Optimal Fluoride Dose
Optimal fluoride concentration in drinking water.
Fluoride is not in pure form when purchased. It is combined with other elements for stability and ease of handling.
Sodium Fluoride - NaF
Sodium Fluorosilicate - Na2SiF6
Fluorosilicic Acid - H2SiF6
Types of Dry Feeders
Acid Feed Systems
Safety Equipment Needed
Day Tanks (Acid)
Operation of Fluoridation Systems
Fluoride Injection Point
Dry Chemical Storage
Fluorosilicic Acid Storage
Analysis of Fluorides in Drinking Water
Ion Selective Electrode method
When Running Fluoride Tests:
Determination of Fluoride With Ion-Selective Electrode Method
This method can be used for measuring total solubilized fluoride in drinking waters, natural surface waters and ground waters. ISEs must be used carefully and results must be interpreted cautiously, since an ISE may be affected by numerous analytical interferences which may either increase or decrease the apparent analyte concentration, or which may damage the ISE. Effects of most interference can be minimized or eliminated by adding appropriate chemical reagents to the sample. Obtaining the most accurate results, therefore, requires some knowledge of the sample composition. ISE manufacturers usually include a list of interferences in the instruction manual accompanying an ISE, along with recommended methods for minimizing or eliminating effects of these interferences.
Summary of Method
Total solubilized fluoride is determined potentiometrically using a fluoride ion-selective electrode (ISE) in conjunction with a standard single-junction reference electrode, or a fluoride combination ISE, and a pH meter with an expanded millivolt scale or an ISE meter capable of being calibrated directly in terms of fluoride concentration.
Standards and samples are mixed 1:1 with a total ionic strength adjustment buffer (TISAB). TISAB adjusts ionic strength, buffers pH to 5-5.5, and contains a chelating agent to break up metal-fluoride complexes. Calibration is performed by analyzing a series of standards and plotting mV vs. fluoride concentration on semilog paper or by calibrating the ion meter directly in terms of fluoride concentration.
Polyvalent cations (e.g., Fe+3 and Al+3) interfere by forming complexes with fluoride which are not measured by the fluoride ISE. Aluminum-fluoride complex ions are approximately ten times more stable than corresponding iron(III)-fluoride complex ions. As the aluminum concentration increases, more fluoride is consumed to form the metal-fluoride complex. Adding TISAB, which contains a strong chelating agent, eliminates this interference by complexing polyvalent cations.
Sample pH is critical. Hydroxide is a significant interferant at concentrations ten times the fluoride concentration. This interference is avoided by adding TISAB which buffers the sample at a pH of 5-5.5. At low pH values, fluoride forms bifluoride (HF ) which is not detected by the 2 - fluoride ISE. Again, adding TISAB prevents this interference by buffering the pH. Temperature changes affect electrode potentials. Therefore, standards and samples must be equilibrated at the same temperature.
The user should be aware of the potential of interferences from colloidal substances and that, if necessary, the samples may be filtered.
Apparatus and Materials
Sample Collection, Preservation, and Handling
If using a fluoride combination ISE, ensure that the ISE is filled with the solution recommended by the manufacturer. Change the solution if the ISE has not been used for a week. If using a fluoride ISE and a separate reference electrode, ensure that the reference electrode is filled with the solution recommended by the manufacturer. In either case, equilibrate the electrodes for 24 hours in a 10.0 mg/L fluoride standard before use.
Calibrate the fluoride ISE using standards that narrowly bracket the expected sample concentration. If the sample concentration is unknown, calibrate with 1.00 mg/L and 10.0 mg/L fluoride standards. Add 20.0 mL of standard and 20.0 mL of TISAB to a 50 mL polyethylene beaker. Add a PTFE-coated magnetic stir bar, place the beaker on a magnetic stir plate, and stir at slow speed (no visible vortex). Immerse the electrode tips to just above the rotating stir bar. If using an ISE meter, calibrate the meter in terms of fluoride concentration following the manufacturer's instructions. If using a pH/mV meter, record the meter reading (mV) as soon as the reading is stable, but in no case should the time exceed five minutes after immersing the electrode tips. Prepare a calibration curve by plotting measured potential (mV) as a function of the logarithm of fluoride concentration. The slope must be 54-60 mV per decade of fluoride concentration. If the slope is not acceptable, the ISE may not be working properly. For corrective action, consult the ISE operating manual. Use only plastic ware (polyethylene) when preparing and analyzing fluoride samples.
Allow samples and standards to equilibrate to room temperature.
Prior to and between analyses, rinse the electrode thoroughly with reagent water and gently shake off excess water. Low-level measurements are faster if the electrode tips are first immersed for five minutes in reagent water.
Add 20.0 mL of sample and 20.0 mL of TISAB to a 50 mL polyethylene beaker. Add a PTFE-coated magnetic stir bar. Place the beaker on a magnetic stir plate and stir at a slow speed (no visible vortex). Immerse the electrode tips to just above the rotating stir bar. Record the meter reading (mV or concentration) as soon as the reading is stable, but in no case should the time exceed five minutes after immersing the electrode tips. If reading mV, determine fluoride concentration from the calibration curve.
When analyses have been completed, rinse the electrodes thoroughly and store them in a 10.0 mg/L fluoride standard solution. If the electrodes will not be used more than one day, drain the internal filling solution, rinse with reagent water, and store dry.
Symptoms of Overexposure
Chronic Toxic Exposure
Acute Toxic Exposure
Determining Feed Rate
Fluoride Chemicals Purity
Ion Concentration Percent (AFI)
A water plant wants a fluoride concentration of 1.00 mg/L in the finished water. If the raw fluoride content is 0.25 mg/L, how much fluoride needs to be added?
Fluoride dose = Total fluoride, mg/L - raw fluoride, mg/L
Fluoride dose = 1.00 mg/L - 0.25 mg/L
Fluoride dose = 0.75 mg/L
Calculating Feed Rate
A water plant treats 750,000 gpd and wants to add 0.9 mg/L of fluoride to the water using sodium fluoride. How many pounds per day will be used?
Calculating Feed Rate
A water plant treats 200,000 gpd and wants to add 0.82 mg/L of fluoride to the water using sodium fluorosilicate. How many pounds per day will they use?
Calculating Feed Rate
A water plant treats 12.0 MGD and wants to add 0.75 mg/L of fluoride using fluorosilicic acid to the water. How many pounds per day will be used?
Feed Rate for Saturator
A water plant produces 250 gpm. Determine the feed rate for a saturator in gal/day if the dosage is 1.0 mg/L?
A total of 25 lbs of sodium fluorosilicate is used to treat 1.75 MGD. What is the concentration of fluoride in the water in mg/L?
A total of 5 lbs of sodium fluoride is used to treat 0.2 MGD. What is the concentration of fluoride in the water in mg/L?
A water plant uses 284 lbs of fluorosilicic acid to treat 6.2 MGD. What is the concentration of fluoride in the water in mg/L?
A water plant uses 9 gallons of sodium fluoride from a saturator to treat 200,000 gpd. There is 0.18 mg/L fluoride in the raw water. What is the total concentration of fluoride in mg/L?
*You can only use NaF in saturators because it has a constant saturation point.