Detailed Description of the Quality Inspection of Pure Water Used in the Experiment

Based on years of experience and the water requirements and pure water production methods of internal laboratories within the industry, Sichuan Youpu Ultrapure Technology Co., Ltd. has summarized the following quality inspection methods for laboratory water:

1. pH Value Test: Take 10ml of water sample and add 2 drops of methyl red pH indicator (color change range: pH 4.2-6.2). The water is considered合格 (qualified) if no red color appears. Separately, take 10ml of water and add 5 drops of bromothymol blue (blue range: pH 6.0-7.6). The water is considered (qualified) if no blue color appears. Alternatively, a precision pH test or a pH meter can be used to determine the pH value.

2. Conductivity Measurement: Conductivity meters used for primary and secondary water testing are equipped with an "online" conductivity cell with an electrode constant of 0.01-0.1 cm⁻¹ and automatic temperature compensation. If the conductivity meter does not have temperature compensation, an "online" exchanger can be installed to control the water temperature at (25±1) degrees Celsius during measurement. Alternatively, record the water temperature and perform the conversion. When the measured water temperature for each grade is not 25 degrees Celsius, its conductivity can be calculated using the following formula: K25 = t(KT - Kρ.t) + 0.00548 Where: K25 — conductivity of the water sample at 25℃, m/m; Kt — conductivity of the water sample at t℃, m/m; Kρ.t — conductivity of theoretically pure water at t℃, m/m; 0.00548 — conductivity of theoretically pure water at 25℃, m/m.

For measuring the conductivity of Grade I and II water, the conductivity cell is installed at the outlet of the water treatment device. The water flow rate is adjusted, and air bubbles are removed from the tube and the conductivity cell before measurement. For measuring the conductivity of Grade III water, a 400ml water sample is placed in a conical flask and the conductivity cell is inserted before measurement.

3. Limit Test for Oxidizable Substances: Measure 1000ml of Grade II water into a beaker, add 5ml of 20% sulfuric acid solution, and mix well. Measure 200ml of Grade II water into a beaker, add 15ml of 20% sulfuric acid solution, and mix well. Add 1ml of 0.01mol·L⁻¹ (1/5KMnO₁) standard solution to the above acidified test solution, mix well, cover with a watch glass, heat to boiling and maintain for five minutes. The pink color of the solution should not completely disappear.

4. Absorbance Determination: Pour water samples into quartz absorption cells with a thickness of 1cm and 2cm, respectively. Using a UV-Vis spectrophotometer, measure the absorbance of the water sample in the 2cm absorption cell at a wavelength of 254nm, using the water sample in the 1cm absorption cell as a reference.

If the instrument sensitivity is insufficient, the thickness of the absorption cell can be appropriately increased.

5. Determination of Evaporation Residue: Measure 1000ml of Grade II water (500ml of Grade III water). Add the water sample in several portions to a 500ml distillation flask in a rotary evaporator, and evaporate under reduced pressure in a water bath (avoid drying). Stop heating when the last 50ml of water sample has evaporated. Transfer the pre-concentrated water sample to a glass dish pre-weighed at (105±2)℃, and rinse the distillation flask 2-3 times with 5-10ml of water sample. Combine the washings with the pre-concentrated water sample, evaporate to dryness in a water bath, and dry to constant weight in an electric oven at (105±2)℃. The residue mass must not exceed 1.0mg.

6. Limit test for soluble silicon: Measure 520ml of Grade I water (270ml of Grade II water) and pour it into a platinum dish. Under dust-proof conditions, evaporate to near boiling until about 20ml, then stop heating. Cool to room temperature, add 1ml of 50g/L ammonium molybdate solution, and shake well. After standing for five minutes, add 1ml of 50g/L oxalic acid solution and shake well. After standing for one minute, add 1 mL of 2 g/L p-aminophenol sulfate solution and shake well. Transfer to a 25 mL colorimetric tube, dilute to the mark, shake well, and incubate in a 60°C water bath for ten minutes. Visually compare the color; the blue of the test solution should not be deeper than the standard.

The standard is prepared by taking 0.50 mL of silica standard solution (0.01 mg/mL), adding it to 20 mL of water sample, and then treating it in the same way as the sample solution starting with the addition of 1 mL of ammonium molybdate solution.

50 g/L ammonium molybdate solution: Weigh 5.0 g of ammonium molybdate [(NH1)6MoO24·4H2O], dissolve it in water, add 20.0 mL of 20% sulfuric acid solution, dilute to 100 mL, shake well, and store in a polyethylene bottle. Discard any solution containing precipitate.

2 g/L para-aminophenol sulfate (Mitor) solution: Weigh 0.2 g of para-aminophenol sulfate, dissolve in water, add 20 g of sodium metabisulfite, dissolve and dilute to 100 ml. Shake well and store in a polyethylene bottle. Store protected from light. Shelf life: two weeks.

50 g/L oxalic acid solution: Weigh 5.0 g of oxalic acid, dissolve in water and dilute to 100 ml. Store in a polyethylene bottle.