## 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 Na can 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 flameemission 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 areavailable, 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 at589 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 bedetermined 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 
, SO 4 2

 and HCO 3  in relatively large amounts. 3) Matrix-match standards and samples by adding identicalamounts 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, anddetermining 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 andpotassium 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.

1d. 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 HNO 3 fol

lowed by several portions of reagent water (¶ 3 a ).

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 formulatedetailed 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, selectand 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 onbracketing standards and sample. Calculate the sodium concentration by the equation in ¶ 5 b 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 ) A 

# D

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 Mg 2 /L, 3.1 mg K /L, 241 mg Cl  /L, 0.25 mg NO 2  -N/L, 1.1 mg NO 3  -N/L, 259 mg SO 4 2

 /L, and 42.5 mg total alkalinity/L (as CaCO 3 ) 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. M ONTGOMERY . 1950. Application of flame spectrophotometry to water analysis. Anal. Chem. 22:667. C OLLINS , C.G. & H. P OLKINHORNE . 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. B URRIEL-M ARTI , F. & J. RAMIREZ -M UNOZ . 1957. Flame Photometry: AManual of Methods and Applications. D. Van Nostrand Co., Princeton, N.J. D EAN , J.A. 1960. Flame Photometry. McGraw-Hill Publishing Co., New York, N.Y. URE , A.M. & R.L. M ITCHELL . 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. D EAN & F.A. S ETTLE , JR . 1981. Instrumental Methods of Analysis, 6th ed. Wadsworth Publishing Co., Belmont, Calif. AMERICAN S OCIETY FOR TESTING AND M ATERIALS . 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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