3500-Na SODIUM*

3500-Na A. Introduction

1. Occurrence and Significance

Sodium (Na) is the third element in Group IA of the periodic

table; it has an atomic number of 11, an atomic weight of 22.99,

and a valence of 1. The average abundance of Na in the earth’s

crust is 2.5%; in soils it is 0.02 to 0.62%; in streams it is 6.3

mg/L, and in groundwaters it is generally 5 mg/L. Sodium

occurs with silicates and with salt deposits. Sodium compounds

are used in many applications, including caustic soda, salt,

fertilizers, and water treatment chemicals.

Sodium is very soluble, and its monovalent ion Nacan reach

concentrations as high as 15 000 mg/L in equilibrium with sodium

bicarbonate. The ratio of sodium to total cations is important in

agriculture and human physiology. Soil permeability can be harmed

by a high sodium ratio. In large concentrations it may affect persons

with cardiac difficulties. A limiting concentration of 2 to 3 mg/L is

recommended in feedwaters destined for high-pressure boilers.

When necessary, sodium can be removed by the hydrogen-exchange process or by distillation. The U.S. EPA advisory limit for

sodium in drinking water is 20 mg/L.

2. Selection of Method

Method 3111B uses an atomic absorption spectrometer in the

flame absorption mode. Method 3120B uses inductively coupled

plasma; this method is not as sensitive as the other methods, but

usually this is not important. Method 3500-Na.B uses either a flame

photometer or an atomic absorption spectrometer in the flame

emission mode. The inductively coupled plasma/mass spectrometric method (3125) also may be applied successfully in most cases

(with lower detection limits), even though sodium is not specifically

listed as an analyte in the method. When all of these instruments are

available, the choice will depend on factors including relative quality of the instruments, precision and sensitivity required, number of

samples and analytes per sample, matrix effects, and relative ease of

instrument operation. If an atomic absorption spectrometer is used,

operation in the emission mode is preferred.

3. Storage of Sample

Store alkaline samples or samples containing low sodium

concentrations in polyethylene bottles to eliminate the possibility

of sample contamination due to leaching of the glass container.

3500-Na B. Flame Emission Photometric Method

1. General Discussion

a. Principle: Trace amounts of sodium can be determined by

flame emission photometry at 589 nm. Sample is nebulized into

a gas flame under carefully controlled, reproducible excitation

conditions. The sodium resonant spectral line at 589 nm is

isolated by interference filters or by light-dispersing devices such

as prisms or gratings. Emission light intensity is measured by a

phototube, photomultiplier, or photodiode. The light intensity at

589 nm is approximately proportional to the sodium concentration. Alignment of the wavelength dispersing device and wavelength readout may not be precise. The appropriate wavelength

setting, which may be slightly more or less than 589 nm, can be

determined from the maximum emission intensity when aspirating a sodium standard solution, and then used for emission

measurements. The calibration curve may be linear but has a

tendency to level off or even reverse at higher concentrations.

Work in the linear to near-linear range.

b. Interferences: Minimize interference by incorporation of

one or more of the following:

1) Operate at the lowest practical concentration range.

2) Add releasing agents, such as strontium or lanthanum at

1000 mg/L, to suppress ionization and anion interference.

Among common anions capable of causing interference are Cl,

SO4

2 and HCO3 in relatively large amounts.

3) Matrix-match standards and samples by adding identical

amounts of interfering substances present in the sample to calibration standards.

4) Apply an experimentally determined correction in those instances where the sample contains a single important interference.

5) Remove interfering ions.

6) Remove burner-clogging particulate matter from the sample by

filtration through a filter paper of medium retentiveness.

7) Use the standard addition technique as described in the flame

photometric method for strontium (3500-Sr.B). The method involves

preparing a calibration curve using the sample matrix as a diluent, and

determining the sample concentration either mathematically or graphically.

8) Use the internal standard technique. Potassium and calcium

interfere with sodium determination by the internal-standard method if

the potassium-to-sodium ratio is 5:1 and the calcium-to-sodium ratio

is 10:1. When these ratios are exceeded, determine calcium and

potassium concentrations and matrix-match sodium calibration standards by addition of approximately equivalent concentrations of interfering ions. Interference from magnesium is not significant until the

magnesium-to-sodium ratio exceeds 100, a rare occurrence.

c. Minimum detectable concentration: Better flame photometers or

atomic absorption spectrometers operating in the emission mode can be

used to determine sodium levels approximating 5 g/L.

* Approved by Standard Methods Committee, 1997. Editorial revisions, 2011.

Joint Task Group: 20th Edition—See 3500-Al.

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d. Quality control (QC): The QC practices considered to be

an integral part of each method can be found in Section 3020.

2. Apparatus

a. Flame photometer (either direct-reading or internal-standard type) or atomic absorption spectrometer operating in the

flame emission mode.

b. Glassware: Rinse all glassware with 1 15 HNO3 followed by several portions of reagent water (¶ 3a).

