DIY Alk test discussion thread

[Randy Holmes-Farley]

Thank you so much! Have ordered calibration weights for my scale, too. Will weigh 1000 grams, then remove 7ml before adding 5ml of the 1.0N NaOH to get the 14 dKH solution. (I found out that 1ml of water weighs a gram. I did not remember that!)

I'd measure the 5 mL with an old hobby syringe if you have one, rather than trying to look at 995 grams plus 5 grams as the error in the 5 mL could be big.

 

[DFW]

I'd measure the 5 mL with an old hobby syringe if you have one, rather than trying to look at 995 grams plus 5 grams as the error in the 5 mL could be big.

Yes sir! Will use my pharmacy syringe. Thank you very much!

 

[jdl513]

This is a tried and true laboratory method for determining alkalinity.
I would suggest purchasing a graduated cylinder to increase sample volume precision.

BRS has a 250 ML one listed for $7.99 that has plenty of room above the 250 ml mark for the HCl

I see that Granger has HCl at 0.1N in one liter bottles available.
Has anyone confirmed the actual purchase through Granger will be allowed to "residences"?
If it does would be good to go.

Upfront costs if you already have a reliable pH meter and old poly syringe: ~$50 for approximately 100 tests.
Price per test would drop considerably getting a larger 0.1N HCl bottle but with a 2 year shelf life it only makes sense if you plan on testing daily.

 

[Reefnjunkie]

Question for Randy;

I've been using an acid titration for a few years now and was curious about the end point of 4.5 you had in your article.

I think it was 3-4 years ago I saw the write up on RC and the dumbed down summary is to take 100ml of tank water, place your ph probe in the water and add 0.1 N sulfuric acid until the PH reads 4.2. You then multiple the mls of acid used by 2.8 and that is your KH

I use 3 mls and that tells me my KH is 8.4, I buy a gallon of acid for 40.00 and get around 1300 tests

Its great to read this method revisited, it's the most accurate way I've come across and for SPS keepers stability in KH is the most important factor

 

[Randy Holmes-Farley]

This is a tried and true laboratory method for determining alkalinity.
I would suggest purchasing a graduated cylinder to increase sample volume precision.

BRS has a 250 ML one listed for $7.99 that has plenty of room above the 250 ml mark for the HCl

I agree. A graduated cylinder would be great. Glad to hear one is readily available from BRS.:)

 

[Randy Holmes-Farley]

Question for Randy;

I've been using an acid titration for a few years now and was curious about the end point of 4.5 you had in your article.

I think it was 3-4 years ago I saw the write up on RC and the dumbed down summary is to take 100ml of tank water, place your ph probe in the water and add 0.1 N sulfuric acid until the PH reads 4.2. You then multiple the mls of acid used by 2.8 and that is your KH

I use 3 mls and that tells me my KH is 8.4, I buy a gallon of acid for 40.00 and get around 1300 tests

Its great to read this method revisited, it's the most accurate way I've come across and for SPS keepers stability in KH is the most important factor

This is a complicated question. The exact endpoint depends on the actual alkalinity. Higher alk leads to a lower endpoint. I have said 4.2 in the past, and almost did for this article. That might be best. Millero (in Chemical Oceanography) says pH 4.2 for normal seawater and I have no reason to doubt it. But many other sources say 4.5 Is a good general average for all waters, including several standard methods.

In reality, you might have a hard time distinguishing these endpoints anyway as the pH is changing so fast at that pH. If you look at my figure in the article, the pH dropped from about pH 4.6 to pH 4.1 with a single addition of 0.2 meq/L (0.5 dKH) so the difference in endpoint is less than that.

 

[Reefnjunkie]

This is a complicated question. The exact endpoint depends on the actual alkalinity. Higher alk leads to a lower endpoint. I have said 4.2 in the past, and almost did for this article. That might be best. Millero (in Chemical Oceanography) says pH 4.2 for normal seawater and I have no reason to doubt it. But many other sources say 4.5 Is a good general average for all waters, including several standard methods.

In reality, you might have a hard time distinguishing these endpoints anyway as the pH is changing so fast at that pH. If you look at my figure in the article, the pH dropped from about pH 4.6 to pH 4.1 with a single addition of 0.2 meq/L (0.5 dKH) so the difference in endpoint is less than that.

Thanks for the reply-I agree and have found the same regarding the drop from 4.5 (or even 4.6) to 4.2

This is a test for anyone looking for an "easy" and accurate method to test KH-I'd prefer math vs color interpretation any day

Thanks again for the info!

 

[Randy Holmes-Farley]

You're welcome and thanks.

Happy Reefing!

 

[H@rry]

Tested

OK, I got around to doing the test this afternoon. Here's the results:

Started out with a Salifert test kit - expires 7/19. It showed 9.7 alk

Calibrated my pH meter with 4.01 and 7.01 solution.

Got a new bottle of .1 N Hydrochloric acid.

