How To Measure Fluoride In Water

Fluorine is one of the essential trace elements for the body. The appropriate fluoride content in drinking water is 0.5-1.0mg/l. Suppose fluoride content is too low and prone to dental caries. If the fluorine content is too high, children can have dental fluorosis.

When there are metal ions such as aluminum ions, iron ions, and lead ions in water samples, or a large number of chloride and sulfate, it will interfere with the determination of fluorine. Therefore, when there are many interfering substances in the water sample, we must treat the water sample in advance by distillation.

The determination of fluorine ions in water

Fluororeagent Spectrophotometry

Fluoride reagent spectrophotometry is one of the primary methods for determining fluoride. It is suitable for determining fluoride in surface water, groundwater, and industrial wastewater.

The principle:

Fluorine ion reacts with fluorine reagent and lanthanum nitrate in acetate buffer medium with pH 4.1 to form a blue ternary complex. The absorbance of the complex at 620 nm wavelength is directly proportional to the concentration of fluoride ion, and fluoride (F -) is quantitatively determined. The high limit of determination is 0.02 mg / L, and the lower limit is 0.08 mg / L.

Pros:

Good accuracy and reproducibility in the analysis of low concentration samples.

Cons:

Complex steps, take time

Steps:

Reagent and instrument fluorine standard solution:

• Weigh 0.2210g sodium fluoride (dry at 500-600 ℃ for 40min).
• Dissolve it in water.
• Transfer it to a 1000ml volumetric flask.
• Dilute it with water.
• Shake it well.

The fluorine content of this solution is 100ug / ml. dilute it to 2ug / ml when in use and store it in a polyethylene bottle.

0.001mol/l fluorine reagent solution: 

3-methylamine alizarin diacetic acid, weigh 0.193g fluorine reagent, add a small amount of water to wet, drop 1mol / L sodium hydroxide solution to dissolve, add 0.125g sodium acetate, adjust pH5 with 1mol / L hydrochloric acid, and dilute to 500ml with water.

0.001mol/l lanthanum nitrate solution:

• Weigh 0.433g lanthanum nitrate.
• Dissolve it with a small amount of 1mol / L hydrochloric acid.
• Adjust pH4 with sodium acetate 1.
• Add water to dilute to 1000ml.

Concentrated sulfuric acid: put 300ml concentrated sulfuric acid into a 500ml beaker, heat it on an electric heating plate until it boils, keeps it for one hour to remove trace fluorine, cools it, and bottle it for standby.

PH4. 1 buffer:

• Weigh 35g of anhydrous sodium acetate and dissolve it in 800ml water.
• Add 75ml glacial acetic acid.
• Dilute it to 1000ml with water.
• Adjust the pH value to 4.1 with a pH meter.

Mixed developer: take 0.001mol/l fluorine reagent solution, and pH4 one buffer, acetone, and 0.001mol/l lanthanum nitrate solution are mixed according to the volume ratio of 3:1:3:3 and newly prepared when used.

Ultraviolet-visible spectrophotometer

Analysis steps:

put the sample (depending on the fluorine content) into a 25ml colorimetric tube, accurately add 10ml of the mixed developer, dilute it with water to the mark, shake it well, and place it for 30min.

Use a 3cm cuvette, place it at the 620nm wavelength of the UV spectrophotometer, take the blank reagent as the reference, measure the absorbance, and calculate the fluorine content from the standard curve.

The standard curve is drawn in six 25ml colorimetric tubes. Add 0.0, 2.0, 4.0, 6.0, 8.0, and 10.0ug of fluorine standard solution respectively, add water to 10ml, and accurately add 10ml of the mixed developer. Draw the standard curve according to the measurement steps below.

Note: the absorbance of the blue complex is greatly affected by the pH value, the amount of acetone, and the amount of buffer.

Dual-wavelength coefficient Spectrophotometry

The principle of this method is the same as that of fluorine reagent spectrophotometry. However, it adopts dual-wavelength spectrophotometry, which can eliminate the influence of reagent background, improve sensitivity, save reagent dosage and reduce environmental pollution.

Ion-selective electrode method

Lanthanum fluoride single crystal is selective for fluoride ions. There is a potential difference between different concentrations of fluorine solutions on both sides of the lanthanum fluoride electrode membrane. This potential difference is called membrane potential， which is related to the ion activity of fluoride solution.

