. Maghemite, prepared in the usual way (but with exclusion of silica, e.g., from glassware) by precipitation of non-stoichiometric magnetite Fe3-,O,-, in aqueous MOH (M = Na, K) followed by air oxidation, picks up moisture from the air to reach the limiting composition MsH,-sFe50,. where 6 -0.02-0.03 for fresh material, but changes under hydrothermal conditions because of ion exchange. Despite the role of absorbed moisture in stabilizing maghernite, formation of the latter from Fe3-,O,-, is markedly retarded, and its decomposition to u-Fe,03 greatly accelerated, under hydrothermal conditions relative to the rates of the corresponding reactions of the dry solids. The rate of hydrothermal decomposition of maghemite is strongly retarded by silica. Over the range 160-187°C at least, silica-free maghemite decomposes in water according to the empirical equation -In ( I -a ) = (kt)", whereu is the fractional extent of decomposition. and 11 -2.5 for neutral water (with k = 1.3 x s-I at 160°C and 3.1 x 10-5s-' at 175°C) but approaches unity, without major effects on the overall time-scale of reaction, at high [MOH]. The mechanistic significance of these and previous results are considered; the hydrothermal conversion of maghemite to hematite evidently proceeds by ii dissolution-reprecipitation sequence. THOMAS WILSON SWADDLE et P~~I L L I P~L T M A N N .Can. J . Chem. 58, 1763 (1980). La maghemite. preparee de manitre habituelle (mais avec elimination de la silice par exemple celle de la verrerie) par precipitation de la magnetite non stoechiometrique Fe,-,O,_, dans du MOH aqueux (M = Na, K) suivie d'une oxydation a I'air, absorbe I'humiditC de I'air pour atteindre une composition limite de MsHI+,Fe5O, ou 6 -0.02-0.03 pour le produit fr-ais, mais il se produit des changements dans des conditions hydrothermiques pa; suite de I'echange d'ions. En depit du r6le de I'humidite absorbee en tant que stabilisatrice de la maghernite, la formation de cette derniere B partir de Fe3+,0,_, est fortement retiirdee et sa dCcomposition en Fe203 u est grandement accClerCe dans des conditions hydrothermiques s'apparentant aux vitesses des reactions correspondantes des solides secs. La vitesse de decomposition hydrothermique de la maghemite est fortement retardee par la silice. Au moins dans I'intervalle de 160-187"C, la maghemite depourvue de silice se decompose dans I'eau selon I'equation empirique In (1 -a ) =

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PHILL~P OLTMANN and DOUGLAS E. RYAN. Can. J. Chem. 63, 2585 (1 985). Reactive silicon in waters was determined indirectly by counting 1°'Tc after neutron activation of molybdosilicic acid (MSA) extracted with methyl isobutyl ketone (MIBK). The detection limit (defined as twice the standard deviation for the blank) was 0.2 ng mL-I. The relative standard deviation was 3% at the 10 ng mL-' level and 30% at the 1 ng mL-' level. Phosphate, arsenate, and ferric ions did not interfere by themselves. Interference from the reduction of molybdophosphoric acid by tannic acid was eliminated through the use of dichromate. The salt effect normally encountered in the analysis of sea water was examined with particular reference to silicate content of reagent salts used in preparing synthetic sea water.