alnovo@inwind.it
Date: 09/26/01-12:36:32 AM Z
>potassium bromide: (all in 1 lb) $11.25 (1.16 x potassium = sodium bromide)
>sodium bromide: $33.95
>
>potassium hydroxide: $13.89
>sodium hydroxide: $5.95 (1.4 x sodium = potassium hydroxide)
>
>potassium metabisulfite: $10.95
>sodium metabisulfite: $5.25
>
>potassium thiocyanate: $89.95
>sodium thiocyanate: $49.95.
>
>Are all sodium and potassium compounds interchangeable? Why the huge price
>difference? Do they, in substitutions, truly work the same? And, what
>makes catechol so stinky?
>And what, in layman's terms, is the difference between a sulfate and
>sulfite and I assume they are not interchangeable? Someone already answered
>this a while ago but I lost my notes.
>Chris
>
There are some differences between sodium and potassium salts, but chiefly from the biochemical point of view instead of the photographic one.
For our common use, they are perfectly interchangeable if not used at high concentration, because in this case the different solubility could be an important factor.
I said "perfeclty", but the different ionic radius of potassium ion makes this one slightly more active in solution than the sodium ion. This effect leads to a slightly more pronounced alkalinity of the carbonate and hydroxide solutions.
Moreover, one need to take in account the different molecular weight of the two ions, potassium one being heavier.
These are the molecular weights of the anhydrous salts:
pot. bromide = 119.01
sod. bromide = 102.9
pot. hydroxyde = 56.11
sod. Hydroxyde = 40.0
pot. metabisulphite = 222.33
sod. metabisulphite = 190.13
pot. thiocyanate = 97.18
sod. thiocyanate = 81.07
As you can see, there is a difference of 16.11 between the pot and the sodium salt (metabisulphite carries two pot. or sodium, so the difference is doubled).
To find how substitute their weights, divide the grams of a substance by its molecular weight, then multiply by the m.w. of the one you want to substitute for.
In the case of hydrated salts, you need to add 18 * (the number of molecules of water) to (the mol. weight of the anhydrous substance).
The different prices are due to the easier way to prepare sodium salts but the bromide, which comes more or less directly from the sea salt deposits.
About the difference between sulphate and sulphite, look at the Archives. There are a lot of posts there.
Alberto
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