Detection method of calcium and magnesium ions in reclaimed water

   The detection principle of calcium and magnesium ions in regenerated water is to spray the water sample into the flame through atomization, and the calcium and magnesium ions are pyrolyzed into ground state atoms. The absorbance of calcium and magnesium atoms was measured by air-B-flame method. Adding strontium chloride or lanthanum chloride can inhibit the interference of various coexisting elements in water and water treatment chemicals. When measuring calcium and magnesium with nitrous oxide-ephrine flame, adding raffinate chloride can suppress the ionization interference of calcium and magnesium ions.

Detection reagent

1 hydrochloric acid
2 Nitric acid
3 Hydrochloric acid solution: 1+1
4 Hydrochloric acid solution: 1+99
5 Lanthanum chloride solution: containing lanthanum 20g/L.
  Weigh 24.0g of lanthanum oxide, put it in a 200mL beaker, add 20mL of water, add 50mL of hydrochloric acid to dissolve, transfer to a 1000mL volumetric flask, and dilute to the mark with water.
6 Strontium chloride solution: 50g/L of strontium.
  Weigh 152.0g of strontium chloride, put it in a 200mL beaker, add 20mL of water, add 20mL of hydrochloric acid to dissolve, transfer to a 1000mL volumetric flask, and dilute to the mark with water.
7 Cesium chloride solution: containing 20g/L of passivation.
  Weigh 25.0g of absolute chloride, put it in a 100mL beaker, add 50mL of hydrochloric acid solution (1+99) to dissolve, transfer it to a 1000mL volumetric flask, and dilute to the mark with hydrochloric acid solution (1+99).
8 Calcium standard stock solution: 1mg/mL.
  Weigh 2.4970g of high-purity calcium carbonate pre-baked at 105℃-110℃ to constant weight, accurate to 0.2mg. Put it in a 100mL beaker, add 50mL of water and 10mL of hydrochloric acid solution (1+1), dissolve it and transfer it to a 1000mL volumetric flask, dilute to the mark with water, and shake well.
9 Calcium standard solution: 0.05mg/mL.
  Pipette 5.0mL of calcium standard stock solution, put it in a 100mL volumetric flask, dilute to the mark with water, and shake well.
10 Magnesium Standard Stock Solution: 0.1mg/mL
11 Magnesium standard solution: 5mg/L.
  Pipette 5.0mL of magnesium standard stock solution, put it in a 100mL volumetric flask, dilute to the mark with water, and shake well. The solution is used now.

Instruments required for testing

1 Atomic absorption spectrometer: equipped with calcium hollow cathode lamp, magnesium hollow cathode lamp, air-acetylene premix burner and nitrous oxide premix burner, printer or recorder.
2 Acetylene cylinder: the pressure shall not be lower than 500 kPa.
3 Air compressor.

Instrument debugging

  Adjust the instrument to the best state according to the instrument manual, and adjust the calcium measurement wavelength to 422.7 nm and the magnesium measurement wavelength to 285.2 nm. The instrument is turned on and ignited, and it is stable for 5min-10min before measuring.

water sample collection

  The water sample collection container should be a polyethylene bottle or a polypropylene bottle.
  After the water sample is collected, it should be acidified by adding hydrochloric acid immediately to prevent the precipitation of calcium carbonate. Usually, adding 8 mL of hydrochloric acid per liter of water sample is sufficient. When there are many suspended solids in the water sample, it can be filtered with medium-speed quantitative filter paper, and the filtrate is stored in a polyethylene bottle (the sample can be stored for 2 weeks).

Detection process

Detection of calcium content
  Pipette an appropriate amount of the sample solution or its diluent and place it in four 150mL conical flasks, so that the calcium ion concentration contained in it meets the range of 0.1mg/L-3.0mg/L. 5.0 mL of nitric acid was added to each, and water was added until the solution volume was about 30 mL. Put it on an adjustable electric stove and heat it slowly, keep it slightly boiling until the remaining volume is about 5mL (the time is about 20min), and remove it. After cooling, transfer to four 50mL volumetric flasks, add 0.0mL, 1.00mL, 2.00mL, 3.00mL calcium standard solution, and then add 5.0mL strontium chloride solution or 2.0mL lanthanum chloride solution (if selected Add 5.0 mL of cesium chloride solution), dilute to the mark with hydrochloric acid solution (1+99), and shake well. Take another 50L volumetric flask, add an equal amount of strontium chloride solution or lanthanum chloride solution, dilute to the mark with hydrochloric acid solution (1+99), shake well, and use this solution as a blank.

  At a wavelength of 422.7 nm, the absorbance of each solution was measured with a blank set to zero. Take the content of calcium standard solution (mg/L) in the solution as the abscissa, and the corresponding absorbance as the ordinate to draw a curve, extend the curve inversely, and the intersection with the abscissa axis is the calcium content in the solution.

Determination of magnesium content
  Pipette the suitable sample solution or its diluent and place it in four 150mL conical flasks respectively, so that the magnesium ion concentration contained in it meets the range of 0.05mg/L-0.50mg/L. Add 5.0 mL of nitric acid to each, and make up with water until the solution volume is about 30 mL. Put it on an adjustable electric stove and heat it slowly, keep it slightly boiling until the remaining volume is about 5mL (the time is about 20min), and remove it. After cooling, transfer to four 50mL volumetric flasks, add 0.0mL, 1,00mL, 2.00mL, 3.00mL magnesium standard solution, and then add 5.0mL strontium chloride solution or 2.0L lanthanum chloride solution (if a Diamine oxide-acetylene flame, then add 5.0 mL of cesium chloride solution), dilute to the mark with hydrochloric acid solution (1+99), and shake well. Take another 50mL volumetric flask, add the same amount of strontium chloride solution or lanthanum chloride solution, dilute to the mark with hydrochloric acid solution (1+99), shake well, and use this solution as a blank.

  At a wavelength of 285.2 nm, the absorbance of each solution was measured by zeroing with a blank. Take the content of the magnesium standard solution (mg/L) in the solution as the abscissa and the corresponding absorbance as the ordinate to draw a curve, extend the curve in the reverse direction, and the intersection with the abscissa axis is the magnesium content in the solution.

  The content of calcium and magnesium ions in the water sample can be calculated according to the corresponding formula.

The above content comes from 《HG/T 4325-2012 Determination of Calcium and Magnesium Content in Reclaimed Water by Atomic Absorption Spectrometry》