Determination of Calcium, Magnesium, Sodium & Potassium in Waters by Flame Aas

Determination of Calcium, Magnesium, Sodium & Potassium in Water by Flame AAS

Version:03

Page:1 of 6

Issue Date:4 December 2001

LABORATORY METHODS MANUALAuthorised by:Marjorie Dickenson

DETERMINATION OF CALCIUM, MAGNESIUM, SODIUM & POTASSIUM IN WATERS BY FLAME AAS

1.SCOPE

This method applies to the determination of calcium and magnesium in riverine waters where calcium is in the range 0.1 to 40 mg/L, magnesium 0.02 to 5 mg/L, sodium 0.1 to 10 mg/L and potassium is in the range 0.1 to 4 mg/L.

2.PRINCIPLE

Aqueous samples are nebulized into a flame of an atomic absorption spectrometer and atomised. The intensity of absorption of light at characteristic wavelengths for either element is proportional to the concentration of the element in solution.

3.REACTIONS AND INTERFERENCES

Extensive interferences exist for these elements in flame atomic absorption analysis. Many inter-element effects for calcium and magnesium are overcome by the use of the hotter nitrous oxide-acetylene flame. The stability of the element oxides for calcium and magnesium must be overcome by the use of lanthanum solution (releasing agent), and the tendency for ion formation is overcome by the use of caesium solution. Sodium and potassium are also partially ionised in an air-acetylene flame, this is overcome by the addition of caesium.

4.REAGENTS

4.1.General

Unless otherwise specified, all reagents shall be of analytical reagent grade, and high-purity water used shall comply with Type I water as specified in ISO 3696 (16.7M/cm resistivity; 0.06S.cm conductivity).

4.2.Nitric acid (HNO3) solutions

4.2.1.Nitric acid (2+3 dilution = 40% (v/v))

Slowly, and with continuous stirring, add 100mL of high-purity nitric acid (20=1.42g/mL) to 150mL of high-purity water in a clean 250mL plastics bottle. Allow to cool before use.

4.2.2.Nitric acid (1+9 dilution = 10% (v/v))

Slowly, and with continuous stirring, add 100mL of high-purity nitric acid (20=1.42g/mL) to 900mL of high-purity water in a clean 1000mL plastics bottle. Allow to cool before use.

4.3.Releasing Agent and Ionisation Buffers

4.3.1.Caesium / Lanthanum solution (ca 100 g/L of Cs and La)

(a)Record all preparation information on laboratory proforma.

(b)Tare a clean, dry 250mL plastics bottle then weigh in 25.3g caesium chloride (CsCl) and 62.4g lanthanum nitrate (La(NO3)2.6H2O).

(c)Add 200g of water, stir to dissolve and mix well. Label with the following information -

• identification (i.e. Method 06-058 Caesium+Lanthanum);
• date; and
• preparer's initials.

4.3.2.Caesium Chloride solution (ca 100 g/L of Cs)

(a)Record all preparation information on laboratory proforma.

(b)Tare a clean, dry 250mL plastics bottle then weigh in 25.3g caesium chloride (CsCl).

(c)Add 200mL water, stir to dissolve and mix well. Label with the following information -

• identification (i.e. Method 06-058 Caesium);
• date; and
• preparer's initials.

4.4.Standard Solutions (1000mg/L). Commercially available individual standard solutions are required for this method.

NOTES:

(d)Complete the solution preparation proforma associated with this method every time any of the solutions are prepared, and file in Standard Solution Log.

(e)Refer to Laboratory Method Manual, method 06-001 for the management and handling of commercial standard solutions.

4.4.1.Calcium (Ca).

4.4.2.Magnesium (Mg).

4.4.3.Sodium (Na).

4.4.4.Potassium (K).

4.4.5.Composite solution

(a)Record all preparation information on laboratory proforma.

