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It is shown that in the assay of pyrethrum flowers the customary use of warm solvent (soxhlet) gives erroneously high results for pyrcthrin 11, especially when the higher-boiling grades of ligroin arc used. Cold exvaction gives lower results for pyrethrin 11, and largely eljminrtcs the variations in the results obtained with diffcrent grades of ligroin. It is also shown that cold ligroin extract:c all the inse:ticidally-active pyrethrics, and that the additional pyrcthrins exrncted by hot ligroin are non-toxic to house-flies. Chloroform. is"shown to extract a particularly large quantity of such non-toxic pyrcthrins." The use of cold ligroin both for the assay, and for the commercid extraction, of pyrethrum flowers is therefore recommended; and it is also suggested that the flowers should be standardized only on their content of pyrethrin I.Pyrethrum flowers are usually analysed by the Seil'. method, or, less frequently, by the mercury r e d~c t i o n~.~ method. In a recent paper (Mitchell, Tresadern and Wood)6 the accuracy of these methods was studied, using the pure chrysanthemum acids for this purpose. It was confirmed that the mercury reduction method gave satisfactory results, provided that it was carried out under carefully standardized conditions; a small modification, designed to improve the results for pyrethrin 11, was proposed. It was also confirmed that the Seil method gave low results for pyrethrin I, and it was shown that these low results were not caused by the presence of mineral acid, as had previously been supposed, but were due to D temperature effect in the steam distillation. Experiments suggested that the .apparent loss of chrysanthemum monocarboxylic acid was due, not to decarboxylation, but to direct hydration to a saturated hydroxy-acid. This latter was much less readily voIatile in steam than chrysanthemum monocarboxylic add, and was also sparingIy soIubIe in Iigroin. A modified Seil method which eliminated these low results was described. A series of comparative analyses of commercial pyrethrum extracts confirmed the accuracy of the above results; but the methods were not applied to the analysis of pyrethrum flowers.In the Seil, mercury reduction, and most of the earlier methods which have been proposed for the assay of pyrethrum flowers, the crushed drug is extracted with ligroin in a soxhlet, or other continuous extractor. In such extractors the drug is extracted with the o u~y l n solvent at a temperature just below its boiling point. In commercial extraction of the flowers, however, cold solvents are generally employed, and the yields of total pyrethrins are somewhat lower than those calculated from the laboratory assays using the soxhlet method. Thus, large-scale experiments on Kenya pyrethrum flowers, the authors have found that cold ligroin rarely extracts more than 95% of the total pyrethrins on the basis of the laboratory test (Seil, or . mercury reduction). Using old flowers, or those appearing to have been carelessly dried or stored, the yields have been only 8 0 %~ or even lo...

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Comprising pyrethrin I and cinerin I, but calculated as pyrethrin I-hence the inverted commas, which have similar significance for "pyrethrin 11" (now known also to include jasmolin 115) and for "total pyrethrins.

The authors have examined in nvo series of expcriments the effect of varying the conditions under which the reduction of DcnigPs rcagcnt by chrysanthemum monoarboxylic acid is allowed to proceed. They have shown that both tcmperaturc and time of reaction influence appreciably the extent of thc reduction ; and have confirmcd the result previously obtnined by Graham and LaForge, that I ml. 0.01 nl-potassium iodate solution is equivalcnt to 5.7 mg. pyrethrin 1 when the reduction takes place at 25 * 2' for I hour. Differences of 5-6% of the general mean value were shown in the second series despite careful standardisation of conditions. When Wilcoxon published (1936)' his method for the determination of pyrethrin I, based upon the reduction of Denigks reagent by chrysanthemum monocarboxylic acid, he gave 4-4 mg. pyrethrin I as the equivalent of I ml. of 0.01 M-potassium iodate solution. This factor was universally adopted without question until 1943, when Graham and LaForgez published the results of an investigation showing the equivalent to be 5-7 mg. In view of the importance attaching to the alteration of the factor by such a substantial amount, a collaborative study of the factor value was undertaken by Rothamsted Experimental For the purpose of this study, xz*131 g. chrysanthemum monocarboxylic acid, were dissolved in the cold in IOO ml.N -S O~~U~ hydroxide solution and distilled water was added to bring the volume up to z liues. The final solution was mixed thoroughly. 500 ml. portions were distributed to the four laboratories where the study was to be undertaken.The following determinations were carried out in duplicate at each laboratory, employing the reduction with Denigis rcagcnt and subsequent operations as described below. 25"do. do.do. 30:As for 2.5 ml.. In each determination su5icient 0.05 N-sodium hydroxide solution was added to make the total volume of monocarboxylic acid solution and sodium hydroxide solution up to 10 ml.The alkaliinc solution of rnonocarboxylic acid was then treated with 10 ml. of Deniges reagent and the reaction allowed to proceed for the appropriate time. 20 ml. of acetone and 3 ml. of saturated salt solution were then added and the solution heated just to boiling. The solution was then filtered through a 7 cm. No. 'I Whatman paper. The calomel precipitate was ' BRAY Et AI.-PYRETHRIN I FACTOR 275 afterwards washed by decantation with 10 ml. of hot acetone followed by t\vo portions of 10 ml. of hot chloroform.The drained paper was placed in a glass stoppered flask, which had been used for the precipitation, and 30 ml. of concentrated hydrochloric acid added, followed by 20 ml. of water. The whole was cooled and 6 ml. of chloroform and I ml. iodine monochloride added. The solution -was then titrated with 0.01. hi-potassium iodate solution until the pink colour in the chloroform layer was removcd.The results obtained by the collaborators are given in Table I. They are cxpresscd as mg. pyrethrin I per I ml. 0'01 hl-pOtaSsium iodate solution. This transformation is based on the fact ...