Sodium ion error

True Solutions were analysed for sodium and silicon by atomic adsorption spectroscopy pH measurements used a Radiometer Type G202B glass electrode, which was corrected for the (low) sodium-ion error. 

For weak adds and bases with double and triple dissociations. Equations 2 4b and 2-4c are used, respectively, in place of Equation 2-4a in Equation 2-4h. Appendix F has a FORTRAN subroutine listing that finds the pH that satisfies the charge balance for i adds and bases with single, double, or triple dissodations by interval halving, The pKj, and pK coeffidents and the solution density should be corrected for process temperature and composition. The effect of changes in the activity coefficients with ionic strength should also be factored in [Ref. 2.6]. The reader is directed to References 2.4 and 2.8 for a more detailed discussion on the effect of ion concentrations on activity coeffidents. The polynomial equation for acid and sodium ion error at die end of the subroutine should be replaced with one that fits the data from the glass manufacturer. 

The caustic solution would absorb carbon dioxide which would add a buffering effect that may not be in the process, the number of data points may be insufficient near the equivalence point due to the steep slope, and the error at the high pH end of the scale may be different than the field due to a sodium ion error. 

Sodium ions can penetrate into the sihcon/oxygen network of the glass electrode and cause a potential response error. Table 7.8 shows the correction for sodium ion error. K is the selectivity coefficient that relates the ion-exchange equilibrium of the sodium ion compared with that of the hydrogen iort, and S is the Nerrrst factor, equal to 2.3 R77nF (R and F are constants, n is the charge on the iorr. 

EXAMPLE 5-5 Calculate the error caused by sodium ion, aNa = 0.01, in the measurement of lithium, au = 0.01, using a lithium ion-selective electrode... 

Figure 23.8 shows the readings of a glass electrode [the measured values of of a cell of the type (23.5)] as a function of solution pH. In the range from acidic to neutral solutions, this curve perfectly obeys Eq. (23.7) (i.e., the potential varies linearly by 0.06 V per unit of pH). However, in alkaline solutions the curve departs from this function ( alkali error of the glass electrode ) in strongly alkaline solutions the readings of the electrode are practically independent of solution pH. This is due to violation of the selectivity conditions. At a pH value of 10 and a sodium ion... 

Because soda-glass membranes contain a high proportion of sodium ions, they exhibit a marked response to sodium ions in solution. The effect becomes increasingly significant as the hydrogen ion activity decreases, i.e. at high pH, and it is sometimes referred to as the alkaline error. At pH 12, the error is about 0.3 of a pH unit if the solution is 0.1 M with respect to sodium ions, and 0.7 of a pH unit if the solution is 1 M in sodium ions. Other monovalent cations such as lithium and potassium have a similar but smaller effect. By replacing the sodium in... 

Finally, the measured emf contains a response from ions other than the proton. Of these other ions, the only one that is commonly present is sodium. This error is magnified at very high pH (> 11) when very few protons are in solution, and is known as the alkaline error ... 

For the buffer solution Ph = 5 6 the variation of interfacial tension with the strength does not exceed the experimental error, but in the more alkaline solution we must conclude that either sodium ions or phosphate ions (or both) are positively adsorbed according to the equation... 

On the other hand, errors emerge regarding the mass ratio for deriving formulas, i.e. the ion number ratio. Schmidt [33] gives us the following example according to the ratio of sodium ions to carbonate ions in sodium carbonate solutions. Students tend to inappropriately set the mass ratio m(Na) m(C03) equal to the number ratio n(Na+ ions) n(C032 ions) in the compound. 

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