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  1. #41

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    Quote Originally Posted by gainer View Post
    I contend there is no chemical test to distinguish one solution from the other if they are made with equally pure ingredients.
    Both Sodium Bisulfite and Metabisulfite in solution (at 50g/L) are about 3.5-5 pH. The difference between them is that there is more bisulfite (or sulfite if you wish, but at that pH it is as the single protonated or hydrosulfite form, HSO3-).

    The difference is that you get more sulfite/bisulfite with the metabisulfite per gram of material than with the bisufite.

    And as you like to point out Pat, you probably can't point out the difference in solution, unless you happened to have an atomic absorption (AA)spectrophotometer or an inductively coupled plasma spectrophotometer (IPC) or any of the other methods of measuring the amount of sodium that is in solution.

    Metabisulfite also goes by the archaic name of "pyrosulfite", which was used when a compound could be made into another compound through the act of intense heating. The heating drives off only water from an "ortho" acid or it's salt, like sulfuric acid forms pyrosulfuric acid (2H2SO4 - H2O => H2S2O7), pryophosphoric acid from phosphoric acid (2·HPO3 => P2O5*H2O), and of course, our topic of converstation, sodium metabisulfite from bisulfite (2·HNaO3S - H2O => Na2O5S2).

    This is not water of hydration, which would also give a compound of higher "concentration" in the dry form, say like making a decahydrate salt into the anhydrous salt, but it also resulted in a different molecular formula, hence the new name for the new compound.

    So metabisulfite is a distinctly different chemical with distinctly different properties and formula and molecular structure from bisulfite. The "pyro" form has more of the ortho acid anion than the original ortho acid form does.

    But as Pat says, once they are put into water and are allowed to rehydrate, they are back to the original starting ortho acids or their corresponding anions.

  2. #42

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    By the way, an "ortho acid" is the compound that is a fully hydrated acid or it's corresponding salt. A good example of this is phosphoric acid H3PO4 and the various forms it takes, the simplest and "fully hydrated" for is orthophosphate, PO4---. See this page for a good comparison of the various forms it can take:
    http://en.wikipedia.org/wiki/Phospho...and_phosphates

  3. #43
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    All photo Alchemists,
    I sure wish I could remember whether I can use "sodium bisuphite" and "sodium metabisuphite" interchangeably in black and white photo chemistry. I ASS u ME I can, since, to me, the gist of your erudite discussions is that so bi is actually so met, except when it is both, or perhaps neither, except when.... or........oh, sheesh!! (vbg)
    John, Mount Vernon, Virginia USA

  4. #44
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    Quote Originally Posted by Anscojohn View Post
    All photo Alchemists,
    I sure wish I could remember whether I can use "sodium bisuphite" and "sodium metabisuphite" interchangeably in black and white photo chemistry. I ASS u ME I can, since, to me, the gist of your erudite discussions is that so bi is actually so met, except when it is both, or perhaps neither, except when.... or........oh, sheesh!! (vbg)
    In most cases where we need the acid sulfite, the amount is not very large and a small error in measurement would not be critical, especially considering that if it's a new formula to the user, there will usually be a test or two before serious usage. If you don't know which one you have, you could probably split the difference. If you have the meta but thought you had bi, you will be less than 5% in error on the high side, and less if the stuff is old and has been exposed to atmosphere. If it is so critical as to cause worries, you should know what pH you're looking for and have a good pH meter. You could try a practical test using the sulfite instead of either of the acid sulfites, or maybe no sulfite. Ascorbic acid sometimes works as well as sodium bisulfite.
    Gadget Gainer

  5. #45
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    Gadge,
    My needs are very simple: occasionally D25R and lots of EK F-24 fixer--so all this arcana is beyond my needs as well as my comprehension.

    And i can always use the substitution chart in my old Ilford Encylopedia of Photography for rough equivalents on most anything I shall ever require. Thus, and meaning no disrespect, it's pretty much all fart gas to me. For my own needs it is always a case of KISS. About wine and beer, howemsoever......:rolleyes:
    John, Mount Vernon, Virginia USA

  6. #46

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    Quote Originally Posted by Anscojohn View Post
    Gadge,
    My needs are very simple: occasionally D25R and
    lots of EK F-24 fixer--so all this arcana is beyond
    my needs as well as my comprehension.
    Most importantly is the bisulfite's reducing prowess
    vs the metabisufite's. Ian has implied a greater prowess
    for the meta. That greater prowess though is associated
    with the amount of bisulfite vs the meta.

    For example, with respect to the precipitation of gold
    from a solution of gold chloride, either one will do.
    I doubt the meta has any thing over the bisulfite
    with regard to it's reducing potential. With both
    that potential has to do with the oxidation of
    sulfur or as often put the SO2 radical.

    On evaporation the normal salt in solution leaves
    a normal salt. With the bisulfite in solution the
    metabisulfite is left. IMO pyro overstates and
    meta is the more appropriate term. Dan

  7. #47

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    Quote Originally Posted by dancqu View Post
    IMO pyro overstates and
    meta is the more appropriate term. Dan
    You may be right the use of "Pyro" overstates, but you needo to keep in mind the archaic name systems that were in use when these compounds were being discovered and what was in common use before IUPAC (International Union of Pure and Applied Chemistry) naming conventions were adopted in the 20th century.

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