Reef Chemistry Question of the Day #204 Alkalinity Testing

[Randy Holmes-Farley]

Hmm...option 3 appears to best describe the chemistry that causes a shift in the equilibria between the dissociated and protonated forms of the indicators causing a color change, but missing is any description of the chemistry associated with the titration...and I assume an "ordinary" test kit utilizes a tritration.

Yes, ordinary alkalinity titration kits used by hobbyists.

 

[Bruce Burnett]

3

 

[TL Reefs]

I have a related question. I have seen two articles written by you about DIY alkalinity testing. One written around 2002 and the other around 2015. One says the target pH is 4.5 and the other article says 4.2. Should I be concerned about which is right? Does it really matter? Yesterday I started testing alk with hydrochloric acid instead of my usual Salifert kits, and they don't match at all, but it is much closer if I use 4.2 as my target pH. Salifert reads 8.9 and DIY reads 8.0 or 8.4 depending on whether I go to 4.5 or 4.2 pH.

 

[Rick Mathew]

#3

 

[Mindi]

number 4

 

[Kayvon]

1

 

[madweazl]

Yes...

 

[Ridgeway]

Non of the above. Based on 30yrs of using around 10 brands of Alk test I conclude that the process is based on pure wizardry and is as accurate as weather forecasting, some how OK although often considerably out.....

 

[Alfrareef]

Non of the above. Based on 30yrs of using around 10 brands of Alk test I conclude that the process is based on pure wizardry and is as accurate as weather forecasting, some how OK although often considerably out.....

Agreed. Same wizardry goes for several other things in reef chemistry and RHF his like Merlin the wizard....

 

[Ridgeway]

Agreed. Same wizardry goes for several other things in reef chemistry and RHF his like Merlin the wizard....

I have an imagine in my mind of an "experienced" Gentleman, with a pointy stick that talks to test kit vials and things just happen.

OK back on topic.

Are the 4 options genuinely the only way that we could measure Alk in hobby kits ?

 

[Randy Holmes-Farley]

I have a related question. I have seen two articles written by you about DIY alkalinity testing. One written around 2002 and the other around 2015. One says the target pH is 4.5 and the other article says 4.2. Should I be concerned about which is right? Does it really matter? Yesterday I started testing alk with hydrochloric acid instead of my usual Salifert kits, and they don't match at all, but it is much closer if I use 4.2 as my target pH. Salifert reads 8.9 and DIY reads 8.0 or 8.4 depending on whether I go to 4.5 or 4.2 pH.

It is a surprisingly difficult thing to pin down exactly, for reasons I'm not sure of. I'd aim for 4.2 or a bit higher.

We recently discussed it in detail here, starting at post 5:

https://www.reef2reef.com/threads/salifert-kh-alk-testing-help.311941/#post-3840976

 

[Bruce Burnett]

I change my answer to 1

 

[TL Reefs]

It is a surprisingly difficult thing to pin down exactly, for reasons I'm not sure of. I'd aim for 4.2 or a bit higher.

We recently discussed it in detail here, starting at post 5:

https://www.reef2reef.com/threads/salifert-kh-alk-testing-help.311941/#post-3840976

Thank you!

 

[Randy Holmes-Farley]

And the answer is...

3. There is a dye in the test that can bind to a proton (H+) when enough protons are present in solution, and which gives a different color when bound in that way. You add acid (H+), which converts the dye to a different color. When you have added enough H+ to bind to a sufficient number of the dye molecules, the color changes and you are done.

Explanation:

Total alkalinity in a marine aquarium is simply a measure of the amount of acid (H+) required to reduce the pH to the carbonic acid endpoint (and hence is "sort" of a measure of the bicarbonate and carbonate present; the acid needed approximately equals the bicarbonate concentration plus twice the carbonate concentration, plus a few minor things like borate).

That endpoint is defined as the pH as which all of the carbonate and bicarbonate are converted into carbonic acid. That pH is around pH 4.2 or so.

CO3-- + 2H+ --> H2CO3
HCO3– + H+ --> H2CO3

In an alkalinity test kit, there is a dye (or more than one dye, often) that changes color at or near this pH. Many, such as the Hach alkalinity test kit, use bromcresol green and methyl red together:

The transformations happening to these dyes as H+ (acid) is added (pH is lowered) are shown below:

Bromcresol green:
https://en.wikipedia.org/wiki/Bromocresol_green
yellow at lower pH<-------------------------------> blue at higher pH

methyl red:

https://en.wikipedia.org/wiki/Methyl_red

yellow at higher pH <----> red at lower pH
(note that this picture is oriented in the opposite direction from the one above. Low pH on the right for methyl red instead of low pH on the left for bromcresol green. I'm just using what's available without redrawing))

So as you add H+ in the titration, the dye molecules combine with some of the H+ added, changing the color of the solution when a sufficiently high fraction of them have become protonated.


Happy Reefing.

 

[beaslbob]

3

the other three are all interrelated.

But thehn what do I know.

Hey

I got this one right for all the wrong reasons. ;Muted

 

[Randy Holmes-Farley]

Hey

I got this one right for all the wrong reasons. ;Muted

Better to be lucky than smart. lol

 

[JimWelsh]

OK, we can discuss that later. lol

My issue is a minor one with the wording. I would rephrase it to be something like (my edit in italics), "There is a dye in the test that can bind to a proton (H+) when enough protons are present in solution, and which gives a different color when bound in that way. You add acid (H+), which converts the dye to a different color. When you have added enough H+ to leave enough free H+ after reacting with the carbonate and bicarbonate to bind to a sufficient number of the dye molecules, the color changes and you are done." You may have considered my edit to be implied, but I think it should be elaborated.

 

[flagg37]

Am I right in thinking that this does not apply to the way the Hannah checker works?

 

[SashimiTurtle]

Am I right in thinking that this does not apply to the way the Hannah checker works?

I'm wondering this also as the Hanna checker has a different color each time and reads the color as the total Alk... Wait maybe when you add 1ml of the reagent, the acid and dye react to give a certain color left for the amount of bicarbonate left in your sample. You're adding a fixed amount of acid, so the color would be different. It's blue for high alk and green for low.

 

[Randy Holmes-Farley]

Am I right in thinking that this does not apply to the way the Hannah checker works?

It is a little different but close. It adds a bunch of acid and effectively measures pH via a dye (or more than one) after the single shot of acid.