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  1. #1

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    Chemistry Question

    I'm not sure if this is the right place to post this, but here goes. Could someone please explain to me how I can calculate the amount of grams or milligrams per liter there would be of a specific element or compound, by looking at the compound it is contained in, the weight of that compound and its dilution? For example, if 40 grams of potassium ferricyanide is dissolved in one liter of water, how would I calculate the amount in grams per liter of cyanide in that solution? Same for chromium where the dissolved salt is potassium dichromate. Apart from those examples, I'm really interested in the methodology, which I assume has something to do with molecular weights - and where to find the needed data - so I can also work on other examples.

    This all has to do with my getting my mind around some environmental rules and how best to deal with them.

    Many thanks.

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    You need 3 pieces of information; the molecular weight of the chemical, its molecular formula, and the atomic weight of the particular element. For potassium dichromate these are 294.2 g/mole, K2Cr2O7, and 52 g/mole. Using this information you calculate the percentage of chromium in a molecule of potassium dichromate, 2 atoms of Cr in potassium dichromate x 52 g/mole / 294.2 g/mole = 35.35%. Continuing with your example you then multiply the percentage times the total weight, 35.35% x 40 g = 14.14 g. Dissolve 40 g of potassium dichromate in a liter of water and 100 ml of this solution will contain 100 ml/1000 ml/l x 14.14 g/l = 1.414 g of chromium.

    For potassiuum ferricyanide the only change would be to use the molecular weight of the cyanide ion which is equal to the sum of the weights of the two elements carbon and nitrogen. The rest of the calculation should be obvious. BTW there is NO free cyanide in potassium ferricyanide as it is all tightly bound to the iron atom. So this calculation is rather meaningless.

    The molecular weight and formula can be obtained from the MSDS for the compound, the atomic weight from the Periodic Table.
    Last edited by Gerald C Koch; 02-13-2013 at 12:45 PM. Click to view previous post history.
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  3. #3
    Ian Grant's Avatar
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    Potassium Ferricyanide doesn't contain free Cyanide, so you can't calculate content that's not there.

    Ian

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    Interesting, Ian. So if the local water treatment plant says cyanide in effluent must be below 3ppm, do I conclude it must be below that because it is bound to the iron molecule and not free? That would be an awesome result.

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    Quote Originally Posted by newcan1 View Post
    Interesting, Ian. So if the local water treatment plant says cyanide in effluent must be below 3ppm, do I conclude it must be below that because it is bound to the iron molecule and not free? That would be an awesome result.
    That rule applies only to cyanide itself, which is a very toxic chemical. When you bond another atom or another molecule to an existing molecule, you COMPLETELY change its chemical properties. It's toxicity changes, its appearance changes, its reactivity changes, etc.
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    Ian Grant's Avatar
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    You'd need to check but here in the UK ferricyanide, and thiocyanatres are not treated as Cyanide for effluent disposal. They test for free cyanide, I was on secondment to a water treatment plant back in 1972 when a local company discharged a quanity of Cyanide which killed two treatment beds.

    If you're worried about disposal of ferricyanide bleach just dilute well before flushing away.

    Ian

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    The naming of substances in chemistry is very important. The cyanATE ion is different from the cyanIDE ion. The LD50 for sodium cyanate is 1500 mg / kg which means you would have to ingest quite a bit of it before any toxic symptoms occured.
    A rock pile ceases to be a rock pile the moment a single man contemplates it, bearing within him the image of a cathedral.

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    Ian Grant's Avatar
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    Quote Originally Posted by Gerald C Koch View Post
    The naming of substances in chemistry is very important. The cyanATE ion is different from the cyanIDE ion. The LD50 for sodium cyanate is 1500 mg / kg which means you would have to ingest quite a bit of it before any toxic symptoms occured.
    While that's true both Ferricyanide and Thiocyanate can liberate free Cyanide under extremes. That won't happen with home disposal by dilution but does raise issues where spent solutions are carried in a vehicle - the issue here is in an accident where battery acid could come into contact with the chemicals liberating Cyanide gas. I came up against this at work and the regulations are quite strict, you need to be a licensed carrier (which we were).

    Ian

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    Quote Originally Posted by Ian Grant View Post
    While that's true both Ferricyanide and Thiocyanate can liberate free Cyanide under extremes. That won't happen with home disposal by dilution but does raise issues where spent solutions are carried in a vehicle - the issue here is in an accident where battery acid could come into contact with the chemicals liberating Cyanide gas. I came up against this at work and the regulations are quite strict, you need to be a licensed carrier (which we were).

    Ian
    Thanks Ian. Which is why I encourage everyone to read the MSDS for every chemical they are intending to use. Such information about dangerous decomposition products is included in this data. This information is easily available on the net.
    A rock pile ceases to be a rock pile the moment a single man contemplates it, bearing within him the image of a cathedral.

    ~Antoine de Saint-Exupery

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    Jerry, it is also necessary to remember that Thiocyanate will only release CN gas with acid and lots of heat. The same is true of Ferri and Ferro Cyanides. Usually the biggest concern with these two ions is the COD (Chemical Oxygen Demand) which uses up oxygen in water and kills wildlife and the BOD (Biological Oxygen Demand), which is the Oxygen needed to break down the chemical by organisms in the water.

    Both Thiocyanate and Ferri or Ferro cyanides are controlled more for their BOD and COD than the formation of Cyanide itself.

    We went through this all with the government when converting from Ferricyanide to Ferric EDTA and then the possible use of Thiocyanate in Blixes. School Finishers of Webster NY was shut down for excess Ferricyanide in Irodequoit Creek and thus became the first trade trial of Ektaprint 3 chemistry to avoid the Ferricyanide COD and potential COD. We removed the Thiocyanate from the blix for the same reason.

    PE

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