(b)To a clean 1000mL volumetric flask containing about 400mL of Type I water and 10mL 40% nitric acid (4.2.1), pipette the following volumes of standard solutions:

Element / Solution / Volume (mL)
Ca / 4.4.1 / 250
Mg / 4.4.2 / 25
Na / 4.4.3 / 25
K / 4.4.4 / 25

(c)Dilute to 1000mL with water and mix well.

(d)Store in a 1000mL polyethylene bottle. Label with the following information -

• identification (i.e. Method 06-058 Composite Solution);
• date;
• preparer's initials;
• symbol and concentration (mg/L) of each element in the solution; and
• discard date (12 months after preparation).

(e)Store on laboratory bench.

4.4.6.Calibration solutions

(a)Record all preparation information on proforma.

(b)To five separate, clean 250mL volumetric flasks containing approximately 100mL Type I water, 10mL 10% HNO3 (4.2.2) using a 50 mL A-grade burette (5.6), add the following volumes of Composite solution (4.4.5).

Solution / Volume of Composite
(mL) / Ca
mg/L / Mg Na K
mg/L
FAAS0 / 0.00 / 0.0 / 0.00
FAAS1 / 12.5 / 12.5 / 1.25
FAAS2 / 25.0 / 25.0 / 2.50
FAAS3 / 37.5 / 37.5 / 3.75
FAAS4 / 50.0 / 50.0 / 5.00

(c)Dilute to the mark with Type I water.

(d)Transfer to a 250 mL polyethylene bottle labelled with the following information -

• identification (e.g. Method 06-058 - FAAS1);
• date;
• preparer's initials;
• symbol and concentration (mg/kg) of each element in the solution; and
• discard date (2 months after preparation).

(e)Store on laboratory bench.

4.5.Reference Material

4.5.1.NIST 1643d River Water

4.5.2.NIST 1643d River Water (1 + 4 = 25% v/v)

(a)Record all preparation information on the laboratory proforma.

(b)Accurately transfer 5mL of River Water (4.5.1) by pipette to a clean 25mL volumetric flask.

(c)Use a micropipettor to deliver 0.4mL of caesium/lanthanum solution (4.3.1) into the vial for Mg determination.

(d)If Na is required, dilute another portion of standard as in step (b) and use a micropipettor to deliver 0.4mL of caesium solution (4.3.2) into the volumetric flask.

5.APPARATUS

5.1.Vials 30 mL capacity, polycarbonate

5.2.Atomic absorption spectrometer Conforming to AS 2134.1

5.3.Polyethylene bottles

250mL capacity, prepared according to AS 2031:2001; all bottles in use for the preparation of calibration and sample solutions should be polyethylene only, acid-soaked with 10% HNO3 for at least three days and rinsed thoroughly with high-purity water before use.

5.4.Micropipettor - 5mL adjustable volume.

5.5.Burette - A-grade 50mL.

6.SAMPLE PREPARATION

The sample must be filtered and made 0.4% with respect to nitric acid as per Laboratory Method 06-025.

7.SAFETY

7.1.General

(a)This method includes the use of harmful and corrosive substances. Before proceeding, familiarity should be gained with the Material Safety Data Sheets (MSDS) on each reagent used with the method.

(b)Users of this method must be familiar with the laboratory’s safety precautions and emergency procedures, and with the safety instructions in the handbooks from the instrument manufacturers.

7.2.Nitric Acid

Personal protective clothing including safety glasses, gloves and laboratory coats must be worn at all times when this acid is being used.

Contact with combustible materials may cause fires. Nitric acid causes severe burns to skin and if taken by mouth will cause severe irritation and damage. Treat exposed site by flooding with large amounts of water. Eyes should be irrigated using an eyewash bottle for up to 20 minutes. Consult the Section Supervisor. Medical advice should be taken depending on the severity of the case. Neutralise spills with soda ash and mop up with large amounts of water to dilute.

8.PROCEDURe

Use laboratory proformasto record values associated with this method as necessary. This form must be used to check off the addition of the caesium/lanthanum solution and dilution of over-range solutions.

