Determination of sodium, potassium, magnesium, and calcium in precipitation

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Determination of sodium and potassium by AAS and AES

4.6 Determination of sodium, potassium, magnesium, and calcium in precipitation
Sodium, potassium, magnesium, and calcium in precipitation can be analysed by atomic spectroscopic methods or with ion chromatography. Both flame (AAS and AES) and plasma (ICP-AES and ICP-MS) based methods can be used, but only the flame methods are described in this manual. For these ions ion chromatography have no special advantage concerning sensitivity, precision and accuracy over the spectroscopic methods, but analysis of all the ions in one run is not possible with flame AAS or AES.
4.6.1 Determination by flame atomic spectroscopy (AAS or AES)
4.6.1.1 Field of application
The method can normally be used for the determination of sodium, magnesium, potassium and calcium in precipitation within the range 0.01-2 mg/l, but this will depend to a certain degree on the commercial instruments used.
4.6.1.2 Principle
The ions in the sample solution are transformed to neutral atoms in an air/acetylene flame. Light from a hollow cathode or an electrodeless discharge (EDL)-lamp is passed through the flame. The light absorption of the atoms in the flame, which is proportional to the ion concentration in the sample, is measured by a detector following a monochromator set at the appropriate wavelength. The described principle holds for the measurement performed in the AAS-mode. In the AES-mode, the light emitted from the atoms exited in the flame is measured. Most commercial instruments can be run in both modes. Sodium may be measured more favourably in the emission mode.
4.6.1.3 Interferences
In atomic absorption spectroscopy both ionization- and chemical interferences may occur. These interferences are caused by other ions in the sample, and result in reduction of the number of neutral atoms in the flame. The ionization interference is avoided by adding a relative high amount of an easily ionized element to the samples and calibration solutions. For the determination of sodium and potassium, caesium is added. For the elimination of chemical interferences from e.g. aluminium and phosphate, lanthanum can be added to the samples and calibration solutions.
4.6.1.4 Instrumentation
Atomic absorption spectrophotometer with a digital readout, suitable recorder or a PC. The wavelength range must be 200-800 nm. Preferably the spectrophotometer should also have the possibility to be run in the emission mode.
EDL or hollow cathode lamps for Na, K, Mg and Ca. Single element lamps are preferred, but multi-element lamps may be used. EDLs are more intense than hollow cathode lamps, and is preferred for K and Na. When performing analyses in emission mode, no lamps are needed.
Pipettes and volumetric flasks in various sizes.
4.6.1.5 Chemicals

Deionized water
Hydrochloric acid (HCl), suprapur, 37%
Caesium chloride (CsCl), suprapur or Cs-solution specially produced for AAS
Lanthanum oxide (La₂O₃), 99.99% or La-solution specially produced for AAS
Sodium chloride (NaCl), spectrapure
Potassium chloride (KCl), spectrapure
Magnesium oxide (MgO), spectrapure
Calcium carbonate (CaCO₃), spectrapure

Compressed gas and pressure-reducing valves. Both acetylene and air are needed. The air may be supplied from a compressor with a cleaning unit.
4.6.1.6 Reagents
Caesium-Lanthanum-solution, 100.000 mg Cs/l + 50.000 mg La/l
Transfer 5,865 g La₂O₃ and 12,67 g CsCl to a 100 ml volumetric flask. Add about 50 ml deionized water and 25 ml suprapure HCl, and dilute to the mark with deionized water.
Commercial available solutions specially produced for AAS may be used.
It is very important that the caesium and lanthanum-solutions used have a low content of sodium, potassium, magnesium and calcium since a relative high concentration of this solution is added to the sample.
4.6.1.7 Calibration solutions and stock solutions
Na, 1000 mg/l:
Transfer 2,542 g NaCl, dried at 140 °C for 1 hour before weighing, to a 1000 ml volumetric flask, add 50 ml of deionized water and 1 ml HCl and shake until all is dissolved. Dilute to the mark with deionized water. Store the solution in a polyethylene bottle.
K, 1000 mg/l:
Transfer 1,907 g KCl, dried at 110 °C for 1 hour before weighting, to a 1000 ml volumetric flask. Add 50 ml of deionized water and 1 ml HCl and shake until all is dissolved. Dilute to the mark with deionized water. Store the solution in a polyethylene bottle.
Mg, 1000 mg/l:
Transfer 1,658 g MgO to a 1000 ml volumetric flask. Add 10 ml HCl and shake until all is dissolved. Dilute to the mark with deionized water. Store the solution in a polyethylene bottle.
Ca, 1000 mg/l:
Transfer 2,497 g CaCO₃, dried at 180 °C for 1 hour before weighting, to a 1000 ml volumetric flask. Add 50 ml of deionized water, and dissolve slowly with a minimum of HCl. Dilute to the mark with deionized water. Store the solution in a polyethylene bottle.
Working standard solution, Na, K, Mg and Ca 10 mg/l:
Pipette 10,0 ml of each of the stock solutions Na, K, Mg, and Ca 1000 mg/l to a 1000 ml volumetric flask. Dilute to the mark with deionized water. Store the solution in a polyethylene bottle. The solution should be made fresh each time the calibration solutions are prepared.
Calibration solutions for Na, K, Mg and Ca:
Pipette 1, 2, 5, 10, 15, 20, 40, and 50 ml of the working standard solution, 10 mg/l to each of eight 100 ml volumetric flasks. Add 1 ml of the Cs-La-solution and dilute to the mark with deionized water. The concentrations in the solutions will be 0,1, 0,2, 0,5, 1,0, 1,5, 2,0, 4,0 and 5,0 mg/l respectively. A solution with 1 ml Cs-La- solution diluted to 100 ml is used as a blank.
The calibration solutions and the blank should be stored in polyethylene bottles and made fresh the day of analysis.
4.6.1.8 Calibration of the instrument
After a warm-up time of the instrument, set the wavelength for the element to be analysed as given in Table 4.6.1, and the slit width and the air/acetylene ratio as given in the instruction manual for the instrument. Ignite the flame. Adjust the reading of the instrument to zero by spraying the blank into the flame. Run the calibration solutions and read the absorption (or emission) signals from the readout. Plot the calibration graph.
The instrument should be recalibrated after every 20-30 samples. A control solution should also be run after each calibration.

Table 4.6.1: Wavelength settings for the analyses.

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