Making Sulfuric 0.1N from 2N

[Garf]

This will be easy for some of you I'm sure. Ive got some Lab Sulfuric 2N that I have diluted 1:18 with RODI by weight, not volume (not got any accurate volume glassware and the internet seemed to think 1:18 was right, not the expected 1:19). I'm thinking I've made it a bit thick and 1:19 was right because using it i'm getting around 5 to 10% difference in alkalinity using the pH meter titration method as opposed to Salifert Alk test. I know 5 or 10% isn't really a big deal, but I may as well make it right.
Thanks for reading

 

[Randy Holmes-Farley Verified Community Expert]

1:19 is correct by volume, and weight is pretty close since the density of 2 N sulfuric is about 1.05 g/mL.

By weight you’d use

1.05 g 2N sulfuric
19 g water

 

[Garf]

1:19 is correct by volume, and weight is pretty close since the density of 2 N sulfuric is about 1.05 g/mL.

By weight you’d use

1.05 g 2N sulfuric
19 g water

Thank you Randy.

 

[Garf]

On the same topic, but different, I've charted my titrant additions against pH and seem have have an equivalence point of near 4pH, which is quite a bit lower than I expected. Freshly calibrated probe, 4&7.

 

[Randy Holmes-Farley Verified Community Expert]

On the same topic, but different, I've charted my titrant additions against pH and seem have have an equivalence point of near 4pH, which is quite a bit lower than I expected. Freshly calibrated probe, 4&7.

The endpoint is just above 4. Seems right. I show my graph here:
https://reefs.com/magazine/chemistry-and-the-aquarium-what-is-alkalinity/

 

[Garf]

The endpoint is just above 4. Seems right. I show my graph here:
https://reefs.com/magazine/chemistry-and-the-aquarium-what-is-alkalinity/

Thanks again. I've seen that chart countless times over the years and just assumed it was near 4.5, as that's the recommended endpoint for the Alk test using a pH meter. That little tiny amount of added extra acid would ensure maximum responsiveness of the pH probe presumably, and give a higher accuracy?

 

[Randy Holmes-Farley Verified Community Expert]

Thanks again. I've seen that chart countless times over the years and just assumed it was near 4.5, as that's the recommended endpoint for the Alk test using a pH meter. That little tiny amount of added extra acid would ensure maximum responsiveness of the pH probe presumably, and give a higher accuracy?

There was a thread here a while back where Jim Welsh and I and others discussed the endpoint, which seems to vary somewhat by author. It will vary a small amount by both the actual alk, the CO2 level, and whether any or all of the CO2 generated during fhe titration is allowed to
leave the solution or not. There are also different pH scales sometimes used in seawater vs fresh.

The true endpoint is the inflection point in the curve, assuming it is determined correctly.

That said, this sort off discrepancy is what happens when one looks behind the curtain on the nitty gritty details of chemistry where the simple assumptions that are usually made begin to break down.

Don’t be discouraged. I expect it happens in nearly all complicated situations in all walks of life that get simplified for every day use. lol

 

[Garf]

The true endpoint is the inflection point in the curve, assuming it is determined correctly.

Understood

That'll be right around here then at 4.5, lol.

 

[Randy Holmes-Farley Verified Community Expert]

That fits with Jim’s Comment, and there is more discussion at the link:

Over the last few years, I've done numerous alk titrations using 100 mL of sample and 0.01N HCl as the titrant. I'm using a just-calibrated pH probe, and am taking data points throughout the titration, and determining the endpoint via the inflection point of the third-order polynomial of the curve. For a typical titration, where the sample is between 8 and 9 dKH, it takes between 29 and 32 mL of titrant, so the sample is being diluted by the titrant in a way that is not insignificant, which does affect the pH of the endpoint. The pH of the inflection point is typically right at 4.41, with a standard deviation of around 0.01 to 0.02 pH units
https://www.reef2reef.com/threads/diy-alkatronic-reagent.639360/page-15#post-11028054

 

[Garf]

That fits with Jim’s Comment, and there is more discussion at the link:

Over the last few years, I've done numerous alk titrations using 100 mL of sample and 0.01N HCl as the titrant. I'm using a just-calibrated pH probe, and am taking data points throughout the titration, and determining the endpoint via the inflection point of the third-order polynomial of the curve. For a typical titration, where the sample is between 8 and 9 dKH, it takes between 29 and 32 mL of titrant, so the sample is being diluted by the titrant in a way that is not insignificant, which does affect the pH of the endpoint. The pH of the inflection point is typically right at 4.41, with a standard deviation of around 0.01 to 0.02 pH units
https://www.reef2reef.com/threads/diy-alkatronic-reagent.639360/page-15#post-11028054

Todays ridiculously tedious Alkalinity test result, lol;

 

[Randy Holmes-Farley Verified Community Expert]

Nice. Not sure what alk that translates to, but it makes it clear the difference is only about 1%, which may be less error than most people would get from the tank water volume measurement. :)

 

[Randy Holmes-Farley Verified Community Expert]

I have been taking mine down to 4.2. Might be a slight overestimate of alk, but it would be consistent. Next time I do it I’ll keep track of how much difference I see in the low 4’s.

pH calibration and measurement errors may also contribute to these small errors. I don’t generally recalibrate my pH meter for this purpose, and since I calibrate at 7 and 10 for tank measurement, the pH 4 range is less accurate.

 

[Garf]

Nice. Not sure what alk that translates to, but it makes it clear the difference is only about 1%, which may be less error than most people would get from the tank water volume measurement. :)

I'm using a regular weighed 287 grammes sample, so 1ml of titrant equals 1 DKH +or- whatever errors I have going on.