8.1.Quality control

8.1.1.General

While the procedure in Clause 8.2 is written in terms of a single determination, any convenient number of test samples, with accompanying replicates, blanks and reference materials, should be batched for maximum efficiency.

8.1.2.Reference material

Suitable approved certified reference materials must be carried through the procedure.

8.1.3.Blank determination

Use high-purity water and follow the procedure in Clause 8.2 as for test samples.

8.1.4.Number of determinations

(a)Reference materials must be processed at the rate of one, plus one in twenty or part thereof.

(b)Test samples should be prepared in replicate at the rate of one, plus one in ten or part thereof.

8.2.Test Procedure

(a)Ensure that the test samples have been filtered and acidified in accordance with Laboratory Method 06-025.

(b)Pipette 25.0 mL of test solution into a polycarbonate vial.

(c)If Ca and Mg are required, use a micropipettor to deliver 0.4 mL of caesium/lanthanum solution (4.3.1) into the vial, and mix the contents well.

(d)If Na and K are required, use a micropipettor to deliver 0.4 mL of caesium solution (4.3.2) into the vial, and mix the contents well.

(e)Prepare calibration standards as outlined in step (b) with addition of the appropriate buffer, either step (c) or (d).

8.3.Measurement

8.3.1.General

Operate the flame atomic absorption spectrometer in accordance with AS 2134.1.

Results which are above the range of the highest concentration calibration solution should be diluted with blank test solution to an estimated extent approximating the concentration of calibration solution FAAS2.

8.3.2.Operating parameters

Na and K Use a stoichiometric acetylene-air flame (stiff blue flame just without greenish colouration) with the nitrous oxide burner for each of Na and K.

The N2O burner with an air flame is essential to achieve acceptable linearity of the calibration curve. Ca and Mg Use a 5-10 mm red feather acetylene-nitrous oxide flame.

Instrument conditions Use three 3 second integrations with the following wavelengths -

FLAME INSTRUMENT WAVELENGTH

Element / Wavelength (nm)
Ca / 422.7
Mg / 285.2
Na / 589.6
K / 769.9

Background correction is not possible with Ca, Na and K due to the wavelengths used being longer than 350 nm, and is not required with Mg because of the use of the nitrous oxide flame.

9.CALCULATIONS

The concentration of analyte in the test solution may be taken directly from the spectrometer printout where the calibration curve has been prepared in accordance with AS 2134.1 and where this is presented with clear indication of sample identification for each test result.

When a dilution has been carried out, the result should be calculated using InfoLAB.

The equation used for dilution correction is, simply -

canalyte = ctest x f

where:

canalyte=concentration of analyte in test sample, in milligrams per litre

ctest=concentration of analyte in test solution, in milligrams per litre

f=dilution factor (if any) of the test solution for measurement

10.QUALITY CONTROL

Update the Quality Control chart for the reference material(s) used in InfoLAB.

11.REPORTING

File all worksheets and printouts associated with each application of this method and report results in the job file.

Report dissolved metal results according the table below.

Element / Reporting limits
Ca / nearest 1 < 100
nearest 10 >100
Mg / nearest 1 < 100
nearest 10 >100
Na / nearest 1 < 100
nearest 10 >100
K / nearest 1 < 100
nearest 10 >100

12.References

(a)ISO 3696:1987 Water for analytical laboratory use; Specification and test methods.

(b)AS 2134.1:1999Recommended practice for chemical analysis by atomic absorption spectrometry - Flame atomic absorption spectrometry.

(c)AS/NZS 2031:2001Selection of containers and preservation of water samples for microbiological analysis.

(d)APHA Standard Methods for the Examination of Water and Wastewater 20th edition (1998):

• Method 3500-Ca(B) - Calcium - Atomic absorption spectrometric method.
• Method 3500-Mg(B) - Magnesium - Atomic absorption spectrometric method.
• Method 3500-Na(B) - Sodium - Atomic absorption spectrometric method.
• Method 3500-K(B) - Potassium - Atomic absorption spectrometric method.

(e)Flame atomic absorption spectrometer operators manual

06-058