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We have reports that alkalinity measurements made with reagent taken from a previously used bottle of the Hanna alkalinity reagent differ from the measurement obtained with reagent taken from a newly opened bottle. Also reported is that this discrepancy is eliminated when the reagent is refrigerated between uses. I was unable to reproduce this effect with an eight month stability test. Before getting any deeper into this discrepancy, I performed a series of stress tests on a newly open bottle of reagent in an attempt to make the reagent misbehave. I looked at how evaporation, overcharges, and heat affected alkalinity measurements. The data is presented below.
The first measurement is the control. The reagent from a newly opened reagent bottle was used to messure the alkalinity of Instant Ocean. From this measurement I calculated Hanna’s expected range in value for the measurement and plotted it as two red lines on the plot. The next test was just putting 1.5 mL of reagent in the evaporating dish and then immediately using it. This shows that moving the sample in snd out of the evaporating dish does not change the reagent. The subsequent samples with increasing amount of percent evaporation seemed to show a an upward trend in alkalinity. Only at 19% evaporation do I feel comfortable saying that I am not observing noise in the data (this seems to say that the Hanna Checker measurement is insensitive to reagent evaporation).
I also looked at the effect of screwing up the reagent addition. In the first screw up I added an extra 0.1 mL of reagent (slight effect) and the second I really screwed things up by undercharging the sample by 0.5 mL and adding an extra 0.5 mL of reagent. In the latter case I saw a strong effect, but not necessarily large enough to notice during a routine measurement, maybe.
In the last test, I placed the reagent in boiling water for 10 minutes. After cooling the sample to room temperature and confirming no evaporation occurred, I performed an alkalinity measurement. There was no effect.
Question: which real life variable(s) are my stability and stress studies missing that exist in the real life situations where aquarists observe reagent instability?
The first measurement is the control. The reagent from a newly opened reagent bottle was used to messure the alkalinity of Instant Ocean. From this measurement I calculated Hanna’s expected range in value for the measurement and plotted it as two red lines on the plot. The next test was just putting 1.5 mL of reagent in the evaporating dish and then immediately using it. This shows that moving the sample in snd out of the evaporating dish does not change the reagent. The subsequent samples with increasing amount of percent evaporation seemed to show a an upward trend in alkalinity. Only at 19% evaporation do I feel comfortable saying that I am not observing noise in the data (this seems to say that the Hanna Checker measurement is insensitive to reagent evaporation).
I also looked at the effect of screwing up the reagent addition. In the first screw up I added an extra 0.1 mL of reagent (slight effect) and the second I really screwed things up by undercharging the sample by 0.5 mL and adding an extra 0.5 mL of reagent. In the latter case I saw a strong effect, but not necessarily large enough to notice during a routine measurement, maybe.
In the last test, I placed the reagent in boiling water for 10 minutes. After cooling the sample to room temperature and confirming no evaporation occurred, I performed an alkalinity measurement. There was no effect.
Question: which real life variable(s) are my stability and stress studies missing that exist in the real life situations where aquarists observe reagent instability?
I’m going try that for all my Hanna reagents.
