Hanna alk checker or Red Sea

[Hanna Instruments]

Let me ask a different sort of question. If you have a reef tank with a total alkalinity of 7 dKH at a pH of 8.5, does your method give exactly the same result (7 dKH, within the claimed error) as when the alkalinity is 7 dKH and the pH starts at pH 7.7?

The total amount of CO2 is obviously very different, and that is known to impact the proper pH endpoint of aa total alkalinity titration, making any particular pH endpoint not exactly right for both cases.

But I'm wondering how it impacts your method which is not simply a pH titration.

TIA.

The method should be expected to perform similarly to the titration to a single set endpoint (that is, the more CO2 in the sample, the lower the alkalinity measured). The Hanna alkalinity reagents contain both an acid and an indicator which changes in intensity based on the pH. The best way to think of it is by putting a quantitative color measurement to the intensity of the indicator in a titration. Of course, in this method, we cannot compensate for changes in endpoint.

EPA 310.1 states that the titration goes to pH 4.5 (except for in rare cases). Standard Methods 2320B (AWWA, APHA, WEF) suggests changes based on alkalinity values. You are right - the pH value at which the equivalence point occurs changes based on the amount of CO2 in the solution. In the cases of typical instrumentation though, it’s challenging to pick a set pH value for all samples. The “best” way to determine alkalinity would be to determine the equivalence point directly, by way of an automatic (or manual) derivative plot of the pH over volume. This way, we can choose the inflection point regardless of the effect of CO2 on our samples.

 

[JimWelsh]

The “best” way to determine alkalinity would be to determine the equivalence point directly, by way of an automatic (or manual) derivative plot of the pH over volume. This way, we can choose the inflection point regardless of the effect of CO2 on our samples.

I concur 100%.

 

[Randy Holmes-Farley Verified Community Expert]

The method should be expected to perform similarly to the titration to a single set endpoint (that is, the more CO2 in the sample, the lower the alkalinity measured). The Hanna alkalinity reagents contain both an acid and an indicator which changes in intensity based on the pH. The best way to think of it is by putting a quantitative color measurement to the intensity of the indicator in a titration. Of course, in this method, we cannot compensate for changes in endpoint.

EPA 310.1 states that the titration goes to pH 4.5 (except for in rare cases). Standard Methods 2320B (AWWA, APHA, WEF) suggests changes based on alkalinity values. You are right - the pH value at which the equivalence point occurs changes based on the amount of CO2 in the solution. In the cases of typical instrumentation though, it’s challenging to pick a set pH value for all samples. The “best” way to determine alkalinity would be to determine the equivalence point directly, by way of an automatic (or manual) derivative plot of the pH over volume. This way, we can choose the inflection point regardless of the effect of CO2 on our samples.

Do you know how far off the alk measurement gets with the range of pH/CO2 values that reefers experience, say pH 7.7 to 8.5?

 

[Hanna Instruments]

Do you know how far off the alk measurement gets with the range of pH/CO2 values that reefers experience, say pH 7.7 to 8.5?

The margin of error in alkalinity measurements should not exceed that listed in the accuracy statement (±0.3 dKH ±5% of reading). This error of uncertainty is conservatively given to compensate for any deviation which might occur due to pH/Co2 values.

 

[Randy Holmes-Farley Verified Community Expert]

The margin of error in alkalinity measurements should not exceed that listed in the accuracy statement (±0.3 dKH ±5% of reading). This error of uncertainty is conservatively given to compensate for any deviation which might occur due to pH/Co2 values.

Great, thanks.

 

[Hanna Instruments]

I need to watch that to make sure I'm doing it correctly. That's one of the harder tests IMO...or easier to mess up.

For clarification, the values between the HI755-11 and HI772-11 are different. The HI772-11 is 5.0 dKH +/- 0.3 dKH while the HI755-11 is 100 ppm +/- 10 ppm CaCo3. These are not the same and we recommend sticking the the standard meant for its corresponding Checker.

 

[Chad_P]

The HI772 (dKH) and HI755 (ppm) Marine alkalinity Checkers utilize an absorbance curve for seawater and thus in order for the accuracy statement to be maintained its recommended to utilize full strength seawater for those two Checkers. We do have our HI775 Freshwater Alkalinity Checker which is based off of the colorimetric method but has a different range and different reagent. Also the HI775 needs our HI93755-53 Chlorine Remover Reagent if chlorine is present in the freshwater sample. The color change is also slightly different between the marine Checkers and the Freshwater Alkalinity Checker.

For the Marine our manual states:
Colorimetric method. The reaction causes a distinctive range of colors from yellow to green to greenish blue to develop

For freshwater our manual states
Colorimetric method. The reaction causes a distinctive range of colors from yellow to blue to develop. This meter has been developed to work with fresh water samples.

Although the meters are similar we recommend uses the HI755/HI772 with full strength seawater and the HI775 with freshwater samples.

Hope this helps, please reach out anytime! We'd love to hear from you!

I just purchased the alk standard checker for the 755 model...I'm consistently testing outside of the standard range of 90-110, at 87 and 88 after 3 readings. Does this mean my checker is bad?