How To Measure Ozone Concentration In Water

Ozone(O3) is a strong oxidant with strong characteristics of sterilization, disinfection, bleaching and deodorization. Therefore, it is widely used in drinking water disinfection, food processing, sterilization and purification, and medical and household. However, it will harm the body if there is excessive ozone. So how to measure the residual amount of ozone in the water?

There are spectral analysis and electrochemical analyses for measuring ozone in water. The common determination methods are iodometry, sodium indigo disulfonate spectrophotometry, and UV Spectrophotometry. Spectrophotometry has the advantage of being small in size, cost-effective, and easy to carry. It is suitable for application in rural or county-level laboratories.

Indigo sodium disulfonate spectrophotometry is to oxidize indigo rapidly by ozone under acidic conditions to make it fade, and the decrease of absorbance is linear with the increase of ozone concentration.

Chemical test methods

Iodine method

Ozone (O3) is a strong oxidant which can free iodine by reacting with potassium iodide (KI) aqueous solution. After sampling and acidifying the solution, titrate the free iodine with 0.1000mol/l sodium thiosulfate (Na2S2O3) standard solution and starch solution as an indicator, and calculate the odour oxygen according to the consumption of sodium thiosulfate standard solution.

The reaction formula:

O3+2KI+H2O=O2+I2+2KOH————— (1)

I2+2Na2S2O3=2NaI+Na2S4O6————(2)

Reagent:

1. Potassium iodide (KI) solution (2%): dissolve 20g of potassium iodide in 1000ml of distilled water-cooled after boiling, and store it in a brown bottle in the refrigerator for at least one day. 1.00mL of this solution contains 0.020g potassium iodide.

2. (1 + 5) sulfuric acid (H2SO4) solution: measure concentrated sulfuric acid (P = 1.84; analytical purity) and dissolve it in distilled water 5 times the volume.

3. C (Na2S2O3 • 5H2O) = 0.1000mol/l sodium thiosulfate standard solution; Accurately weigh 24.817g sodium thiosulfate (Na2S2O3 • 5H2O; analytical purity) with an analytical balance and dissolve it in a 1000ml volumetric flask with freshly boiled and cooled distilled water. Alternatively, take 25g of sodium thiosulfate (Na2S2O3 • 5H2O; analytical purity) and dissolve it in 1000ml of newly boiled and cooled distilled water.

The concentration of sodium thiosulfate in this solution is about 0.1mol/l. Then add 0.2g sodium carbonate (Na2CO3) or 5ml trichloromethane (CHCl3);

Calibration: adjust the concentration to 0.1000mol/l and store it in a brown bottle. If the storage time is too long, it needs to recalibrate before use.

4. Starch solution: weigh 1g soluble starch, mix it into suspension slurry with cold water, then add about 80ml boiled water, stir while adding, and dilute to 100ml; After boiling for a few minutes, place the precipitate overnight, take the supernatant and use it. If it needs to store for a long time, add 1.25g salicylic acid or 0.4g zinc chloride.

Step:

Measure 1ml of potassium iodide solution, pour it into a 500ml absorption bottle, and then add a certain volume of solution to be tested- add 1ml (1 + 5) sulfuric acid solution (reduce the pH value to below 2.0), shake it well, and stand for 5min. Titrate with 0.1000mol/l sodium thiosulfate standard deep solution, add a few drops of starch solution (about 1ml) when the solution is light yellow, and continue to titrate carefully and quickly until the colour disappears. Record the dosage of sodium thiosulfate standard solution.

Calculation

Co3=VNa × C × 2400/V0(mg/L)

CO3 — ozone concentration, mg / L

VNA — dosage of sodium thiosulfate standard solution, ml

C — concentration of sodium thiosulfate standard solution, mol / L

V0 — sampling volume of ozone water, ml

Pros：

Intuitive， low-cost equipment

Cons:

Easy to be disturbed by oxidants such as non-oxidant and ci2.

Chromatography

Colourimetry is a method to determine ozone concentration according to the ozone’s colour or decolourization level and different chemical reagents. Artificial colourimetry and photometric colourimetry are mainly used to measure the concentration of dissolved ozone in water.

For the determination of the dissolved concentration of ozone in bottled water, we can use potassium iodide and o-methylamine. The method is to determine the ozone solubility value (0.05 ~ 0.08mg/l) of the sample by comparing the sample colour liquid tube and then measuring it by spectrophotometry.

Based on this principle, the ozone concentration detection instrument is very convenient for on-site sampling. When the wavelength ratio is 600nm, the concentration is 0.05 ~ 0.75nm/l, and the accuracy is ± 0.01nm/l, less disturbed by other oxidants.

Colourimetry

It determines the ozone concentration according to ozone’s colour change reaction procedure and different chemical reagents. According to the colourimetric method, there is an artificial colour sample and photometer colourimetry. It is mainly used to measure water dissolved ozone solution.

In General, We can directly determine ozone in water by potassium iodide DPD on-site colourimetry when drinking water or natural water disinfected by ozone with the range of 0.02mg/l-2.50mg/l.

