Alkalinity titration using methyl orange indicator

[WillpoleReefers]

I plan to try titration to measure saltwater alkalinity and as I don’t yet have a pH meter thought I might see how it went using an indicator. Still waiting on supply of standard 0.1N HCl so I haven’t tried this yet. Here in the UK an indicator TA4 is marketed for alkalinity testing, after ordering some I later learned this is just methyl orange. Reading up on methyl orange I see it should just start to change from yellow to orange at pH 4.4. It’s a fairly narrow range indicator though this is still above the pH endpoint when it would become red. So I guess the method is to titrate until the solution just becomes orange, at which point I assume we have a reasonably accurate measure of alkaline buffer neutralisation?

I am not sure how accuracy of the method would compare to a pH meter based titration, I imagine it is slightly reduced but probably is way better than using a test kit and counting drops? Does anyone have any measurements?

Equipment here is a 50ml burette and flask. My understanding based on reading here is that if I dilute the acid to 0.01N (I have volumetric flasks) and titrate 10ml seawater I would need a volume that equals alkalinty in mEq. Low for accurate use of a 50ml burette. I suspect it is best to either dilute the acid to 0.001N (introducing greater inaccuracy) or perhaps use 100ml samples with 0.01N HCl , giving me an approx 25ml acid requirement. Appreciate some expert checking of my logic here, the last titration I did was at school!

Steve

 

[KrisReef]

I plan to try titration to measure saltwater alkalinity and as I don’t yet have a pH meter thought I might see how it went using an indicator. Still waiting on supply of standard 0.1N HCl so I haven’t tried this yet. Here in the UK an indicator TA4 is marketed for alkalinity testing, after ordering some I later learned this is just methyl orange. Reading up on methyl orange I see it should just start to change from yellow to orange at pH 4.4. It’s a fairly narrow range indicator though this is still above the pH endpoint when it would become red. So I guess the method is to titrate until the solution just becomes orange, at which point I assume we have a reasonably accurate measure of alkaline buffer neutralisation?

I am not sure how accuracy of the method would compare to a pH meter based titration, I imagine it is slightly reduced but probably is way better than using a test kit and counting drops? Does anyone have any measurements?

Equipment here is a 50ml burette and flask. My understanding based on reading here is that if I dilute the acid to 0.01N (I have volumetric flasks) and titrate 10ml seawater I would need a volume that equals alkalinty in mEq. Low for accurate use of a 50ml burette. I suspect it is best to either dilute the acid to 0.001N (introducing greater inaccuracy) or perhaps use 100ml samples with 0.01N HCl , giving me an approx 25ml acid requirement. Appreciate some expert checking of my logic here, the last titration I did was at school!

Steve

I do titration fairly frequently but I just follow the instructions on my test kits.

The master @Randy Holmes-Farley wrote them?

 

[taricha]

It looked like it should work: chem.libretexts.org
But I haven't seen anybody use it for that purpose and I remembered the color change being pretty subtle with methyl orange.

I had some standard acid, Methyl Orange, and a calibrated pH meter nearby so I gave it a look.
It does shift color at the right pH, but the color shift is real subtle.

left is a couple of drops before the endpoint, right is just after the endpoint.

 

[KrisReef]

It looked like it should work: chem.libretexts.org
But I haven't seen anybody use it for that purpose and I remembered the color change being pretty subtle with methyl orange.

I had some standard acid, Methyl Orange, and a calibrated pH meter nearby so I gave it a look.
It does shift color at the right pH, but the color shift is real subtle.

left is a couple of drops before the endpoint, right is just after the endpoint.

Looks like it goes from "Dehydrated" to "Dangerously Dehydrated." This reminded me of why I struggled in chemistry lab. I have pispoor eyesight.

Cool set up, pics, and write up! @taricha

 

[WillpoleReefers]

It looked like it should work: chem.libretexts.org
But I haven't seen anybody use it for that purpose and I remembered the color change being pretty subtle with methyl orange.

I had some standard acid, Methyl Orange, and a calibrated pH meter nearby so I gave it a look.
It does shift color at the right pH, but the color shift is real subtle.

left is a couple of drops before the endpoint, right is just after the endpoint.

