This guide will help you prepare calibration standards using stock standards of a higher concentration.
Overview
For two-point calibrations on ISE, it is recommended to use at least 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.
*For direct entry using dilution table (calculation) on the concentration needed and ISA volume, please download the ISE dilution calculation.
What’s Needed
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)
Abbreviation
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
Dilution Formula
The general dilution formula for preparing calibration standards from stock standards is given by the following:
Vstock*Cstock=Vcalibration*Ccalibration
This formula can be used for any concentration that is below that of the stock standard such that Ccalibration<Cstock.
Example
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.
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