What is the recommended procedure for running a standard for Chlorine using an Amperometric titrator?

Dokument-ID

Dokument-ID TE12889

Veröffentlichungsdatum

Veröffentlichungsdatum 07.04.2021
Frage
What is the recommended procedure for running a standard for Chlorine using an Amperometric titrator?
Zusammenfassung
Instructions for preparing and running a primary Chlorine standard on an amperometric titrator such as the AT1000 or AutoCAT9000.
Antwort
  1. Run two titrant purge cycles of the syringe/burette.
  2. Perform an electrode cleaning. (See: What are the cleaning instructions for the IntelliCAL MTC695 Double Platinum Electrode for AT/KF1000 RedOx Titrations? )
  3. While the electrode cleaning is in progress take that time to pretreat all the glassware for Chlorine demand.
    • Rinse all the beakers that will be used with tap water three times.
    • Rinse all the beakers with lab Deionized (DI) water three times.
  4. After the Cleaning is done then run 3 blanks. These blanks can be done with lab DI water. It does not need to be Chlorine demand free, just Chlorine free. Run this on the low range Total Chlorine program regardless of the program that is being checked using the standard. (Even if you are running a Free Chlorine standard, or checking a higher range Total Chlorine program, run your blanks on the low range Total Chlorine). This is for the probe to make sure the probe is cleaned and conditioned.
    • The expected result for the blanks is 0.000 or below detectable limit (BDL). An error of flat signal is fine, but an insufficient curve resolution error is not. In the case of an insufficient curve resolution error, or a result greater than 0.000 after three blanks, run through another cleaning cycle and run 3 more blanks. Repeat as needed until at least the last of the three blanks gives you a BDL result.
  5. Prepare your standard.
    • Rinse the beaker 2x with lab DI. Then a 3rd time with Organic Free water. Not much Organic Free water is needed, just enough to swirl, then rotate the beaker while slowly pouring out the rinse water to ensure the walls of the beaker are also rinsed.
      • If a source of reagent water that has gone though a treatment system that includes organic removal is not available, Organic Free dilution water can be purchased: Dilution Water, Organic Free, 500 mL Product # 2641549.
    • Fill the beaker with approximately 200 mL of Organic free water. (This does not need to be precise, it's ok to be approximate based on the volume graduation on the beaker)
    • Put the beaker with Organic free water onto a balance and add the additional reagent at this time (Phosphate buffer for Free Chlorine, Acetate buffer and KI for Total chlorine). Then zero the balance. The balance needs to have a max weight of greater than 300 g and should have a resolution of 0.01 or greater (the better the resolution the more precise the nominal concentration will be.)
    • Best practice is to make sure that the standard stays cold. The colder the standard is the less Chlorine is lost to volatility. So it would be recommended to make a shallow ice bath the ampule can be in with the volume level of the ice bath outside of the ampule only going up about half way. This is to ensure that none of the ice bath water can contaminate the solution in the ampule. Wait until just before the standard is pulled from the ampule to break it open.
      • The recommended ampules to prepare standard from can be purchased: Chlorine Standard Solution, 50-75 mg/L as Cl2, pk/16 - 10 mL Voluette Ampules (NIST) Product # 1426810.
    • Using a sterile transfer pipet, fill the pipet with standard from the ampule and dispense directly into a sink. This is to remove what little chlorine demand there may be from the pipet itself.
      • Sterile transfer pipets can be purchased: Pipet, Sterile, Transfer, 15/pk Product # 2232512
    • Calculate the correct amount of standard to add, then using the sterile transfer pipet add solution down to drop wise to beaker on balance until the desired standard volume is displayed as g.
      • Example: If the desired standard concentration is ~0.2 mg/L Cl2, and your COA value for the lot of standard is 68.5 mg/L Cl2. The final volume used in the result calculation is 200 mL.
        • Volume of standard to use = (desired concentration x 200)/COA value.  
        • Volume of standard to use = (0.2*200)/68.5= 0.58
        • Based on this we would round to the nearest 0.1 and would want to use near 0.60 g of standard added to the beaker.
  6. Run analysis on the prepared standard.
    • Put the beaker on the titrator. Add the stir bar (without touching the stir bar. Best practice is to rinse the stir bar, blot it dry using a paper towel or wipe, then while holding the stir bar with the towel/wipe add it to the beaker)
    • While the standard is running use the standard amount in g from when the standard was prepared to calculate a nominal concentration.
      • Nominal concentration = (Weight of standard added as measured by balance * COA value)/200.
      • Example: Following the example values from the standard preparation step, if during the preparation the weight displayed on the balance when the beaker was removed was 0.61 g.
      • Nominal concentration= (0.61*68.5)/200= 0.209 mg/L Cl2
    • Once the measurement is complete, calculate recovery.
      • Percent Recovery = (result/nominal concentration)*100
      • Example:Following the previous examples, if the result of the analysis was 0.199 mg/L Cl2
      • Percent Recovery = (0.199/0.209)*100 = 95.2%
  7. Transition steps between replicates.
    • Lift electrode head and set beaker to the side. Rinse the electrodes with lab DI and blot dry.
    • Using a stir bar retriever (not hands) remove the stir bar from the beaker and pour the solution into a waste container. Then put the stir bar back into the beaker.
      • Stir bar retrievers can be purchased: Stir Bar Retriever Product # 1523200
    • Using a wash bottle with lab DI, rinse the stir bar and walls of the beaker. Then swirl the stir bar in the rinse water. Then remove the stir bar with the stir bar retriever and set on paper towel/wipe.
    • Pour the rinse water into the waste container as well. Then rinse an additional time with lab DI. Then rinse the beaker an additional time with the organic free water rotating the beaker while slowly pouring out the solution to rinse the walls of the beaker.
    • Then fill with ~200 mL of organic free water to prepare next standard.
  8. If determining a Method Detection Limit (MDL), the MDL is calculated by standard deviation, not by recoveries. The variation between replicates is what is used to calculate the MDL. It has little to nothing to do with the standard concentration being used in terms of how high or low the standard is. The more variation between replicates the higher the MDL will be, and the less variation between replicates the lower the MDL will be. It's possible to get a falsely low MDL by getting consistent results on a lower range standard. And a higher range standard may have more variance and therefore running a higher concentration standard can be better for performing an MDL. The standard concentration needs to be within 2-10 times the concentration of the EDL.
This procedure is specific to titrations and could not be used for colorimetric determination. The Benefit of this method for standard preparation is that only one volume measurement matters, and that's the volume of standard added to the beaker. This volume is measured by weight which is more accurate of a way to measure volume than with a pipette or other volume graduation. The error from the accuracy of volume measurement in a volumetric flask to prepare the standard is removed. The accuracy of volume measurement of sample that is titrated is removed. The titration is a moles to moles reaction with the mg of titrant and the mg of Chlorine. The only place volume comes into play to calculate the concentration is in the 200 mL sample volume (which is assumed to be accurate even if in preparation it isn't), the volume of titrant to reach the end point (measured by the titrator during the titration), and the concentration of the titrant. The actual final volume in the beaker does not matter if we add the standard directly to the beaker instead of transferring a volume of prepared standard from a volumetric flask.

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