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-ex-
change 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 spectromet-
ric 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 qual-
ity 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 concentra-
tion. Alignment of the wavelength dispersing device and wave-
length 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 aspirat-
ing 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 identical
amounts of interfering substances present in the sample to cali-
bration standards.
4) Apply an experimentally determined correction in those in-
stances 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 graph-
ically.
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 ⱖ5:1 and the calcium-to-sodium ratio
is ⱖ10:1. When these ratios are exceeded, determine calcium and
potassium concentrations and matrix-match sodium calibration stan-
dards by addition of approximately equivalent concentrations of inter-
fering 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.
1
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-stan-
dard 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 (¶ 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 so-
dium 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 recom-
mendation for selecting proper photocell and wavelength, ad-
justing 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 ¶ 4dbelow.
d. Bracketing approach: From the calibration curve, select
and prepare sodium standards that immediately bracket the emis-
sion 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 concen-
tration by the equation in ¶ 5band 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 ⫽
冋
(B⫺A)(s⫺a)
(b⫺a)
⫹A
册
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
Ca
2⫹
/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.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., Prince-
ton, 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, rubid-
ium, 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, Fluores-
cence, 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.
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