6.1 Apparatus

6.1.1 25-mL Class A burette with Teflon stopcock.

6.1.2 Glass volumetric pipettes.

6.1.3 Micropipettes with tips.

6.1.4 125-mL Erlenmeyer flask.

6.1.5 Polargraphic analyzer - model 374 or 384 manufactured by Princeton Applied Research (PAR) or equivalent. 

6.1.6 Static mercury drop electrode - PAR 303 or- equivalent.

6.1.7 15-mL glass or polyethylene polargraphic cells.

6.1.8 Nitrogen purification apparatus.

6.2 Reagents - All chemicals should be ACS reagent grade or equivalent, and the dilution water must be deionized. 

6.2.1 0.0575 M Titanium Oxysulfate: Add 4.6 g TiOSO₄, 20 g (NH₄)₂SO₄ and 100 mL concentrated H₂SO₄ to a 500 mL beaker. See precautions in 6.3.1. Heat gradually for several minutes until the chemicals are dissolved. Cool the mixture to room temperature and pour carefully into about 350 mL deionized water in a 500 mL volumetric flask. Filter the solution through an HA filter to remove any particulates, and dilute to 500 mL. The solution should be stable for 6 months.

6.2.2 0.00575 N Titanium 0xysulfate: Take a 1-10 dilution of the 0.0575 M TiOSO₄ stock solution by adding 10 mL of the stock solution (6.2.1) to a 100 mL volumetric flask and diluting to volume with deionized water. This solution should be made fresh monthly.

6.2.3 0.00575 M Titanium 0xysulfate: Take a 1:50 dilution of the stock TiOSO₄ solution (6.2.1) by adding 2 mL of the stock to a 100 mL volumetric flask and diluting to volume with deionized water. This solution should be made fresh monthly.

6.2.4 Supporting electrolyte: Add 53 g (NH₄)₂SO₄, 38 g EDTA, and 75 mL of 28.8% (NH₄)OH to about 500 mL deionized water in a 1000 mL volumetric flask. Let cool, then dilute to 1000 mL with D.I. water.

6.2.5 4 N Sulfuric Acid: Slowly add 112 mL H₂SO₄ to about 500 mL deionized water in a 1 L volumetric flask, stir and let cool. See precautions in 6.3.1. Dilute to 1 L with deionized water.

6.2.6 Starch indicator solution: To 5 g starch add a little cold water and grind in a mortar to a thin paste. Scrape into 1 L of boiling distilled water, stir, and let the covered solution settle overnight. Decant the clear supernate into a brown bottle and preserve with 4 g zinc chloride.

6.2.7 0.1 M Sodium Thiosulfate: Add 24.82 g Na₂S₂O₃·H₂0 to about 500 mL deioized water in a 1000 mL volumetric flask and let dissolve. Dilute to volume deionized water. Add two or three mL chloroform to minimize bacterial decomposition.

6.2.8 1 M Ammonium Molybdate: Add 20.6 g (NH₄)₆Mo₇O₂₄ to about 50 mL deionized water in a 100 mL volumetric flask and dissolve. Dilute to volume with deionized water. Store in glass. 

6.2.9 1 M Potassium Iodide: Add 33.2 g of KI crystals to 100 mL deionized water, dissolve, and dilute to 1 L. Store in a brown bottle. 

6.3 Precautions

6.3.1 When handling mercury, hydrogen peroxide, or sulfuric acid, gloves and safety glasses must be worn. Extreme care must be observed to avoid splashing or spilling on skin. Add sulfuric acid to water very carefully and never add water to sulfuric acid. Sulfuric acid gives off a great deal of heat when added to water and can splatter or boil violently. To prevent tile heat from shattering the volumetric flask, place the flask in a cool water bath and add the sulfuric acid a little at a time. 

6.3.2 Refer to the polarographic instruction manual for instrumental safety precautions (7.2, section I-1, and 7.3 section I-1). 

6.4 Sample Preparation 

6.4.1 Open the collection vial and measure the sample volume using a graduated cylinder. Take an aliquot of sample and transfer to a 15 mL polarographic cell. The sample aliquot size will depend on the intensity of the color of the collecting solution. If the sample is very yellow, use a 1 mL aliquot of sample and add 4.0 mL of the 0.00115 N TiOSO₄ (6.2.4) If the sample is colorless, use a 5 mL aliquot. 

6.4.2 Add 5 mL of the supporting electrolyte (6.2.4) to give a total volume of 10 mL and analyze by differential pulse polarography. 

6.5 Standard Preparation

6.5.1 A hydrogen peroxide stock solution is prepared by placing 2 mL of 30% H₂O₂ in a 500 mL volumetric flask and diluting to volume with deionized water. This is approximately 1200 µg/mL H₂O₂. 