The main principle is that ozone in water can react with potassium iodide to generate free iodine. When the pH value is 6.2-6.5, free iodine can react with N, N diethyl p-phenylenediamine (DPD) to generate a red compound that can selectively absorb 520nm wavelength light. You can determine ozone concentration according to the level of light absorption of water samples.

In addition, too many iron and copper ions in the water will interfere with the final test results. We can add 0.1ml disodium EDTA solution to the water sample to eliminate it. Suppose there is too much residual chlorine in the water sample. In that case, it will also significantly impact the results, which can be eliminated by adding 0.2ml glycine solution to the water sample.

Instruments and reagents

Instrument: 1 Spectrophotometer 2 Colorimetric bottle

Reagent:

1. Potassium iodide reagent tube

2. DPD reagent tube

3. Potassium iodide (KI)

4. EDTA disodium solution

Weigh 4g disodium EDTA, dissolve it in pure laboratory water to 1000ml and mix well.

5. Glycine solution

Weigh 10g glycine and dissolve it in pure water in the laboratory, dilute it to 100ml with pure water and mix evenly.

6. Potassium iodate standard stock solution

Weigh 0.1486g of high-grade pure potassium iodate baked at 120 ℃ – 140 ℃ for 2h, dissolve it with pure laboratory water to 100ml, and mix well.

7. Potassium iodate standard solution

Accurately take out 1.00mL of potassium iodate standard stock solution into a 100ml Brown volumetric flask, add about 20ml of water, add about 1g of potassium iodide and a drop of 20% sulfuric acid solution, shake well, stand for 2min, then fix the volume with pure water in the laboratory and mix well.

Prepared temporarily. Potassium iodate solution with a concentration of 14.86mg/l is equivalent to ozone with a concentration of 10.0mg/l.

Steps

1. Use a pipette to move the measured water sample to the 10ml scale mark of the colourimetric bottle, wipe the outer wall of the colourimetric bottle, and put the colourimetric bottle into the colourimetric tank to lock it.

2. Adjust zero and conduct blank measurement. The calibration is completed when the instrument displays “0.00”.

3. Add a potassium iodide reagent into the colourimetric bottle, move the measured water sample to the scale mark of the colourimetric bottle with a pipette, add a DPD reagent, tighten the locator of the colorimetric bottle, shake to dissolve the reagent, wipe the outer wall of the colorimetric bottle, lock it in the colorimetric tank and place it for 3min.

4. Conduct concentration measurement. According to the built-in standard curve of the instrument, the value displayed on the instrument is the mass concentration of ozone in the water sample (unit: mg/l).

Detection tube Method

The ozone oxidation discolorable reagent is impregnated on the carrier as a reagent and encapsulated in a glass tube with a standard inner diameter as a detection tube. When in use, cut off both ends of the detection tube and connect the air extractor to the outlet end to absorb quantitative ozone gas.

The ozone concentration is directly proportional to the discolouration length of the reagent column in the detection tube, and read the concentration value through the scale value.

Ultraviolet spectrophotometry

The principle

The light wave absorption by a gas or liquid follows the beer. Lambert theorem: = ioexp (- KLC)

IO is the radiation intensity of the light beam;

I is the intensity of the light beam after penetrating the sample (gas or liquid);

L is the length of the optical path through the sample;

C is the amount and concentration of the absorbing substance in the sample;

K is the absorption coefficient of the absorbing substance to the wavelength of light.

Ozone absorbs Hartley band ultraviolet light in the short wave ultraviolet region (200 ~ 300 nm). When the wavelength is 253.7 nm, it is the maximum absorption value, and the ozone absorption coefficient is 302.4 (L • cm-1 • mol-1). Therefore, we can get the ozone concentration from the formula when I, lo, K and L are known.

Measuring method

We can generally use a UV spectrophotometer with a double light column and additional compensation for light and temperature for ozone analysis.

The display of the reference light column of the photometer is the value when there is no ozone in the sample. We can get the ozone concentration in water by comparing the absorbance of the measured sample with the absorbance of the reference light column.

Pros:

• can continuous online detection
• high detection accuracy
• good stability due to rarely disturbed by other oxidants

Cons:

High cost

Spectrophotometry is monochromatic light with high purity, which has a high accuracy of the analysis results, and the relative error is 2%~ 5％.

Although it is hardly disturbed by other oxidants in water, its sensitivity is lower than that of iodometry. The main reason is that the dissolved organic and inorganic substances in the sample will absorb ultraviolet light of the same or similar wavelength as ozone;

In addition, It will also be affected by the suspended solids in water, which is unsuitable for when the turbidity is above 30 NTU, so this method is commonly used for clarified water samples.

When UV spectrophotometry is used to measure the ozone concentration in water, its measurement range is 1.2 ~ 12 mg / L, and sensitivity is higher than the traditional sodium thiosulfate standard solution titration method.

In order to improve the accuracy, we can replace iodine with a boric acid solution containing potassium iodide and use the replaced iodine as the equivalent ozone standard when using ultraviolet spectrophotometry.

It avoids the positive interference of possible oxidizing substances in the sample and the possible negative interference of reducing substances.