We disolved diatoms in NaOH solutions for icp-ms analysis and the dissolution was ok. After one day, we saw silicate polymers floating in the tube. Any ideas on how to prevent this phenomena?
A highly alkaline solution of silica should be fairly stable unless the concentration get too high. Unless there are metal ion impurities the most likely silicates would be sodium metasilicate nine hydrate, but this is quite soluble in water. In James Vail, ACS [Rheinhold] 1952 there is a wealth of information on silica solubility. My paper Silica in Biology also has many references. If you provide more info on composition I might be able to say more. [email protected]
It is likely that the samples contains polyvalent cations, i.e. Mg+2, Ca+2. These will form polymers with silica at alkaline conditions. This is a somewhat slow process, therefore you don't see anything until the following day. The process can be significantly slowed (perhaps indefinitely) by complexation of the cations. You could try to add simple alcohols such as propanol, or simple carboxylic acids such as acetic acid. Probably even better are salicylic acid or sugars (for example sucrose) as they can form several bonds to the cations. The amount that should be added is depending on the concentration of cations and silica. Ideally it should be as low as possible to prevent viscosity problems with the icp-ms but still significantly higher than the concentration of silica and cations. 0.1-1% of complexating agent can perhaps be a starting point.
I think work related to Alkali Silica Reaction in concrete may give you some answers. This deliberation I made in a paper to the 13th ICAAR , Trondheim 2008. (Hagelia 2008)References given below are, however, most relevant to your problem:
"The electrostatic energy between Ca2+ and O-, i.e. the affinity for reaction with hydrated silica polymers, is greater than Na+ and K+ (cf. [a, b)]). Hence, Ca should be more strongly attracted to the hydrated negative silica ions in solution than the alkali ions, e.g. opposite to the behaviour in electrical double layers. This mechanism seems directly supported by the experiments of Gaboriaud et al. [ c] who demonstrated that in a strongly basic silico-alkaline solution the presence of Ca2+ ions was promoting cross-linking of silicate ions in the gelation process"
a) Dove, PM and Rimstidt, JD (1994): Silica – water interactions. In: Heaney P.J., Prewitt, C.T. and Gibbs, G.V. (eds.), Silica. Reviews in Mineralogy, (29): 259-308.
b) Wang, H, and Gillott, JE (1991): Mechanism of alkali-silica reaction and the significance of calcium hydroxide. Cement and Concrete Research, 21, 647-654.
c) Gaboriaud, F, Nonat, A, Chaumont, D and Craievich, A (1999): Aggregation and gel formation in basic silico-calco-alkaline solutions studied: A SAXS, SANS and ELS study. Journal of Physics and Chemistry, 103, 5775-5781.
Thank you all for your answers. I will look up these references and try to select a different dissolution method.
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