Air Pollution Continuous Monitoring Technology in Japan
 Examples of simple measurement

5. DThe Methods of Measuring SO2 by Lead Peroxide Candles

A method for measuring sulfation involves use of lead peroxide "candle." A paste made of lead peroxide in a gum tragacanth solution is applied to a cotton gauze wrapped around a glass or porcelain form ; the product is the lead peroxide candle. The candle is exposed to the ambient air in a louvered shelter for an extended period of time, usually a month, and the lead sulfate formed in the candle is then determined. It is obvious that the method cannot give any information on short-term variations in sulfur oxides pollution levels and that it is only an empirical estimate of the average concentration. The method measures sulfur-containing compounds which can form sulfate.
The primary application of the lead peroxide candle is for mapping sulfur pollution in a given area as related to sources and meteorology. It is convenient since a simple shelter requiring no electrical power is used. A major problem with the lead peroxide candle is that results are influenced by wind movement and humidity. Another major drawback to the lead peroxide candle is that the laboratory preparation and analysis is quite time consuming. Also, the lead peroxide candle provides intelligence on the oxidizable sulfur compounds in the atmosphere which seldom can be directly related to sulfur dioxide. It has been observed that in many cases monthly averages of sulfur dioxide rise and fall somewhat parallel to monthly sulfation values.

5.1. Reagent


(1) Lead dioxides(PbO2): Use a standard product of lead dioxides. (Standard product should be used since the activity varies strikingly with products.)
(2) Tragacanth gum solution: Dissolve and stir 2 g of tragacanth gum powder in 10 ml of ethanol and add 190 ml of distilled water.

5.2 Instruments


(1) Biscuit cylinder: 10 cm in circumference and 15 cm long.
(2) Instrument shelter or cylinder cover: To see Figure 6.5.1 and Figure 6.5.2
(3) Lead dioxide adhesive cotton:
(4) Adjustment of lead dioxide cylinder: Paste 5 ml of a tragacanth gum solution on cotton cloth and let the cotton cloth dry by wrapping it around the biscuit cylinder. Use a flat brush to uniformly apply the paste made with the mixture of 4 g of lead dioxide and 2.5 ml of a tragacanth gum solution to the dried cotton cloth with a tragacanth gum solution. Make sure that the size of pasted area is 100 cm2. After the paste was dried enough, the cylinder shall be put in the dessicator.

5.3. Placement of the PbO2 cylinder


The PbO2 shall be left cylinder inside the instrument shelter or be used the cylinder cover to protect it from rain. Make sure the location of the PbO2 cylinder is not close to a chimney or any other local pollution source. A location where there is no protruding obstacle above an angle of approximately 45 degrees elevation from the horizon, should be selected.


A: Instrument shelter
B: Zinc louver door
C: Porcelain cylinder
D: Porcelain cylinder wrapped with cloth
E: Cylinder pasted with PbO2 paste
F: Wooden column
G: Nail
Figure 6.5.1 Lead dioxide method apparatus (Shelter type)


Figure 6.5.2 Lead dioxide method apparatus (Cylinder cover type.)

5.4. Quantification


(1) Gravimetric analysis method

a. Adjustment of the testing solution
The lead dioxide cylinder on which lead dioxide was exposed to the air for a fixed period of time shall be removed, from the instrument shelter or the cylinder cover, and the pasted area of lead dioxide on the cotton cloth should be accurately measure. To measure the area, you can measure the height of the pasted surface on the cylinder and the diameter of the cylinder, or peel the lead dioxide portion of the cotton cloth from the cylinder and measure the area. Put the lead dioxide portion of the cotton cloth into a beaker. Add 100 ml of sodium carbonate solution to the beaker. Stir the solution inside the beaker occasionally over a boiling water bath, heating it for one hour. Then, after adjusting the total volume to 100 ml with added water, filter it through and wash it thoroughly with warm water. Combine the filtered liquid with the washing water to create a testing solution.

b. Quantification operation
Add a drop of methyl orange to the testing solution and neutralize it with hydrochloric acid. Add to it 0.5 ml of concentrated HCl acid. Next, heat it up over a boiling water bath. Completely expel carbonic acid gas from the solution and adjust the liquid volume to approximately 100 ml (hydrochloric acid acidity of 0.02 to 0.1 N). Stir the solution well while dripping a 10 % barium chloride solution heated to approximately 80deg.C. After leaving this solution for a few hours, gradually filter the deposit of barium sulfate through a glass filter. Wash the deposit with warm water a few times and confirm that there are chloride ions in the washing solution. After confirmation of no chlorine ion, dry the deposit with a heat of approximately 100deg.C and measure the weight.

c. Calculation
The extent of contamination by sulfur oxide is expressed with a unit of mg of SO3/day/100cm2 PbO2. The extent of the contamination is calculated using the following formula:

W × 0.343 × 100/a × 1/n
where w, n and a represent the weight of barium sulfate (mg), the number of days for which the PbO2 cylinder is left alone and the pasted area of the PbO2 cylinder (cm2), respectively.

(2) Colorimetric analysis

a. Reagent

1) 2, 5 - dinitrophenol indicator: Dissolve 0.1 g of 2, 5 - dinitrophenol (superior quality) in 50 ml of ethanol and add water until the solution reaches a volume of 100 ml.
2) Buffer solution: Mix 36 ml of 0.2 M sodium acetate with 164 ml of 0.2 M acetic acid (pH 4).
3) Barium chloranil acid: Use one of superior quality.
4) Sulfuric acid standard solution: Ammonium sulfate (superior quality), dried with a heat of 100deg.C, is measured to a precise weight of 0.825 g and dissolved in water to a total volume of 1 liter. 1 ml of standard solution = 0.5 g SO3
b. Adjustment of the testing solution
The cotton cloth, on which PbO2 has been exposed to the air for a fixed period of time shall be peeled, from the cylinder and put it in a beaker. Add 100 ml of 1 % solution of sodium carbonate into the beaker. Stir the contents of the beaker over a bath of water and heat it for an hour. Cool it and put the contents into a 250 ml stoppered cylinder, and add water for a total volume of 250 ml. The testing solution is obtained after filtering the solution in the cylinder.

c. Quantification operation
Put 25 ml of the testing solution into a 50 ml volumetric flask. Using 2, 5 - dinitrophenol as an indicator, neutralize it with 1 N hydrochloric acid and expel carbonic acid gas from the solution. Repeat the neutralizing process and add water for a total volume of precisely 50 ml.
Put 5 ml of this solution and 5 ml of sulfuric acid ion standard solution in a stoppered test tube. Add 5 ml of a buffer solution, 10 ml of ethanol and approximately 0.1 g of barium chloranil acid. Shake the mixture for 10 minutes and filter it. The filtered solution is measured in terms of A and A0 using an approximate wave length of 530 m micro. Repeat the same operation on the lead dioxide cylinder stored in the desiccator to the comparison liquid.

d. Calculation
The extent of contamination by PbO2 is expressed with a unit of mg of SO3/day/100 cm2 PbO2. The extent of the contamination is calculated using the following formula.

0.5 × (A/A0) × (50/25) × (250/n) = (A/A0) × (250/n)
where n is the number of days the PbO2 cylinder is left alone.
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