I need to figure out the molecular weight for tri-potassium di-citrato bismuthate (De-Nol, Yamanouchi, Europe) and was wondering if any of you may have an idea? I’ve made a few guesses, but my chemistry is pretty rusty and I don’t really trust my abilities in that area.
I have found the compound in the Merck Index, but it appears that they don’t know what the formula is either. Any input would be greatly appreciated.
Thanks.
As a guess
Potassium - 39.0983
39.0983 x 3 = 117.2949 for tripotassium.
Sodium Bismuthate
279.97
279.97 - 22.989770 = 256.98023 for Bismuthate
Do you mean citrate or citrato?
The chemical name is citrato and not citrate. I’m not sure if it really matters though. I figure that it has three potassiums, two citrate groups and a bismuth oxide. I’m just not sure what the citrate part is or how many bismuth oxides are floating around.
The compound has been used as an antiulcer drug for 25 years or so in Europe, but I haven’t had any luck finding a formula in the literature. I’m baffled, you’d think that something like a formula would be important as there are quite a few bismuth compounds that have been used over the past couple of hundred years for medical purposes ie Pepto Bismol, Zantac etc.
Thanks for looking into it.
Wesley Clark:
I see what you’ve done to get the Bismuthate part. That could be useful. Pretty smart.
This stuff isn’t a well-defined molecular solid, it’s a mishmash of bismuth hydroxide with other ingredients. The ratio of bismuth to the other ingredients is not set in stone, so it’s impossible to express a formula weight. Why?
Perhaps this will help. http://content.karger.com/ProdukteDB/produkte.asp?Aktion=ShowPDF&ProduktNr=229900&ArtikelNr=77618&filename=77618.pdf
It’s a pay per view or subscribe site, and I’m not willing to go that far to find out exactly what it says, but essentially:
CBS, also called tripotassium dicitrato bismuthate, is a complex salt of bismuth
and citric acid. Its formula is as follows: Bi X (OH) y (C 6 H 5 O 7 ) z. The
group C 6 H 5 O 7 represents the citric acid radical.
It’s important to note, however, that formulations vary between companies, and even between lots made by the same manufacturer. It’s not a nice and neat reaction, you see.
I’d like to know what the formula weight is so that I can compare two different bismuth compounds on a molar basis. Looks like it’ll be more complicated than I thought it would be. Nothing worthwhile is easy though, I guess! 
Thanks for all of your help guys.
Why is it called “tripotassium” if there aren’t any potassium’s in the formula?
Possibly a counter-ion. I’m having a little trouble figuring out if X is a stocheometric amount or an indeterminate atom.
Tripotassium dicitrato bismuthate is also known as “colloidal bismuth subcitrate.” Bismuth (III) citrate (citric acid chelated with bismuth) is the actual complex of interest, but isn’t a soluble compound. It can be solubilized with either ammonium or potassium hydroxide buffers. For years, either formulation was known as colloidal bismuth subcitrate, but to distinguish between the solubilizing agents, the terms tripotassium dicitrato bismuthate and ammonium bismuth citrate were coined. Anyway, I believe that the correct chemical formula is Bi(KOH)3C6H5O7. There will also be a hodgepodge of products such as BiC6H5O7(insoluble), (OH)3C6H5O7, etc. formed. Hope this helped.
It’s ‘citrato’ and not ‘citrate’ because this is a coordination compound, one where bonding to the metal is primarily through coordinate (aka ‘dative’) bonds. Coordinate bonds are a type of bond where one participant in the bond provides two electrons to the other participant, rather than the two participants each contributing one electron to a shared pair (a covalent bond). In a coordination compound, the units that bond to the metal are called ligands, so the citric acid here is called a ‘citrato ligand’.
Citrato is a tridentate ligand, which means that it’s capable of ‘biting’ into a metal three times – that is, it forms three coordinate bonds. (At least it can; that doesn’t mean it always does.) A compound called tripotassium dicitratobismuthate(III) should have two citrato ligands bound to the bismuth in an octahedral fashion. Each ligand has a (3-) charge, assuming it is bound as a tridentate ligand, and the overall complex also therefore has a (3-) charge. This negative charge would be balanced with three potassium ions, which are not part of the complex, but are bound to it as in any salt. At least, that’s what it would be if they were using the name correctly, but that’s not what’s happening. The molecular formula of a compound correctly named tripotassiumdicitratobismuthate(III) gives the formula C[sub]13[/sub]H[sub]12[/sub]BiK[sub]3[/sub]O[sub]14[/sub], MW 718.5, but again that’s not correct.
The Merck Index (11th Edition) says explicitly that this name is incorrectly applied to a potassium and ammonium salt of oxohydroxocitratobismuthate(III), and it gives a formula no one else has given. (That may be because I’m looking at an older edition.) The ‘approximate formula’ given in the 11th edition is K[sub]3/sub[sub]2[/sub][Bi[sub]6[/sub]O[sub]3/sub[sub]5/sub[sub]4[/sub]] (ci = citrate). I suspect that something closer to correct has been reported in the literature (which I can’t access now), though it seems to be a mixture of products. Bismuth compounds (according to Cotton & Wilkinson, which means this information is Literally True) aren’t formed as predictably as compounds of the transition metals, and the most important thing in determining the structure is the number of covalent bonds.
Wow! Thanks for all of the info.
If the production of this compound isn’t always exactly reproducible then how are they able to use it as a drug? Do they have to toss out batches or recalculate the amount of active ingredient with each batch? Seems like it would cause some problems.
How common is it to see these “sloppy” reactions where A + B doesn’t always equal C?
Thanks