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  1. #1

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    Potassium Thiocyanate

    How much is potassium thiocyanate toxic?
    I know it must not come in contact with strong acids, but what about strong alkalis?
    As far as I know it could potentially be life-threatening (-CN fumes)...

  2. #2
    Photo Engineer's Avatar
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    It is virtually harmless in acid or alkali. It does not release cyanide fumes at all.

    You would have to boil it with acid or alkali to cause it to decompose.

    PE

  3. #3
    georgegrosu's Avatar
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    Complex combinations of cyanide (KSCN, K3[Fe(CN)6] ) are less toxic than simple salts of hydrocyanic acid.
    http://www.merck-chemicals.com/unite...search+results KCN
    http://www.merck-chemicals.com/unite...-105124/p_uuid KSCN
    http://www.chemical.ro/files/produse...%20POTASIU.pdf K3[Fe(CN)6
    George

  4. #4
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    Thiocyanate is not related to Cyanide in any direct way chemically! SCN is not CN! The latter is a very violent poison, but the former is used in many harmless OTC photo chemicals and in some toothpastes. SCN is not a "complex combination" of cyanide, rather it is a separate chemical compound that is stable and rather harmless unless mistreated.

    This argument played out over several months on Photo Net about 5 years ago and I am rather tired of the repetitive nature of this question coming up over and over by non-chemists. Please look at Kodak's C-41 RA Fixer MSDS. It contains SCN salts and is rather harmless. http://www.kodak.com/eknec/documents.../PF790AENG.pdf or: http://www.3eonline.com/ImageServer/...2a296bca45.pdf

    In fact, the MSDS of SCN containing fixers lists the Sulfite as being of greater concern than the SCN!

    Remember, KCN is not KSCN. Nor are the two directly related. They are only very indirectly related.

    PE

  5. #5
    Ian Grant's Avatar
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    Quote Originally Posted by Photo Engineer View Post
    Thiocyanate is not related to Cyanide in any direct way chemically! SCN is not CN! The latter is a very violent poison, but the former is used in many harmless OTC photo chemicals and in some toothpastes. SCN is not a "complex combination" of cyanide, rather it is a separate chemical compound that is stable and rather harmless unless mistreated.

    This argument played out over several months on Photo Net about 5 years ago and I am rather tired of the repetitive nature of this question coming up over and over by non-chemists. Please look at Kodak's C-41 RA Fixer MSDS. It contains SCN salts and is rather harmless. http://www.kodak.com/eknec/documents.../PF790AENG.pdf or: http://www.3eonline.com/ImageServer/...2a296bca45.pdf

    In fact, the MSDS of SCN containing fixers lists the Sulfite as being of greater concern than the SCN!

    Remember, KCN is not KSCN. Nor are the two directly related. They are only very indirectly related.

    PE
    Will still probably kill you if you mix Thiocyanates with conc Sulphuric Acid especially if it's absorbed moisture, it'll boil isntantly, so basic safety is an issue. Storing Acids and Alkali's safely (and separately) away from dry or pre mixed chemicals.

    A lack of very basic Chemical safety missing in this post, Allessandro realises the potential issues.

    Ian

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    Ian;

    I always advise safety, and you are quite aware of that fact.

    I just am unhappy at the regular confusion between Thiocyanate and Cyanide and am answering that point.

    Also, boiling in acid is very unlikely to produce cyanide according to my sources. The reverse reaction is more likely, that is, cyanide can form thiocyanate in the presence of sulfur. I have no references that suggest that the reverse takes place under normal conditions up to 100 C either in water or alone.

    In summary: since pyrolysis (heating melted thiocyante salts at about 500C) can produce cyanide, it is best to avoid dropping any on a hotplate. Adding thiocyanates to strong acid is also not advised, but it does not form HCN. Heating Thiocyanates to their melting point does not produce any harmful effects but if they are heated to 500C they decompose.

    PE

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    Quote Originally Posted by Photo Engineer View Post
    Ian;

    I always advise safety, and you are quite aware of that fact.

    I just am unhappy at the regular confusion between Thiocyanate and Cyanide and am answering that point.

    Also, boiling in acid is very unlikely to produce cyanide according to my sources. The reverse reaction is more likely, that is, cyanide can form thiocyanate in the presence of sulfur. I have no references that suggest that the reverse takes place under normal conditions up to 100 C either in water or alone.

    In summary: since pyrolysis (heating melted thiocyante salts at about 500C) can produce cyanide, it is best to avoid dropping any on a hotplate. Adding thiocyanates to strong acid is also not advised, but it does not form HCN. Heating Thiocyanates to their melting point does not produce any harmful effects but if they are heated to 500C they decompose.

    PE
    PE: I'm perfectly aware of the differences between -CN and -SCN groups.
    The point imho is another: reading properly a MSDS sheet requires some lab practices if one has to understand fully and correctly what's written in them.

    "Thiocyanate is produced by the reaction of elemental sulfur or thiosulfate with cyanide:

    8 CN− + S8 → 8 SCN−
    CN− + S2O32− → SCN− + SO32− "

    So it's chemically related to cyanide.

    I was asking only if under common photographic chemistry handling the use of thiocyanates poses particular problems.
    For example, some paper developer (Tetena Eukobrom I'm currently using) contains NaOH.
    Can I safely use thiocyanates with that developer?

  8. #8
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    You can safely use Thiocyanate containing photographic processing solutions if you use the same precautions you use for handling and storage of any other processing solution.

    It is not worse than anything else in the lab, and does not approach Cyanide in any way for toxicity.

    And, as an analogy for you, you can easily produce Sulfiric acid from Sulfur Dioxide, but this does not mean that you can easily produce Sulfur Dioxide from Sulfuric acid. That is the sense in which I was referring to your above equations.

    PE

  9. #9
    georgegrosu's Avatar
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    PE, you are right. KSCN is not a complex combination.
    PE, I put here KCN, KSCN and K3 [Fe (CN) 6] for the following reason:
    KCN is highly toxic (referance) and not used in photographic chemistry.
    KSCN and K3 [Fe (CN) 6] are substances which are used in photographic chemistry.
    I hope that you can compare the lethal doses of different chemical and toxicological evaluation can be made.
    From what I understand from KSCN (Merck) LD50 lethal dose is 854 mg / kg.
    This value is about 40 times higher than the lethal dose for KCN (not used in photo).
    About 40 years ago a colleague wanted to kill. What do you think he chose from the lab?
    K3 [Fe (CN) 6]. Not dead. At low concentrations is a diuretic. She done wash gastric.
    I saw that at low concentrations is used to clarification of Wine!
    George

  10. #10
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    George;

    Thanks. I have the LD50 values here in my own Merck Index, thanks. That is part of my reference material.

    I can name many benign substances in the lab that can be very toxic in acid or base.

    Na2SO2 solution with acid generates a poison gas below about pH 4.5.
    Na2S solution generates a poison gas in acid solution below about pH 6.
    NH4OH solution generates a poison gas if NaOH is added.
    Sodium Hypochlorite solution and any Amine solution such as Ammonia generates a poison gas.

    I don't expect anyone to test this and I mention it for you all to avoid these mixtures in the interest of safety. Also, the above are generalizations. For example, Na2S continually exudes a minor amount of the poison gas H2S even in alkaline solution.

    PE

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