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Solubility and toxicity of different fluorides
Consumption of fluoride cannot be avoided. It appears in so many different forms and in so many different environments (in our water supplies, our food, the air we breath, etc.). Therefore, it is important to know exactly how toxic each type of fluoride is and how much is considered safe when exposed to this chemical.
Toxicity is related to solubility. If a compound of fluoride (fluorine + one or more other elements) is very soluble, the fluoride ion is made more readily available. In 1971, former Aston University chemist Malcolm Harris investigated some fluoride compounds and determined the following table of solubility (solubility: "... a critical aspect of toxicity");-
Calcium fluoride [natural] 16 ppm at 18ºc and 17 ppm at 26ºC
Sodium Fluoride [artificial] 42,200 ppm at 18ºC
Sodium fluosilicate [artificial] 6,520 ppm at 17ºC
Hydrofluorosilic acid [artificial] miscible liquid
It can be clearly seen that sodium-fluoride compounds are far more soluble than calcium fluoride. Hydrofluorosilic acid (used to fluoridate water supplies in the UK and [extensively] in the USA) is a "miscible" liquid. A miscible liquid means something which can be mixed (in this instance, with water).
By definition, hydrofluorosilic acid has 6 atoms of fluorine (H2SiF6). Of these six atoms, it is sometimes assumed that all six will become simple fluoride ions. However, there is no known published to demonstrate this actually happens. In fact, the best available knowledge suggests that only four of these atoms will become fluoride ions and the remaining two will form more 'exotic' complexes (depending on the quality of water the compound is mixed with).
More information on solubility:
Lanthanum Fluoride (LaF3) Insoluble in water
Magnesium Fluoride (MgF2) 0.0002g/100g water
Calcium Fluoride (CaF2) 0.0017gm/100gm water at 20Â°C
Strontium Fluoride (SrF2) 0.012g/100g water at 27Â°C
Barium Fluoride (BaF2) 0.17g/100g water at 23Â°C
Lithium Fluoride (LiF) 0.27g/100g water at 20Â°C
Sodium Fluoride (NaF) 4.22g/100g water at 18Â°C
Considering that solubility is "... a critical aspect of toxicity", it is now prudent to examine another table which shows different types of fluoride. Professor Kaj Roholm's table of toxicity gives three categories of inorganic fluorine compounds (it should be noted that Prof. Roholm is the author of the first and most comprehensive monograph on fluorine toxicity);-
Hydrogen Fluoride (anhydrous)
Very Toxic (Easily soluble fluorides and fluorosilicates)
Moderately Toxic (Poorly soluble fluorides)
Hydrofluoric acid and hydrofluorosilicic acid are classified as being extremely toxic. Hydrofluorosilicic acid is used to fluoridate UK water supplies. The similarly dangerous hydrofluoric acid carries the following warning issued by the Health & Safety Executive (2001);-
Hydrofluoric acid poisoning
Recommendations on first aid procedures. Health & Safety Executive: http://www.hse.gov.uk
Information contained within this document is accurate as of 1/1/2001
IMPORTANT. ALWAYS contact the HSE for advice. See Disclaimer (below).
This leaflet is aimed at employers and employees in industries where hydrofluoric acid is used. It provides information on:
 THE INFORMATION PROVIDED ON THIS WEB-PAGE IS PURELY FOR THE PURPOSE OF HIGHLIGHTING THE DANGERS OF HYDROFLUORIC ACID.
 ANYONE WHO WORKS WITH HYDROFLUORIC ACID SHOULD ENSURE THAT THEIR EMPLOYER HAS THE APPROPRIATE MEASURES TO DEAL WITH ANY INCIDENT RELATING TO THE USE OF THIS CHEMICAL.
Hydrofluoric acid is corrosive. It can cause severe burns to the skin and eyes. If it comes into contact with skin, you may not feel pain at once Hydrofluoric acid is also highly irritating to the respiratory system and very toxic if swallowed.
The Control of Substances Hazardous to Health Regulations 1999 (COSHH) apply. A COSHH assessment should be completed. Consider the use of safer alternatives. If there are no suitable alternatives, the assessment should detail appropriate precautions to be taken when using hydrofluoric acid, which include using a safe system of work. Employers should ensure that employees are given adequate information and training on the hazards to health posed by hydrofluoric acid, and the precautions to take to avoid them.
Obviously, hydrogen-related fluorine compounds are extremely dangerous. While hydrofluorosilicic acid breaks down in water, the raw material poses a significant threat to those who come into contact with it.