a)
Citric acid solutions with concentration of 0.2%, 0.4% and 0.6%.
(b)
Calcium lactate solutions with concentration of 0.5%, 1.0% and 1.5%.
(c)
0.4% citric acid solution contained either 0.5%, 1.0% or 1.5% Calcium lactate.
(d)
Untreated sample dipped in tap water for 5 min.