3. Reagents

To minimize sodium contamination, store all solutions in

plastic bottles. Use small containers to reduce the amount of dry

element that may be picked up from the bottle walls when the

solution is poured. Shake each container vigorously to wash

accumulated salts from walls before pouring solution.

a. Reagent water: See Section 1080. Use reagent water to

prepare all reagents and calibration standards, and as dilution water.

b. Stock sodium solution: Dissolve 2.542 g NaCl dried at

140°C to constant weight and dilute to 1000 mL with water; 1.00

mL  1.00 mg Na.

c. Intermediate sodium solution: Dilute 10.00 mL stock sodium solution with water to 100.0 mL; 1.00 mL  0.10 mg Na

(1.00 mL  100 g Na). Use this intermediate solution to

prepare calibration curve in sodium range of 1 to 10 mg/L.

d. Standard sodium solution: Dilute 10.00 mL intermediate

sodium solution with water to 100 mL; 1.00 mL  10.0 g Na.

Use this solution to prepare calibration curve in sodium range of

0.1 to 1.0 mg/L.

4. Procedure

a. Pretreatment of polluted water and wastewater samples:

Follow the procedures described in Section 3030.

b. Instrument operation: Because of differences between

makes and models of instruments, it is impossible to formulate

detailed operating instructions. Follow manufacturer’s recommendation for selecting proper photocell and wavelength, adjusting slit width and sensitivity, appropriate fuel and oxidant gas

pressures, and the steps for warm-up, correcting for interferences

and flame background, rinsing of burner, igniting flame, and

measuring emission intensity.

c. Direct-intensity measurement: Prepare a blank and sodium

calibration standards in stepped amounts in any of the following

applicable ranges: 0 to 1.0, 0 to 10, or 0 to 100 mg/L. Determine

emission intensity at 589 nm. Aspirate calibration standards and

samples enough times to secure a reliable average reading for each.

Construct a calibration curve from the sodium standards. Determine

sodium concentration of sample from the calibration curve. Where

a large number of samples must be run routinely, the calibration

curve provides sufficient accuracy. If greater precision and less bias

are desired and time is available, use the bracketing approach

described in ¶ 4d below.

d. Bracketing approach: From the calibration curve, select

and prepare sodium standards that immediately bracket the emission intensity of the sample. Determine emission intensities of

the bracketing standards (one sodium standard slightly less and

the other slightly greater than the sample) and the sample as

nearly simultaneously as possible. Repeat the determination on

bracketing standards and sample. Calculate the sodium concentration by the equation in ¶ 5b and average the findings.

5. Calculation

a. For direct reference to the calibration curve:

mg Na/L  (mg Na/L in portion)  D

b. For the bracketing approach:

mg Na/L  

(B  A) (s  a)

(b  a)

AD

where:

B  mg Na/L in upper bracketing standard,

A  mg Na/L in lower bracketing standard,

b  emission intensity of upper bracketing standard,

a  emission intensity of lower bracketing standard,

s  emission intensity of sample, and

D  dilution ratio

 mL sample mL water

mL sample

6. Precision and Bias

A synthetic sample containing 19.9 mg Na/L, 108 mg

Ca2/L, 82 mg Mg2/L, 3.1 mg K/L, 241 mg Cl/L, 0.25 mg

NO2

-N/L, 1.1 mg NO3-N/L, 259 mg SO42/L, and 42.5 mg

total alkalinity/L (as CaCO3) was analyzed in 35 laboratories by

the flame photometric method, with a relative standard deviation

of 17.3% and a relative error of 4.0%.

7. Bibliography

WEST, P.W., P. FOLSE & D. MONTGOMERY. 1950. Application of flame

spectrophotometry to water analysis. Anal. Chem. 22:667.

COLLINS, C.G. & H. POLKINHORNE. 1952. An investigation of anionic

interference in the determination of small quantities of potassium

and sodium with a new flame photometer. Analyst 77:430.

MELOCHE, V.W. 1956. Flame photometry. Anal. Chem. 28:1844.

BURRIEL-MARTI, F. & J. RAMIREZ-MUNOZ. 1957. Flame Photometry: A

Manual of Methods and Applications. D. Van Nostrand Co., Princeton, N.J.

DEAN, J.A. 1960. Flame Photometry. McGraw-Hill Publishing Co., New

York, N.Y.

URE, A.M. & R.L. MITCHELL. 1975. Lithium, sodium, potassium, rubidium, and cesium. In J.A. Dean & T.C. Rains, eds. Flame Emission

and Atomic Absorption Spectrometry. Dekker, New York, N.Y.

THOMPSON, K.C. & REYNOLDS, R.J. 1978. Atomic Absorption, Fluorescence, and Flame Spectroscopy—A Practical Approach, 2nd ed.

John Wiley & Sons, New York, N.Y.

WILLARD, H.H., L.L. MERRIT, JR., J.A. DEAN & F.A. SETTLE, JR. 1981.

Instrumental Methods of Analysis, 6th ed. Wadsworth Publishing

Co., Belmont, Calif.

AMERICAN SOCIETY FOR TESTING AND MATERIALS. 1988. Method D 1428-

82: Standard test methods for sodium and potassium in water and

water-formed deposits by flame photometry. Annual Book of

ASTM Standards, Vol. 11.01. American Soc. Testing & Materials,

Philadelphia, Pa.

SODIUM (3500-Na)/Flame Emission Photometric Method

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