The effect of composition on the effectiveness and fixation of CCA and copper/chrome preservatives. Part 1: Effectiveness. Part 2: Selective absorption and fixation
D N R Smith, A I Williams
A preliminary study of a wide range of copper-chrome-arsenic formulations indicates that the most effective are in the region, CuSO4 · 5H2O - 35 to 45 per cent; K2Cr2O7 - 40 per cent and above; As2O5 · 2H2O - 25 to 15 per cent or less. Effectiveness appears to depend more on the copper content than the copper - arsenic. Observations on selective absorption and leaching suggest that the chromium fixes the arsenic preferentially to the copper. Although there appears to be no major amount of fixation of copper by arsenic, at the start of addition of arsenic to copper-chrome formulations there is a small but significant decrease in the leaching of the copper suggesting either a small amount of fixation of copper by arsenic or the formation of an insoluble complex of all three components. Maximum fixation of arsenic was obtained when the Cr/As ratio (as salts) was 1.9 or greater. Maximum fixation of copper is not so simply defined but is round about a Cr/Cu ratio (as salts) of 1.7 . The area of maximum effectiveness is not coincident with the area of maximum fixation of both copper and arsenic but slightly displaced towards a region of higher copper where there is still maximum fixation of arsenic but some loss of copper.