GB/T 6910-2026Analysis methods for boiler water and cooling water - Determination of calcium - Complexometric titration (English PDF)

4.1 Principle

Adjust the pH of the water sample to not less than 12, using calcein-phenolphthalein as an indicator, and EDTA disodium salt standard solution.

The endpoint is reached by titrating with the liquid until the yellow-green fluorescence disappears and the solution turns red; or by using calcium carboxylic acid as an indicator, titrating with EDTA standard solution until red.

The color change to bright blue is the endpoint. The calcium ion content is calculated based on the volume of EDTA standard solution consumed. Under these test conditions, the magnesium ion content...

The molecules form magnesium hydroxide precipitate, which does not interfere with the determination.

4.2 Reagents and Materials

Unless otherwise specified, this document shall use reagents of analytical grade or higher.

4.2.1 Water

Grade II water as specified in GB/T 6682.

4.2.2 Microporous filter membrane

aqueous system, pore size 0.45µm.

4.2.3 Quantitative filter paper

medium speed.

4.2.4 Potassium hydroxide solution (200g/L)

Weigh 20g of potassium hydroxide, dissolve it in water and dilute it to 100mL.

4.2.5 Hydrochloric acid solution

Add 100 mL of hydrochloric acid to 100 mL of water and mix well.

4.2.6 Triethanolamine solution

Add 50 mL of triethanolamine to 100 mL of water and mix well.

4.2.7 L-cysteine hydrochloride solution (10g/L). Weigh 10g of L-cysteine hydrochloride, dissolve it in water and dilute it to 1000mL.

4.2.8

EDTA standard solution [c(EDTA)=0.01mol/L]. Prepared and standardized according to GB/T 601.

4.2.9 Calcium carboxylic acid indicator

Weigh 10g of sodium chloride dried at 105°C±2°C for 2h, add 0.1g of calcium carboxylic acid, mix well and grind finely before storage.

In a desiccator.

Note. Calcium carboxylic acid indicator is also known as calcium indicator or calcium red indicator.

4.2.10 Calcein-Phenolphthalein Indicator

Weigh 0.2g calcein and 0.07g phenolphthalein into a mortar, add 20g potassium chloride, and mix well.

After grinding, store in a ground glass bottle.

4.3 Test Procedure

4.3.1

Collect water samples according to GB/T 6903.If the water sample contains visible suspended solids, filter it with quantitative filter paper or microporous membrane.

4.3.2

Take 50.0 mL of water sample into a 250 mL Erlenmeyer flask, add 50 mL of water and 3 drops of hydrochloric acid solution, mix well, and heat to a gentle boil.

Continue for 0.5 min, then cool to room temperature to eliminate interference from bicarbonate and polyacrylic acid.

4.3.3

Add 5 mL of potassium hydroxide solution to the water sample.

4.3.4

Add approximately 0.04 g of calcein-phenolphthalein indicator or calcium carboxylic acid indicator to the water sample.

4.3.5

While continuously shaking, immediately titrate with EDTA standard solution. The titration speed should be slightly faster at the beginning and slightly slower near the endpoint.

When using calcein-phenolphthalein indicator, titrate against a black background until the yellow-green fluorescence disappears and the solution turns red; the endpoint is reached when using calcium carboxylic acid.

When using an indicator, titrate until the solution changes from purple-red to bright blue as the endpoint. The entire titration process should be completed within 5 minutes, and the amount consumed should be recorded.

The volume of EDTA standard solution is V1, accurate to 0.01 mL. A blank test is also performed simultaneously, recording the volume of EDTA standard solution consumed.

V0, accurate to 0.01 mL.

4.3.6

If iron, aluminum, manganese, copper, or zinc ions are present in the water sample, 2-3 mL of triethanolamine solution can be added before step 4.3.3.

Combined masking with 2 mL to 3 mL L-cysteine hydrochloride solution to eliminate interference.

Note. If the water sample is not red after adding the indicator, or if the indicator does not turn bright blue during the titration process, it is suspected that there may be interference.

5.1 Method Overview

The pH of the water sample was adjusted to be no less than 12, using calcium carboxylic acid as an indicator, and analyzed using an automatic potentiometric titrator and photometric electrode, with EDTA standard.

The solution was titrated to the point of abrupt change in the absorbance of the water sample, and the calcium ion content was calculated based on the volume of EDTA standard solution consumed.

Under these test conditions, magnesium ions form magnesium hydroxide precipitate, which does not interfere with the determination.

5.2 Reagents and Materials

Same as 4.2.1 to 4.2.9.

5.3 Instruments and Equipment

5.3.1 Automatic Potentiometric Titrator

It has the function of photometric titration, with a maximum permissible error of ±1.5% for the instrument reading and a repeatability of no more than 0.2%.

5.3.2 Photometric Electrode

The wavelength value can be set in the range of 600nm to 660nm, with a wavelength error of no more than ±10nm and a full width at half maximum of 30nm.

5.4 Test Procedure

5.4.1

After the water sample is processed according to 4.3.1 and 4.3.2, it is transferred entirely to a titration cup, and then the procedure is performed according to 4.3.3.

5.4.2

Add approximately 0.04 g of calcium carboxylic acid indicator to the water sample and mix thoroughly.

5.4.3

Adjust the wavelength of the photometric electrode to the optimal value within the range of 600nm~660nm, place the titration cup containing the water sample on the titration stage, and turn on the titration switch.

Start the instrument and stirrer, titrate with EDTA standard solution, and stop titrating when a complete abrupt change curve appears. Record the stoichiometric point.

The volume of EDTA standard solution consumed is V1, accurate to 0.01 mL. A blank test is also performed simultaneously, and the volume of EDTA standard solution consumed is recorded.

The volume is V0, accurate to 0.01 mL.

6.1 Method Overview

Adjust the pH of the water sample to be no less than 12, and titrate calcium ions with EDTA standard solution. The potential of the calcium ion selective electrode changes with the concentration of calcium ions.

The degree of change is continuous and decreases, and a sudden jump occurs in the electrode potential near the stoichiometric point. This is based on the cumulative consumption at the time the stoichiometric point is reached.

The calcium ion content was calculated from the volume of the EDTA standard solution. Under these test conditions, magnesium ions formed magnesium hydroxide precipitate, which did not interfere with the calcium ion content.

Measurement.

6.2 Reagents or Materials

Same as 4.2.1 to 4.2.8.

6.3 Instruments and Equipment

6.3.1

Automatic potentiometric titrator, with a maximum permissible error of ±1.5% for instrument readings and a repeatability of no more than 0.2%.

6.3.2

Calcium ion electrode and reference electrode.

6.4 Test Procedure

6.4.1

After processing according to 4.3.1 and 4.3.2, transfer the entire water sample to the titration cup, and then proceed according to 4.3.3.

6.4.2

Place the titration cup containing the water sample on the titration stage, insert the calcium ion electrode into the water sample, turn on the instrument and stir, and use EDTA standard.

Solution titration. Stop titrating when a complete abrupt change curve appears, and record the volume of EDTA standard solution consumed at the stoichiometric point.

Record the volume of EDTA standard solution consumed as V1, accurate to 0.01 mL. Simultaneously perform a blank test and record the volume of EDTA standard solution consumed as V0, accurate to 0.01 mL.