Hach-Lange HQD - Sodium (Na+) Ion Selective Electrode ISENa - Manual (Page 14)

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Checking electrode response
To verify electrode response, measure the electrode potential (in mV) of two Sodium
Standard Solutions one decade apart in concentration, bracketing the expected sample
concentration. For example, use 10 and 100 mg/L Sodium Standard Solutions. The two
solutions should have potentials (difference in mV readings) that are -59 mV apart at
25 ºC. Both solutions should be above 1.4 mg/L Na
+ .
Checking accuracy of sample reading
To verify sample measurement accuracy, add a spike of Standard Sodium Solution with
volumetric pipet. Use the following table and formulas to calculate the percent of recovery.
Typically a percent of recovery of 100% ± 5% is a good indication that the instrument,
technique, and the sample are not contributing to measurement errors.
Percent recovery
Use the following formula to calculate the percent recovery when the sample volume is
25 mL:
E = (C x V
1
) / V
2
R = (A / (E + S)) x 100
Where:
• S = mg/L of Na + in sample (before spike)
• C = concentration of standard used for spiking (mg/L)
• V
1
= spike volume (mL)
• V
2
= spike volume (mL) + 25 mL sample volume
• E = expected concentration of spike (mg/L)
• R = percent recovery
• A = actual reading on meter after spike (mg/L Na + )
Measured sample concentration Volume of standard to add Concentration of standard
1 - 2 mg/L 0.5 mL 100 mg/L
3 - 6 mg/L 1.0 mL 100 mg/L
7 - 15 mg/L 0.3 mL 1000 mg/L
15 - 30 mg/L 0.5 mL 1000 mg/L
30 - 60 mg/L 1.0 mL 1000 mg/L