Guest Editorial Comment
813
J. Clin. Chem. Clin. Biochem.
Vol. 28, 1990, pp. 813-815
© 1990 Walter de Gruyter & Co.
Berlin · New York
Guest Editorial Comment
Determination of Sodium with Ion-Selective Electrodes:
A New Method or a New Quantity?
It is generally accepted that active molality (raNa) is the most important quantity for evaluating and understanding the effect of sodium ions. Other quantities, which are more or less helpful for this purpose, are:
Free molal concentration (wNa), which can be calculated from mNa:
mNa = —^ [mmol/kg H2O]
YNa
yNa = molal activity coefficient of sodium
Total molal concentration (wtNa), which includes free molal and complex bound molal concentration:
= "%a + mNaHco3 + ^Naco3- + mNax + wNaprot [mmol/kg H2O]
WNaHco3'·
tf?Naco3-:
tt*Nax'
^Naprot'
sodium
sodium
sodium
sodium
bound
bound
bound
bound
to
to
to
to
HCO3~
CCV
other anions
proteins
Total molar concentration of a serum (ctNa), which is given by:
CtNa = WtNa ' Qn2o [mmol/1 serum]
QH2(> mass concentration of water in the serum
The above mentioned quantities can be determined more or less reliably as follows.
1. Active molality of sodium (mNa) can be determined directly by ion-selective electrodes without dilution of
the sample, according to Nernst's equation.
2. Free molal concentration (mNa) can be calculated from wNa, assuming a constant activity coefficient, which,
however, varies whithin narrow limits.
3. Total molal concentration (wtNa) is identical to wNa, if one neglects the small amount of sodium bound at
pH 7.4 (1.5% of total). In sera, QH2o is not a constant, and it can vary widely and unpredictably. The
calculation of wtNa by using ctNa is therefore only possible if the mass concentration of the water in the serum
(Qn2o) is also determined.
4. Total molar concentration of sodium (ctNa) is determined by flame atomic emission spectrometry or ionselective electrodes after dilution of the sample.
J. Clin. Chem. Clin. Biochem. / Vol. 28,1990 / No. 11
814
Guest Editorial Comment
A reference method for the determination of the total molar concentration of sodium in serum is well
established (1), whereas reference or even definitive methods for the other quantities are not yet available. A
first approach to this problem was made by van Suijlen et al. (see this issue (2)), who proposed a reference
method for the determination of the total molal concentration of sodium. As expected, this candidate reference
method can be helpful in the evaluation of ion-selective electrodes reporting free molal concentration, since
activity coefficients in sera are rather constant and the amount of bound sodium is small. However, a
systematic error of about 1.5% between the reference method value and value obtained by ion-selective
electrode is inevitable.
Efforts to develop a reference method for the determination of active molality — like that published for pH
(3) — should be encouraged, because this quantity is independent of assumptions about the activity coefficient
and sodium binding. Another step forward would be the development of a reference method for free molal
concentration — such as the reference method which is being developed for ionized calcium (4), in which the
only ambiguities arise from the activity coefficient.
Procedures for calibrating "direct" ion-selective electrodes for the total molar concentration of normal sera
have been proposed by Maas et al. (5) and van Suijlen et al. (2). These procedures are analytically the least
desirable, because they imply the substitution of primary standard solutions by secondary standard samples,
which might have been analysed even by field methods. In addition, interpretation of the results is not easy.
Thus, if calibration is performed as proposed (2, 5), values obtained by ion-selective electrodes and e.g. flame
atomic emission spectrometry can differ widely and unpredictably in individual samples. It is nevertheless
necessary to report, which method was used; one method being independent of the mass concentration of
water ("direct" potentiometry) and the others not (flame atomic emission spectrometry and "indirect"
potentiometry). The reference intervals, which are identical in this case due to calibration, are deceptive. They
may give the impression of identical quantities, but in fact they represent two different quantities. It is even
less evident that the ctNa reference interval is dependent on the macromolecule (proteins, lipids. dextran etc.)
content of the sample.