Used a Pyrex measuring 1 cup measuring cup. Filled it to the 1 cup line as close as I could.
The pH in the tank water measured 8.35. I question that as I'm running a calcium reactor which usually drops the pH to around 7.7 or 7.8. One explanation is that the meter was calibrated with 4.01 and 7.01 solution as opposed to 7.01 and 10.01 which I would normally use. This put the actual pH out of range of the calibration fluids but I though that it would make it more accurate at the target pH of 4.5 that I would be shooting for.
I used a 5 ml syringe from an old test kit. Dumped the whole syringe in to start with as I could see from the chart that it would take more than that. I refilled the syringe and started adding .1 ml at a time while I was stirring with a spoon. I got to 6.9 ml total and the pH was reading 5.? (I forgot the number and didn't write it down because I didn't think it was significant at the time). I added another .05 ml which brought the pH down to 4.39. I was not expecting it to change that much so quickly. You would almost need an dropper at that point to get it right at 4.5 pH.

From my calculated extrapolations that would put the alk at 8.1.

The explanations for these findings are:

1. At least one of the methods are inaccurate.
2. I didn't measure the cup of water accurately. (looked good to me)
3. The pH meter was inaccurate.
4. The spoon was affecting the pH.
5. I incorrectly interpreted the change from purple to orange on the Salifert test.

My guess would be item 5 above. Anybody got any comments?

 

[JimWelsh]

I've worked with both the Salifert and also the Hach alkalinity burette method (http://tinyurl.com/pdztadq). Both methods use very similar (although not identical) color indicators (I substitute the Hach Bromcresol Green-Methyl Red indicator solution for the powder pillows). The nice thing about the Hach method is the use of the pH 4.5 buffer powder pillow that you can use together with the indicator to get the color you are trying to match in your titration. In my experience that color is actually more of an intermediate lavender color between the purple and the orange/pink. I have found that titrating to the orange/pink color is actually overshooting the endpoint. I have standardized my own acid for the titration, and have compared results between the 4 combinations of Salifert indicator, Hach indicator, Salifert acid, and my standardized acid, and on the same sample, can repeatably get results to within +/- 0.01 meq/L using any combination of the above, if I use that lavender color as the endpoint.

So, in other words, I agree that your item 5https://www.reef2reef.com/forums/usertag.php?do=list&action=hash&hash=5 above may very well be the issue.

 

[Randy Holmes-Farley]

With choice 2, what sort of measuring cup was it?

If you have any others, you might try those to see any difference, and average them.

I don't think 3 or 4 are very likely.

I've never done it, perhaps Jim has, but it would be very interesting for someone to measure and publish the color change of the dyes used in alk kits from pH 5.5 to pH 3.5 to see not only the "correct" color, but how much potential error there is by color.

 

[Randy Holmes-Farley]

Duplicate post

 

[Cory]

Just curious can I use a dye with an acid? I really don't like electronic testers, after finding my Apex conductivity probe to be prone to many errors I have doubts in the reliability of these electronics. I may try this one day with the ph meter.

 

[JimWelsh]

I've never done it, perhaps Jim has, but it would be very interesting for someone to measure and publish the color change of the dyes used in alk kits from pH 5.5 to pH 3.5 to see not only the "correct" color, but how much potential error there is by color.

That is a very good idea in principle, but the challenge here is getting faithful color reproduction, from the color temperature of the ambient light and background when taking the pictures, to the color balance of the camera in processing, to the color calibration of the monitor and/or printer being viewed by the end user.

I would suggest that for anybody using color indicators rather than a pH meter to do alkalinity titrations, the idea of using the Hach (or similar) pH buffer powder together with the indicator being used is a very viable way of guaging the exact color of the endpoint in the ambient light under which the test is being performed. FWIW, the Hach 4.5 pH powder pillows I'm talking about can be found here: Buffer Powder Pillows, pH 4.50, 50 mL, pk/25 | Hach USA - Details | Hach. Simply disolve the contents one of these in 50 mL of RO/DI water, then add an appropriate number of drops of the indicator you are using. The resulting color is the correct titration endpoint color, assuming an endpoint of pH 4.5.

 

[H@rry]

With choice 2, what sort of measuring cup was it?

Used a Pyrex 1 cup measuring cup. Filled it to the 1 cup line as close as I could.

 

[H@rry]

So, in other words, I agree that your item 5 above may very well be the issue.

I think and hope you're right. I never could convince myself that the color was orange yet, still a little lavender.

 

[JimWelsh]

Used a Pyrex 1 cup measuring cup. Filled it to the 1 cup line as close as I could.

Ideally, you should be using a reasonably accurate way of measuring volume, such as a graduated cylinder, syringe, pipette, or even a volumetric flask (depending upon the volume you're dealing with), paying proper attention to how to read the meniscus. Perhaps Pyrex measuring cups aren't accurate enough?

 

[jdl513]

Out of curiosity why not do a standard 3 point calibration? (10, 7 and 4 SU)
Typically would want the standards to bridge the full range of the measurements anticipated but I could be mistaken in this case.

 

[H@rry]

Out of curiosity why not do a standard 3 point calibration? (10, 7 and 4 SU)
Typically would want the standards to bridge the full range of the measurements anticipated but I could be mistaken in this case.

I was under the impression that with the PinPoint pH monitor you either used the 4 or 10 calibration fluids with the 7, not both.

 

[Randy Holmes-Farley]

Different pH meters have different requirements. The majority of meters require pH 7 as one of the standard, and then frequently either pH 4 or 10 for the other point of a two point calibration.

But some meters allow more points, do not require pH 7, and may allow entry of any pH values for standards, such as pH 8.41. I have one of those.