It is widely used for applications, such as determining water samples with high turbidity and chromaticity.

Measuring instruments and accessories

1) PH meter

2) Fluoride ion-selective electrode, saturated calomel electrode

3) Magnetic stirrer, plastic stirrer

4) Polyethylene beaker

reagent

1) Fluoride standard stock solution I (F – = 100) μ g/mL）

2) Fluoride standard solution II (F – = 10) μ g/mL）

3) Total ionic strength adjustment buffer solution (TISAB)

Steps

1. Drawing of the standard curve

1) Suck 2.00ml, 6.00ml, 10.00ml, 20.00ml and 40.00ml fluoride standard solution II into 100ml volumetric flask respectively.

2) Add 10ml total ionic adjustment buffer solution (TISAB) respectively, dilute it to the mark with distilled water, and mix well.

3) move them into polyethylene cups and put a plastic stirrer in each.

4) Insert the electrode in the order of concentration from low to high, continuously stir the solution, and get the stable potential value E (MV) under the stirring state. Before each test, rinse the electrode with distilled water and absorb the water with filter paper.

5) Taking the negative logarithm of fluorine ion concentration as the abscissa ;

The battery electromotive force E (MV) as the ordinate;

The E-LGCF standard curve is drawn on the semi-logarithmic coordinate paper.

Determination of water samples

Suck a 50ml water sample with a pipette and put it into a 100ml volumetric flask. The operation steps are the same as those in drawings (2) and (3) of the standard curve. Then insert the electrode into the polyethylene cup, get the steady-state potential value ex (MV) under continuous stirring, and check the fluoride content from the standard curve.

Calculation

CF-(mg/L)=Cf-*100/V

Where: C F – fluorine content found from the standard curve, mg / L; V – the volume of water sample (ML).

Ion chromatography

The ion chromatography consists of an eluent, high-pressure pump, sample injection valve, protective column/separation column, suppressor, conductivity cell, and data processing system. There are conductivity detectors, ampere detectors, and optical detectors in sensors.

The ion chromatograph adopts the principle of ion exchange to separate various ions in the separation column quickly. The suppressor removes the strong electrolyte in the eluent to deduct the background conductivity. Then the conductivity of the ions to be measured is obtained by continuous measurement by the detector.

The conductivity is directly proportional to the concentration of the ion to be measured at a low concentration. The retention time of the components to be tested is used for qualitative analysis, and the peak height or peak area is used for quantitative analysis.

There are two main functions of the suppressor:

one is to reduce the background conductivity of the eluent；

other is to increase the conductivity of the measured ions and improve the signal-to-noise ratio.

Ion chromatography is a high automation instrument that can analyze the contents of various anions in water at the same time. An automatic sampler and software can control it, automatically inject samples, conduct qualitative and quantitative analysis, and print reports; hence, it can significantly reduce work and improve efficiency.

If you measure fluorine ions under weakly acidic conditions, you can add a little alkaline eluent that can release fluoride ions to improve the accuracy of the determination. However, ion chromatography is expensive, with a high maintenance cost and consumables.

In addition, a high concentration of low molecular weight organic acid in the water sample will interfere with the determination because its retention time is similar to that of the measured component. If Water samples with more suspended solids or severe pollution, we need to treat them before testing.

It is suitable for determining soluble fluoride, chloride, nitrate, and sulfate in drinking water and source water.

Zirconium alizarin colorimetry

It is suitable for determining fluoride in drinking water, surface water, groundwater, and industrial wastewater.

The principle:

In the acidic solution, sodium alizarin sulfonate and zirconium salt produce a red complex. Fluorine ions in the sample can take out the zirconium ions in the complex, generate colorless zirconium fluoride ions, and release yellow sodium alizarin sulfonate.

It is quantified with the standard colorimetry according to the different chromaticities of the solution, from red to yellow. When taking a 50 ml sample and directly determining the concentration of fluoride, the detection limit of determination is 0.1 mg / L, the lower limit of determination is 0.4 mg / L, and the high limit of determination is 1.5 mg / L.

Pros:

low concentration samples, more simple, economic, and accurate

Cons:

 large error.

summary

There are many methods to measure the fluoride determination in water, and each method has a corresponding application and standard operation steps. The use of the technique directly is related to the accuracy of the data.