For two-point calibrations, it is recommended to use two standards that differ by a factor of 10.
For three-point calibrations, high, medium, and low concentration standards should be used.
In either case, the highest and lowest calibration solution concentrations should bracket the expected sample range. For example, if the expected concentrations of the samples are around 30 parts per million (ppm), then 10 ppm and 100 ppm standards should be prepared for a two-point calibration.
NexSens WQSensors software will support up to a three-point calibration for ISE sensors. However, two-point calibrations are sufficient and recommended.
- Concentrated stock standard
- Ionic strength adjuster (ISA)
- Distilled or deionized water for dilution
- Instrument for accurately measuring small volumes of liquid (ex. Pipette)
- Graduated cylinders, beakers, or volumetric flasks (appropriately sized for desired volume of solutions)
Vstock – volume of the stock standard being used
Cstock– concentration of the stock standard being used
Vcalibration – desired volume of the calibration standard
Ccalibration- desired concentration of the calibration standard
The general dilution formula for preparing calibration standards from stock standards is given by the following:
This formula can be used for any concentration that is below that of the stock standard such that Ccalibration<Cstock.
The following example details how to prepare 100 mL of 10 ppm standard and 100 mL of 100 ppm standard from a 1000 ppm stock standard.
1. Begin with the 10 ppm standard. The desired volume of calibration standard is 100 mL. Therefore the volume of stock standard must be computed as shown below:
- Cstock=1000 ppm
- Ccalibration1=10 ppm
- Vcalibration1=100 mL
- Vstock=(100 mL)*(10 ppm)/(1000 ppm)=1 mL
- This means that 1 mL of stock standard should be put into a graduated container and distilled or deionized water should be added until the total volume in the container is 100 mL.
2. Similarly, for the 100 ppm standard
- Cstock=1000 ppm
- Ccalibration2=100 ppm
- Vcalibration2= Vcalibration1=100 mL
- Vstock=(100 mL)*(100 ppm)/(1000 ppm)=10 mL
- This means that 10 mL of stock standard should be put into a graduated container and distilled or deionized water should be added until the total volume in the container is 100 mL.
3. Ionic strength adjuster (ISA) specific to the sensor being calibrated should always be added to both calibration standards and samples. This reduces the margin of error associated with ionic strength variations.
- For adding ISA to standards, a 1:50 ratio should be used. In this example, 2 mL of ISA should be added to both the 10 ppm and 100 ppm calibration standards. ISA is not considered a part of the dilution. For adding ISA to samples, refer to sensor-specific documentation for recommendations.