Methods are described for the determination of phosphorus or arsenic in 30-60 pg. samples of organic compounds. The samples are burned by a modified oxygen-flask method. Phosphorus is determined by precipitation as quinoline phosphomolybdate and alkalimetric titration of the precipitate, or spectrophotometrically as molybdenum blue with iron (11) ammonium sulphate as the reductant. Arsenic is determined spectrophotometrically by a similar method with hydrazine sulphate as reductant.CLASSICAL methods for the determination of phosphorus and arsenic in organic compounds generally depend on wet combustion in a sealed tube with fuming nitric acid. Attempts to determine phosphorus in this way on the microgram scale were unsuccessful; very low results were obtained and it was suspected that some adsorption took place on the surface of the tube. Similar results were obtained when pure potassium dihydrogen phosphate was subjected to the same treatment. Accordingly, a different decomposition procedure was sought. The oxygen-flask method of decomposition has already been applied on the microgram scale to the determination of halogens and sulphur (see Parts X, XIV, XV, and XVII), and this method was therefore examined further. Many Papers have been published on the determination of phosphorus and arsenic by flask methods on other scales of working.Determination of Phos$horzts.-When combustions were done in the special 25 ml. flask with the sample support described previously,l the results obtained were consistently 0.5-1% low. When the original platinum-gauze hinge was formed into a cup so that less heat was conducted away from the combustion zone and the sample did not come into immediate contact with the support during the combustion, the recoveries were much better. Polythene foil was used as the sample wrapper and contained no significant blank. The fuse material was filter paper, because the linen thread previously recommended caused low recoveries (cf. ref. 2). When the spectrophotometric finish was applied, 25 ml. round-bottomed flasks could be used instead of the specially designed flasks, because there was no need to collect the solution in a suitable compartment for titration.An absorption solution of alkaline sodium hypobromite as recommended on the milligram scale3 was found satisfactory. Because it was necessary to keep the volume low (ca. 0.1 ml.) on the microgram scale the absorption time had to be increased to avoid low recoveries.For the final determination of phosphate produced in the reaction, precipitation as quinoline phosphomolybdate was first examined. This is undoubtedly the best of all *
Islands are structurally related to Africa they cannot be regarded as truly oceanic. Leucite, in these continental potash-rich, basic and ultrabasic lavas, is normally an early-formed idiomorphic minoral occurring as both phenocrysts and in the ground-mass as discrete octahedra and does not commonly occur interstitially. In other oceanic suites, under-saturation with respect to Si0 2 appears to lead to t,ho cryste,llization of nepheline and/or melilite, not leucite.It is most unlikely that the late-stage formation of leucite in tho Tristan rocks was by a process similar to that envisaged for tho continental occurrences and it is probable that some unusual physico-chomical conditions prevailed during the cooling history of these rocks; further work is being undortaken on this material and it is hoped to discuss the petrogencsis in detail elsewhere in due course.We thank Prof.
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