Is it possible to use a digital titrator for Alkalinity, Acidity, or Hardness for samples above or below the ranges listed in the methods?


Dokument-ID TE7769


Veröffentlichungsdatum 29.06.2020
Is it possible to use a digital titrator for Alkalinity, Acidity, or Hardness for samples above or below the ranges listed in the methods?
Ways to make adjustments to range options for digital titrator methods.
These two important variables lead to the upper recommended ranges for Alkalinity, Acidity, and Hardness digital titrator methods:
1. The multiplier. The highest range (1,000 - 4,000 ppm (parts per million)) of either method uses a mutiplier of 10 (implied units of ppm/ digit). Generally speaking the closer to 1 ppm/ digit the multiplier is, the more accurate the final result. This is because any percent error in the digit measurement is multiplied by the multiplier as well when calculating the final result. The digital titrator itself has an error of ±1% (see What is the accuracy of the digital titrator? ). Meaning that for a range with a multiplier of 1 ppm/ digit, a 1% error in digit measurement is equal to a 1% error in result. Whereas with a multiplier of 10 ppm/ digit, a 1% error in digit measurement is equal to a 10% error in result. Using the titrant concentrations listed in the methods (which are the highest concentrations that Hach offers in digital titrator cartridges), to increase the range, the sample volume would need to be decreased, which will increase the multiplier to be above 10 ppm/ digit. This would give an error of over 10%, which for most customers is unacceptable and certainly not recommended. The only way to increase the range while not also impacting accuracy would be to both decrease sample volume as well as increase the titrant concentration.
2. The volume of titrant required to reach the end point. There is no true upper limit to any titration range. The only limit is the amount of titrant and time that is available to reach the end point. It's technically possible to get to a high concentration using the sample volume, titrant concentration, and multiplier of the lowest range listed in the method. But it would take a very long time and consume a vary large amount of titrant which makes it far less convenient than following the instructions for the higher range. Looking at the range tables from the methods and doing some quick math shows that the upper limit of each range option is equivalent to 400 digits. This is not a coincidence. Any more than that and the test would take longer to do, and more titrant would be consumed per test causing customers to go through titrant cartridges much more frequently. It's not recommended to use more than 400 digits per test.
Here are the options for testing samples above the recommended range and their trade offs:
1. Dilution. Samples can be diluted and the results can then be multiplied by the dilution factor. The trade off is the additional error in the process of dilution itself as well as (just as with the multiplier as explained above) understanding that all error in the result will be multiplied by the dilution factor. It also increases the time per test by introducing additional steps and requires additional apparatus to do the dilution.
2. Running it as is, following the instructions for the highest range in the method. The trade off here is that the tests will take longer to run and more titrant will be used. For example, 10,000 ppm would be equivalent to 1,000 digits when using the highest range that has a multiplier of 10 ppm/ digit, which is a considerable amount higher than recommended (more than double). However, The accuracy will be the same. If the sample concentration is close to the recommended upper limit of the test, than this option is a great choice. However, if your sample concentration is much higher than the upper range of the test, this option is not recommended. 
3. Use a smaller volume.  As an example, a  500 - 2,000 ppm range has a multiplier of 5 ppm/ digit and a sample volume of 20 mL using the higher concentration titrant available in a cartridge. Using simple ratio math, if 2 mL of sample volume and the same higher concentration cartridge is used, the multiplier would be 50 ppm/ di git and the range would be 5,000 - 20,000 ppm. The trade off is the multiplier of 50 ppm/ digit (instead of 10 ppm/ digit as described above) would make the result 5x less accurate.
4. Use a smaller sample volume as well as a higher titrant concentration. This will be the most accurate solution, but requires the most work. The higher concentration titrant (as stock solution in a bottle) would have to purchased, as well as empty cartridges (See What is the catalog number for an empty digital titrator cartridge? ). These cartridges would need to be filled manually, and the math would need to be done to figure out what the multiplier would be using this custom titrant concentration. The math is simple ratio math based on the titrant concentrations and sample volumes given in the tables in the manual (like shown above), but when using a custom titrant concentration, the multiplier at the end will not be a nice round number.

Important note for samples below the recommended range:

The same arguments and examples cannot be said for concentrations that are below the recommended lower limit as dilution and using more titrant are not viable options. However, a lower concentration titrant in an empty cartridge and higher sample volumes can be used to get results below the recommended lower limit. This is done at a higher risk than getting results above the recommended range because lower concentration titrants are less stable than higher concentration titrants and will inherently have lower accuracies. Also, using higher sample volumes can cause problems with the volume capacity of the vessel the test is done in, as well as limits to the volume capacity of the indicator reagents.

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