alnovo@inwind.it
Date: 03/21/02-01:30:27 AM Z
>
>
> On Wed, 20/03/2002 23:06 Judy Seigel wrote:
>
> >
> > Those "watch glasses" are used because one can wash them while
> > transferring the chemical to the vessel, to be sure that truly *all* the
> > weighted substance goes into the solution. This is particularly needed
> > for an exact weighting, such as for very small amounts.
>
> You mean WASH glasses? You sluice the chemical off with water?.. that
> sounds, um ... messy? & what if you only want a little water? Or alcohol?
>
> Of course the difficulty in getting the chemical off that round surface,
> especially into something narrower than it is (I often transfer into a
> graduate, for instance), would more than cancel out any supposed
> advantage. Transfer from round without a spout shape is another accident
> waiting to happen.
>
> I agree otherwise, except about the kitchen aluminum foil -- too likely to
> crinkle & hold powder...
>
> Judy
YES, that is a normal laboratory practice for a chemist. I am referring to, for example, preparing 1000 ml 0.1N (1 equivalent/litre) silver nitrate solution: I need to weight 16.987 grams (up to the milligram) of silver nitrate, then I transfer all that stuff to a volumetric flask. I will use a funnel on the flask, then using a wash bottle I will gently soak the watch glass many times with small amounts of distilled water, paying attention also to the opposite side of the glass and the borders.
If you want to dissolve your substance with a very small amount of liquid probabily you have a very small amount of substance too. You need to use a watch glass, a vessel and an amount of liquid proportional to the amount of the weighted substance.
However, chemists are used to cut an hair in four (or more...). I don't think this is the case of the normal photographic practice, with some exceptions (gold and platine, for example).
Alberto
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