodies and must be adhered to in order to be labeled A.R. or L.R. Commercial/technical grade does not normally have to meet any of these requirements.
In the case of bright dipping of zinc-plated parts the commercial grade should be more than adequate. But to be on the safe side, request a “typical analysis” to be sure the nitric does not contain large amounts of impurities such as heavy metals.