On the use of probit analysis for assessing the toxicity of wood preservatives
D A Belenkov
To elucidate the general character of the action of the toxic material on the fungus, it is sufficient to carry out preservative tests using the previously described procedure, but in doing so, it is necessary somewhat to change the number of specimens and their arrangement in jars. Five concentrations of toxic material are tested simultaneously. For the testing of one preservative ten jars should be used in each of which are placed 50 specimens, comprising 10 of each of the five concentrations. The proposed arrangement of the specimens in jars makes due allowance both for the variation in the reaction of the individual sections of the mycelium in one jar (ten specimens) and for the variation in the reaction of the fungus in the different jars (ten groups of specimens). The effect of each concentration of toxic material on the fungus may, therefore, be assessed from the result of observing its reaction on a hundred specimens tested in all ten jars. With this procedure the author investigated the toxicity to Coniophora of sodium fluoride, sodium chloride and sodium silicofluoride, copper sulphate, zinc chloride, ammonium fluoride and ammonium pentaborate. The results are given in Table 1. Complete curves of the toxic effect were not obtained for all the formulations, but for the majority of them levels of transition from incomplete to practically complete protection of the timber against destruction by the fungus were established. The test results for sodium fluoride, using the given procedure, are represented graphically in Fig.1 (curve 2). It can be seen from Fig.1 that the change of reaction of Coniophora in timber containing different amounts of NaF is well described by an S-shaped curve, similar to the integral function of a normal distribution. Similar graphs are also obtained when testing other preservatives. The nature of the curve shows yet again that different probit-analysis methods may be used when testing preservatives in timber. Fig.3 shows the probit curves of the effect of sodium fluoride. Since in our case the curve is symmetrical and not lengthy, there was no need to use a logarithmic scale for the axis of abscissae. It can be seen from Fig.3 that, by using a normal distribution as a model for transforming the NaF toxic effect curve, rectilinear graphs are obtained. With such graphs it is easy to find the different levels of timber protection, and also to assess the variation of the reaction of the wood-destroying fungus by computing the mean dose and the standard deviation by the known methods of probit-analysis. From the probit v. NaF content graph (Fig.3) is found the amount of sodium fluoride necessary for protecting the timber of pine sapwood against destruction by Coniophora in 95 per cent of cases. It is 0.168 kg/m³. When determining the said level of protection by the author's proposed procedure, the value obtained was 0.136 kg/m³, i.e. 0.032 kg/m³ less. This is due to the fact that in the latter case the number of specimens tested was comparatively small. The proposed method, therefore, gives an overall picture of the protection of the timber by a specific preservative against the action of wood-destroying fungus and enables the amount of toxic material, which should be selected for a more detailed investigation by the previously reported procedure, to be determined.