6.5.2 A hydrogen peroxide standard solution is prepared by placing 1 mL of the H₂O₂ stock (6.5) in a 100 mL volumetric flask and diluting to volume with deionized water. This is approximately 12 µg/mL H₂O₂.

6.5.3 Prepare a series of standards in the analytical range of 6 to 48 µg by adding the following serial dilutions. Add to the polarographic cell the appropriate aliquot of the H₂O₂ standard solution (6.5.2) and aliquots of deionized water using the calibrated nicropipettes. Add 1 mL of the 0.00575 M TiOSO₄ (6.2.2) and 5 mL of the supporting electrolyte (6.2.4) to make a total volume of 10 mL.

Stock	Aliquot	Aliquot	Final
Solution	H2O2	D.I. H2O	Standard
(ppm)	(mL)	(mL)	(µg)
12	4..0	0.0	48
12	3.0	1.0	36
12	2.0	2.0	24
12	1.0	3.0	12
12	0.5	3.5	6

6.6 Analysis 

6.6.1 Turn on the polarograph, Model 384 and 303 and allow to warm up for at least 30 minutes. 

6.6.2 Analyze the standards and samples by differential pulse polarography using the following instrumental conditions:

Initial Potential	-0.820 V
Final Potential	-1.020 V
Purge Time	300 sec
Scan Increment	2  mV
Replications	1
Drop Time	0.5 seconds
Peak Location	Yes
Peak Potential	-0.940 V
Date	as needed

This method is stored as Method No. 2 in the PAR, Model 384. 

6.6.3 Prepare the samples and working standard solutions as described in sections 6.4 and 6.5. 

6.6.4 Purge each standard and sample for 5 minutes pre-purified nitrogen. 

6.6.5 Analyze the reagent blank, standards, and the samples. A standard should be re-analyzed after every 4 or 5 samples. 

6.6.6 Record the peak current and potential for each  standard and sample in the laboratory notebook. The differential pulse polarogram of hydrogen peroxide gives a peak at approximately -0.940 V. 

6.6.7 If any of the samples have enough hydrogen peroxide to be over the PEL, the 1200 µg/mL stock (6.5.1) must be standardized against the 0.1 H sodium thiosulfate (6.2.7) before a standard curve is prepared. See 6.6.9 through 6.6.12. 

6.6.8 Use any available least square regression program to plot a calibration curve of peak current vs. concentration (ppm, ppb, or total µg) of the standards. 

6.6.9 To standardize the H₂O₂ stock solution, transfer the following solutions to a 125 mL Erlenmeyer flask. 

1. 10 mL stock 1200 µg/mL H₂O₂ (6.5.1)

2. 10 mL 2N H₂SO₄ (6.2.5) 

3. 6 mL 1N KI (6.2.9)

4. 3 drops 1N (NH₄)₆Mo₇O₂₄ (6.2.8)

5. 20 mL D.I. water

6.6.10 The Solution is titrated to a very faint yellow with 0.1 N Na₂S₂O₃ (6.2.7) and then 1 mL starch solution (6.2.6) is added to produce a blue color. The titration is continued until the solution becomes colorless.

6.6.11 The total amount of Na₂S₂O₃ required to reach the endpoint is determined (about 10 mL) and recorded.

6.6.12 Calculate the concentration of the 1200 µg/mL H₂O₂
stock, the 12 µg/mL standard, and the actual concentrations of the standards to be used in the standard curve.

6.7 Calculations 

6.7.1 Subtract the initial volume of sodium thiosulfate from the volume at the endpoint. This is the total volume of Na₂S₂O₃ used. 

Since: 

2 S₂O₃= + H₂0₂ + 2 H⁺ -----> S₄O₆ = + 2 H₂0
 
Then: 

M Na₂S₂O₃  × V Na₂S₂O₃ = 2 × M H₂O₂ × V H₂O₂

or:

0.1 × mL Na₂S₂O₃ used = M H₂O₂ × 2 × 10 mL, and: 

mmoles H₂0₂ = mmoles Na₂S₂O₃ × 1/2 then:

mg H₂O₂ = mmoles Na₂S₂O₃ 1/2 × 34     
            = mmoles Na₂S₂O₃ × 17 

6.7.2 The weight of H₂O₂ in a sample aliquot is determined from the calibration curve. The total weight of H₂O₂ is calculated front the equation: 

µg H₂O₂ = (aliq. µg - blank aliq.)(sample vol. mL)
                    (sample aliquot vol, mL) 

6.7.3 The concentration of H₂O₂ is calculated in µg/L, converted to mg/m³, and then to ppm.

µg H₂0₂/liters sampled = mg/m³ and;

ppm H₂O₂  = mg/m³ × 24.45/34 = mg/m³ × 0.719 ppm