Abstract
Activities of fluoride and foluorosilicate in chromium plating baths were studied by using a fluoride electrode, and the following results were obtained.
(1) In chromium plating baths, the activity of fluoride was less than that of the additional fluoride. The reason for this was that F-+H+→HF.
(2) The activity of fluoride well corresponded with the value calculated from pH of the plating bath and ionization constant of HF, 7.2×10-4.
(3) The activity of fluorosilicate was quite the same as that of fluoride at concentrations of less than 10-3mol/l or at higher pH. The reason was that the fluorosilicate was completely decomposed at low concentrations, and it was decomposed to fluoride at higher pH as follows:
SiF62-+6OH-→6F-+[Si(OH)6]2-
(4) The difference between activities of fluoride and fluorosilicate under other conditions was due to the formation of complex salt by fluorosilicate.
