In this video, we will prepare the chloride standard solution used for calibration of the EXO Chloride Ion Selective Electrode or ISE. You can find these instructions in the EXO manual.
You may also purchase chloride standards from a certified supplier, but please be sure to add the prescribed amount of magnesium sulfate to the purchased standard, as detailed in the EXO manual. Before beginning the chloride standard prep, please read and follow the safety instructions in the MSDS documentation for each chemical. You should wear safety glasses, gloves, and a lab coat.
You will need two one-liter volumetric flasks, a high-accuracy pipette that can measure 10 mL, two labeled storage bottles, a scale, weigh boats, a spatula, deionized water, a transfer pipet, solid sodium chloride, and solid magnesium sulfate.
This procedure references anhydrous sodium chloride and anhydrous magnesium sulfate. If you use a hydrated form of magnesium sulfate, like heptahydrate, make sure you use the molecular weight of your compound to calculate the correct amount to use in the procedure so that you still end up with the same concentration of magnesium sulfate.
In the case of magnesium sulfate heptahydrate, for example, since water makes up about 50% of its molecular weight, double the amount of magnesium sulfate heptahydrate you measure in the procedure to equal the required concentration of anhydrous magnesium sulfate.
So let’s start with the 1000 mg/L chloride standard. First, tare your scale with a weigh boat. Then, weigh 1.655 grams of sodium chloride into the weigh boat. Try to be as accurate as possible. The higher accuracy you can achieve, the less error you will have in your sensor calibration. Transfer the solid sodium chloride to a clean one-liter volumetric flask. Rinse the weigh boat with DI water into the flask to minimize transfer error.
Now weigh 0.5 grams of anhydrous magnesium sulfate. If you are using a hydrated version, this is where you will calculate the appropriate amount to use that is equivalent. In the case of heptahydrate, which is what I have today, this would be about 1.02 grams. The purpose of magnesium sulfate is to add hardness to the DI water, which is essential for the proper function of the Chloride ISE.
Add the magnesium sulfate to the flask. Rinse the weigh boat with DI water into the flask to minimize transfer error. Add DI water to about half the volume of the flask and swirl to dissolve the reagents. Now fill the flask to the 1-L volumetric mark. Use a transfer pipet to get an accurate volume. Cover the flask and invert three times to mix thoroughly. This is our 1000 mg/L standard for chloride ISE calibration. Transfer this to a labeled one-liter bottle.
Next, we will make the 10 mg/L chloride standard by diluting the 1000 mg/L standard. Start with a clean one-liter volumetric flask. We will add DI water to about 90 to 95% of the volumetric mark. Use a pipet to transfer 10.00 mL of the 1000 mg/L standard into the flask. Rinse the pipet tip to minimize transfer error.
Then weigh out 0.5 grams of anhydrous magnesium sulfate, and again, use the equivalent measurement depending on what form of magnesium sulfate you have. Since I have heptahydrate, I will measure out 1.02 grams.
Add the magnesium sulfate to the flask, and rinse the weigh boat with some DI water to get all of the solids transferred. Swirl to dissolve the reagents. Then dilute up to the volumetric mark using a transfer pipet to get one liter. Cover the flask and invert it three times. Then transfer the solution to a labeled bottle.
Now we have our 10 mg/L chloride standard, as well as our 1000 mg/L standard. These standards can be stored for up to six months at room temperature until you are ready to calibrate your Chloride ISE. Do not reuse standards. Prepare fresh standards for subsequent calibrations.
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