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A method has been developed for the determination of Cs137 in sea-water. The acidified sample, containing added ammonium nitrate and caesium carrier, is passed through a column containing ammonium 12-molybdophosphate (AMP) mixed with an equal weight of asbestos to give adequate porosity. The caesium is retained quantitatively on the AMP (which acts as a selective cation exchanger) together with a small fraction of the natural rubidium and potassium present in the original sample. After treatment to separate the rubidium and potassium (both of which contain natural beta-emitting nuclides) the CsI3' is counted.The principal advantage of this method is the high radiochemical purity of the separated C P 7 which may be assayed by beta counting. With the use of low-background beta counters, levels of approximately 0.1 pc/l. may be determined on a 25-1. sea-water sample with a 95 per cent confidence limit of 10.015 pc/l. The results of measurements on sea-water samples collected from the North Atlantic and coastal waters round the British Isles during 1961 and 1962 are reported.

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which was found to vary almost linearly with added acid or alkali.O• l mol. of each of the following was dissolved ~eparately in 250 ml. water, the solutions being mixed m the order shown just prior to use: sodium carbonate, 2-amino-2-methyl propan-1 : 3-diol• (Eastman-Kodak reagent); sodium dihydrogen orthophosphate ; citric acid.Van Slyke 4 and Britton and Robinson• deduce that in mixtures of buffer substances a constant buffering power over the working pH range will be obtained if the consecutive pK values of the acids involved differ by not more than l •2. In the present system the pK values are deduced to be 3 • 5 :