[safewater]

I provide techical assistance to public water systems in the state of Arkansas.  A number of our water systems add fluoride (fluorosilicic acid H2SiF6) to their water as well as sodium hexametaphosphate for iron sequestration.  I'm wondering how these two chemicals may react with one another or cause an interference with fluoride concentration testing using the SPADNS reagent for analysis? 

[Alpha-Omega]

SPADNS Reagent Zirconium Lake-EPA 844 Method 13A (discusses possible interferences for this colormetric analysis)

Link:  http://www.epa.gov/ttn/emc/promgate/m-13a.pdf


Hydrofluorisilic Acid:

The most commonly used fluoridating chemical.  This acid, also known as fluorosilicic acid, hexafluosilicic acid, and silicofluoric acid, is a liquid with the formula H2SiF6.  The liquid may be fed directly into the raw water or may be diluted.  Hydrofluosilicic acid is a popular choice in many water treatment plants because it is usually the least expensive fluoridation chemical and is the easiest to feed.  However, it can be expensive to ship since it is a liquid and is heavier than the other fluoridation chemicals.

Sodium Hexametaphosphate:

Sodium hexametaphosphate is a chemical also known as glassy phosphate, Calgon, and Sodium Polyphos. It is used in corrective treatment to form a protective coating on iron pipes, but may cause corrosion of copper pipes.  Sodium Hexametaphosphate is used as sequestering agent. It is used in the industry of soap, detergents, water treatment, metal finishing and plating, pulp and paper manufacture, synthesis of polymers, photographic products, textiles, scale removal and agriculture.


In the water purification process:

As the raw water enters the plant, it is treated with ferric sulfate and lime. Ferric sulfate is also a coagulant chemical, and it is used to aid the polyelectrolyte in the raw water clarification process. Lime, also known as calcium oxide, is used for pH adjustment, softening, and corrosion control.

After the raw water has been treated with polyelectrolyte, ferric sulfate, and lime, it is gently mixed by large mechanical paddles in two flocculation basins where the suspended particles gather together into larger particles. The flocculated water then travels into two primary settling basins where the particles settle. The settled particles form a sludge layer on the bottom of each primary settling basin. This sludge is pushed into sumps by traveling mechanical rakes, and is then removed from the basins through a series of valves and pumps.

The clarified water then exits the settling basins, and is disinfected by the addition of free chlorine. Anhydrous ammonia is added shortly thereafter, producing chloramine. Chloramine is a compound produced by reacting free chlorine with ammonia, and is used for residual disinfection.

Chloraminated water then enters large secondary settling basins, allowing additional settling time for suspended particles and disinfection contact time. After the water exits the secondary settling basins, it is treated with sodium hexametaphosphate and fluorosilicic acid. The sodium hexametaphosphate is used as a sequestrant, which holds the lime in solution keeping it from depositing on the filter or the media. The fluorosilicic acid is used to add fluoride to the drinking water to aid in the prevention of dental cavities.

They do not interfere.  Check www.epa.gov for additional information.