Sensitivity of copper indicators for detecting preservative penetration

Abstract

Chrome azurol S (CAS), 1-(2-pyridylazo)-2-naphthol (PAN), and rubeanic acid (RA) are the three copper indicators currently used by the timber industry to assess preservative penetration. Although only the American Wood Protection Association (AWPA) standards specify indicator sensitivity, it is unclear how those limits were developed. The sensitivity of each copper indicator was determined in solutions and on wood. The indicators prepared as per the Australian and US standards were added into various concentrations of Cu solutions (from CCA) and colour changes were evaluated using ultraviolet/visible (UV-vis) spectroscopy. End-matched slash pine sapwood cross sections (Pinus elliottii) treated to retentions of 0.6, 1.7, 2.8, 5.7, 14.2, 28.5, 50, 75, 100 ppm Cu were used to evaluate the detection limit of indicators on wood. All samples were photographed in the same place under the same lighting conditions. Multiple visual assessments and CIE L*a*b* parameters were used to characterize surface colour changes. Spectrophotometric results were consistent with visual observations with the indicators in copper solutions with detection limits of approximately 1 ppm. The sensitivities of CAS, PAN, and RA indicators on wood were approximately 28.5, 14.2, and 5.7 ppm Cu, respectively. The results support the detection limits of CAS and PAN suggested in the AWPA standards. However, the RA indicator was more sensitive than the AWPA recommendation. Further investigations are underway to better understand the sensitivity of copper indicators with different wood preservatives.