Endress+Hauser CA76NA, Руководство по эксплуатации

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CA76NA Product description
Hauser 11
The measurement system in the flow direction consists of a sodium electrode (measuring
electrode), a temperature sensor and a pH electrode (reference electrode).
The sodium electrode measures the concentration of sodium ions in the sample. Its ion-
selective glass membrane allows Na+ ions to pass through.
The pH electrode has 2 specific functions:
• It serves as a reference for the sodium electrode.
• It measures the pH value of the sample.
The sample needs to have a pH value of > 10.8. Otherwise, H+ ions in the sample would
influence the measurement of the Na+ ions. The pH value of the sample is increased to
11.0 by adding an alkalization reagent, e.g. diisopropylamine. The amount of alkalization
reagent to be added is regulated by measuring the pH.
The Na electrode system has the following electrochemical structure:
Ag/AgCl(S) – sodium electrolyte – Na
+ -sensitive glass membrane - alkalized measuring
solution - diaphragm - KCl electrolyte gel - AgCl(S)/Ag
The potential of the sodium electrode is measured against the reference of the pH
electrode.
3.1.5 Calibration unit
The calibration unit consists of the following main components:
• Supply vessel with standard solution pump
• Manifold with 3 solenoid valves for draining, circuit flow and lab samples
• Circuit pump for draining, circuit and supply of lab samples
• Standard solution (available for order as an accessory →  63)
3.2 Operating principle
3.2.1 Measuring principle
The analyzer measures the concentration of dissolved sodium ions.
Sodium measurement is potentiometric using ion-selective glass electrodes.
An advanced Nernst equation describes in principal the processes at the ion-selective glass
membrane:
U = U
i 0
2.303 RT
F
log (a + K
Na Na
Σ
·
+
a )
x +
·
+
/z
x
- 1
A0034599
U
i
Measured value in mV
U
0
Standard potential
R Relative gas constant (8.3143 J/molK)
T Temperature [K]
F Faraday constant (26.803 Ah)
a
Na+
Activity of Na + ions
K
Na+
Selectivity coefficient
a
x
Activity of interference ion
z
x
Value of interference ion
The slope of the Nernst equation (2.303RT/F) is known as the Nernst factor and has
a value of 59.16 mV/px at 25 °C.