The quantities proposed for this "fitting" procedure are called e.g. "standard sodium concentration (mmol/1
serum)". This quantity, which is outside the easily understandable rational system of quantities and units (6),
is fictitious and difficult to explain. Although the measuring unit is said to be mmol/1 serum, this is only true
when the macromolecule content is "normal"; otherwise the real volume deviates more or less from 1 litre.
On the whole, this leads to a deterioration in analytical performance without any improvement in the ease
of interpretation of the results (which is no criterion for a reference method). Measurement by e.g. "direct"
ion-selective electrodes and flame atomic emission spectrometry does not represent the measurement of a
single quantity by two methods. Rather, each method measures a different quantity, and the respective values
are fortuitously quite close; almost by chance, they do not differ as much as the values of free and total
calcium. The total molar concentration of sodium is determined by flame atomic emission spectrometry or
ion-selective electrodes after dilution of the sample. Ion-selective electrode measurements of undiluted samples
should be expressed either as active molality wNa or free molal concentration of sodium, a quantity which is
susceptible to ambiguities from the activity coefficient, but a quantity for which a reference or a definitive
method might be more easily developed, and which might be more easily understood.
As ion-selective electrodes for other analytes in serum water are under development, a deliberate decision
concerning the determination of sodium should be made. The method should not introduce fictitious units
of very limited applicability, which impede the development of definitive and reference methods for the
universally applicable rational system of quantities (6).
Wolf-Rüdiger Külpmann, Hannover
J. Clin. Chem. Clin. Biochem. / Vol. 28, 1990 / No. 11
815
Guest Editorial Comment
References
1. Velapoldi, R. ., Paule, R. C., Schaffer, R., Mandel, J. &
Moody, J. R. (1978) Standard Reference Materials: A reference method for the determination of sodium in serum.
NBS Spec. Public. 260-60.
2. van Suijlen, J. D. E., Berrevoets, C. A. & Leijnse, B. (1990)
A candidate reference method for coupled sodium-water
determination in human serum. J. Clin. Chem. Clin.
Biochem. 28, 817-824.
3. Maas, A. H. J., Weisberg, H. F., Burnett, R. W., MüllerPlathe, O., Wimberley, P. D., Zijlstra, W. G., Durst, R. A.
& Siggaard-Andersen, O. (1987) Approved IFCC methods:
Reference method (1986) for pH measurement in blood. J.
Clin. Chem. Clin. Biochem. 25, 281-289.
4. Covington, A. K., Kelly, P. M. & Maas, A. H. J. (1989)
Reference method for the determination of ionized calcium
in serum, plasma or whole blood: experimental aspects. In:
Methodology and clinical applications of ion-selective electrodes (Maas, A. H. J., Buckley, B. M., Manzoni, A., Moran,
R. F., Siggaard-Andersen, O. & Sprokholt, R. eds.) pp.
119-128. Elinkwijk Printing Co., Utrecht.
J. Clin. Chem. Clin. Biochem. / Vol. 28, 1990 / No. 11
5. Boink, A. B. T. J., Buckley, B. M., Christiansen, T. F.,
Covington, A. K., Maas, A. H. J., Muller-Plathe, O., Sachs,
C. & Siggaard-Andersen, O. (1986) Recommendations on
the expression of results of ion-selective electrode measurement of sodium and potassium ion activities in undiluted
serum, plasma of whole blood in clinical practice, in: Methodology and clinical applications of ion-selective electrodes
(Maas, A. H. J., Buckley, B. M., Marsoner, H. J., Saris, N.E. L. & Sprokholt, R., eds.) pp. 137-149. Elinkwijk Printing
Co., Utrecht.
6. Siggaard-Andersen, O., Durst, R. A., Maas, A. H. J. (1987)
Approved recommendation (1984) on physico-chemical
quantities and units in clinical chemistry with special emphasis on activities and activity coefficients. J. Clin. Chem.
Clin. Biochem. 25, 369-391.
Received for publication October 4, 1990
Prof. Dr. W. R. Külpmann
Institut für Klinische Chemie I
Medizinische Hochschule Hannover
Konstanty-Gutschow-Str. 8
W-3000 Hannover 61