The alignment of the poster above with a “balanced” duo of opinion pieces on fluoridation in the recent issue of the Journal of primary healthcare is just too good not to comment on.
The articles are:
- New Zealand drinking water should be fluoridated – Yes by W Murray Thomson BSc, BDS, MA, MComDent, PhD Professor of Dental Epidemiology and Public Health, Sir John Walsh Research Institute, University of Otago, Dunedin, New Zealand, and
- New Zealand drinking water should be fluoridated – No by G Mark Atkin BSc, LLB(Hons) Science and Legal Advisor, New Zealand Fluoridation Information Service, PO Box 9804, Wellington, New Zealand.
Looks a bit uneven, doesn’t it? A Professor of Dental Epidemiology and Public Health ”balanced” against a political activist? Worse, Atkin’s organisation is an astroturf one set up by the Fluoride Action Network of NZ (FANNZ). Rather clumsily, I add, as they use the same office address! (See Anti-fluoridationist astro-turfing and media manipulation).
Still, I want to comment on the chemical arguments used by Atkin. He is effectively clutching at straws, using very naive interpretations of the chemistry of fluorosilicates and fluoride. As a chemist I find such opportunist distortion of chemistry offensive. Problem is, his and similar arguments are often presented by anti-fluoridation activists and lapped up by their supporters.
Fluorosilicic acid and sodium fluorosilicate are common fluoridating chemicals. When diluted in water the fluorosilicate decomposes to form silica and the fluoride anion. Some anti-fluoridation activists, including Mark Atkin, deny this becuase they wish to deny the relevance of studies showing the safety of fluoride at concentrations used in fluoridating drinking water. So they advance the bogey of an especially toxic fluorosilicate species. Atkins condenses two of their arguments in this succinct statement:
“Silicofluorides do not fully dissociate to form free fluoride ions in aqueous solution and revert to the silicofluoride ion in acid stomach conditions.”
Hydrolysis of fluorosilicate.
There is some straw clutching going on here in the discussion of the chemistry of fluorosilicates and fluoride which distorts the real chemistry.
The Nuclear Magnetic Resonance work of Finney et al (2006) (“Reexamination of hexafluorosilicate hydrolysis by 19F NMR and pH measurement”) showed complete decomposition of fluorosilicate species at neutral pH values on dilution. They also showed the presence of surviving fluorosilicate species at low pH values (3 and below) – which is of course irrelevant for water treatment. No one is going to produce drinking water at such acidic values.
Anti-fluoride people are using the observation at low pH values to claim that fluorosilicate species remain in solution at neutral pH values. They sometimes also rely on studies where authors have expressed their results with an indication of precision. For example, Atkin’s activist organisation (the NZ Fluoridation Information Service) claims fluorosilicates:
“do not completely break down into Fluoride ions. This was shown by Crosby in 1969.”
But Crosby (1969) (“Equilibria of fluorosilicate solutions with special reference to the fluoridation of public water supplies”) actually reported:
“sodium fluorosilicate, at the concentration normally present in public water supplies, is dissociated to at least 95%.”
Atkins and his organisation are clutching at a very weak straw there.
Let us be clear – the research indicates that within experimental precision the deocmposition of fluorosilicates is complete and, as expressed by Urbansky (2002), (“Fate of fluorosilicate drinking water additives”):
“equilibrium should have been achieved by the time the water reaches the coinsumer’s tap if not by the time it leaves the waterworks plant.”
Reversion of fluoride to fluorosilicate?
Atkin’s claim of reversion of fluoride to fluorosilicate “in acid stomach conditions” is also incorrect. He is relying on a simplistic misunderstanding of the nature of “dissociation” of fluorosilicate on dilution.
It is important to recognise the “dissociation” of fluorosilicate species into fluoride and silica at neutral pH values is, in effect, a decomposition. Because of the polymerisation of the silica, and the olation and oxolation reactions involved, the equilibrium is driven to completion – in effect the silica is removed from the reaction. (While it may remain in “solution” or “suspension” for a time it is effectively inert – due to olation – as far as the equilibrium is concerned).
What do I mean by olation? While monomolecular Si(OH)4 is formed on dissociation of the fluorisilicate it rapidly undergoes reactions which lead to exclusion of water and the transformation of Si-OH bonds to Si-O-Si bonds.
(HO)3Si-OH + OH-Si(OH)3
↓ Olation
(HO)3Si-OH-Si(OH)3 + OH
↓ Oxolation
(HO)3Si-O-Si(OH)3 + H2O
Eventually this leads to formation of colloidal and solid silica. But even while in solution olation and oxolation reduces the reactivity of the silica species.
Just as fluorosilicate species do not reform in your tap water, they do not reform in your stomach. Even if the silica is still in suspension it is no longer present as mono-molecular Si(OH)4 and is effectively inert. So despite the low pH there is no simple equilibrium. Remember too, your drinking water will contain silica derived from other sources besides fluorosilicate (which is probably a minor contributor).
Sure, one can prepares solutions in the laboratory at pH values of 3 or less that contain fluorosilicate species – but once decompostion (involving loss of silica reactivity) occurs at neutral pH values the reaction is not easily reversed. Especially considering the time lapsed between decomposition of the fluorosilicate and drinking water entering one’s stomach.
HF in stomach
But Atkin still has a fallback postion – if the fluorosilicate doesn’t get you the hydrofluoric acid will. He aserts:
“that 40% of ingested fluoride is absorbed through the stomach wall as molecular hydrofluoric acid (a known mutagen). This negates the ‘all fluoride ions are the same’ deception.”
Yes, in the acid conditions of your stomach F anions will exist in equilibrium with the protonated HF species.
H+ + F- ↔ HF
This is also true for other weakly acidic anions – even sulphate – that is just simple chemistry. But, the real danger of the solution in your stomach is that it is very acid, it has a low pH – a very high concentration of hydrogen ions. It is the hydrogen ions that are corrosive. (If anything, the presence of weakly acidic anions like fluoride will actually lower the acidity by removing some of the hydrogen ions). One does not put one’s hand, or any other sensitive tissues, into acidic solution like this. However the stomach is built to handle these conditions.
I understand the molecular species involved in the transfer of fluoride across the stomach wall cells into the blood stream is HF. (Once in the blood HF will convert to fluoride because of the higher pH). So clearly the low pH assists in uptake of F by the body. Don’t forget the concentrations of fluoride, and therefore HF, is actually very low. The protonated species in your stomach solution are not equivalent to the HF (or HCl and H2SO4) chemicals we are used to in their concentrated forms in the laboratory.