Hey thanks so much for making that demo! Yea the initial colour change does look subtle. Yet that is the pH we want to see. I will play once the acid arrives, Christmas post issues apply

Steve

 

[WillpoleReefers]

Looked at the curve in the link. Seems we are titrating a weak base with a strong acid , so the additional acid change with pH even down into the 3s is minimal. Looks promising even if the very first hint of orange is missed based on that info

Steve

 

[Randy Holmes-Farley]

Some test kits use two dyes to get a sharper and more obvious color change at the endpoint.

 

[Randy Holmes-Farley]

Looked at the curve in the link. Seems we are titrating a weak base with a strong acid , so the additional acid change with pH even down into the 3s is minimal. Looks promising even if the very first hint of orange is missed based on that info

Steve

I show the actual titration of seawater here:

A DIY Alkalinity Test: By Randy Holmes-Farley - REEFEDITION

Alkalinity is one of the most important measurements that a reef aquarist can make. It can become rapidly depleted in many aquaria, requiring frequent measurement in order to maintain stable levels.

www.reefedition.com

Figure 1. A pH titration of pure water and water from my reef aquarium using 0.1 N HCl.

 

[WillpoleReefers]

I show the actual titration of seawater here:

A DIY Alkalinity Test: By Randy Holmes-Farley - REEFEDITION

Alkalinity is one of the most important measurements that a reef aquarist can make. It can become rapidly depleted in many aquaria, requiring frequent measurement in order to maintain stable levels.

www.reefedition.com

Figure 1. A pH titration of pure water and water from my reef aquarium using 0.1 N HCl.

Ah thanks for the info Randy. So seawater is not exactly an ideal vertical drop in the titration curve. I can from the curve see you could significantly overuse acid if missing a pH in the 4s. I’m not yet sure exactly what’s in my “TA4” indicator the company were a bit brief in response to my message. I think the acid has arrived and I will have a play tomorrow. Bromocresol green also looks interesting in context of that pH range, it is a bit more expensive to acquire here though. Wonder what mixes well with methyl orange,

Steve

 

[taricha]

I was also going to suggest bromocresol green.
I suspect multiple hobby alk kits (blue-green-yellow) use that as the indicator
this looks much easier on the eye than yellow-orange to orange-yellow.
https://images.app.goo.gl/v8geHkV7qw8AVtQR8

 

[WillpoleReefers]

I was also going to suggest bromocresol green.
I suspect multiple hobby alk kits (blue-green-yellow) use that as the indicator
this looks much easier on the eye than yellow-orange to orange-yellow.
https://images.app.goo.gl/v8geHkV7qw8AVtQR8

Yes I saw that image. Just green would appear to indicate pH 4.5. Suppliers seem to be selling this one solid in quantities in excess of likely need. Will continue to search if the methyl orange one is tricky

Steve

 

[WillpoleReefers]

Hmm, wonder what is actually in my TA4 indicator! It certainly is not just methyl orange as claimed in the brief manufacturer reply I got. I titrated tankwater down with 0.01N HCl and got a fairly sharp switch from blue to pale pink at 21.7ml = 2.17 mEq, about right for old water no recent water changes I guess. This may be a very useful indicator. Pursuing the manufacturers as we speak. Though I might need a pH meter to make sense of things which kinda defeats the object! Colours below:

 

[WillpoleReefers]

Ah … if anyone reads this … reading around indicators revealed the truth. The original manufacturer reply re TA4 was clearly wildly wrong. This is not a methyl orange based product! It is now obvious this is bromocresol green/methyl red combo, which provides one of the narrowest colour change ranges possible and becomes fully pink at about pH 4.5. Seems perfect for alkalinity titration and I hope makes a pH meter pretty redundant for practical aquarist use. See the colour chart below

 

[taricha]

Mmmm... that'd be a nice indicator to have.

 

[WillpoleReefers]

Mmmm... that'd be a nice indicator to have.

Here’s the formulation of the Sigma Aldridge version, they seem to be dissolving the dyes in methanol, not sure how critical that is for lower strength solutions

 

[WillpoleReefers]

Seems the formulation of Bromocresol green/methyl red can be in methanol, ethanol or water

 

[Sral]

I show the actual titration of seawater here:

A DIY Alkalinity Test: By Randy Holmes-Farley - REEFEDITION

Alkalinity is one of the most important measurements that a reef aquarist can make. It can become rapidly depleted in many aquaria, requiring frequent measurement in order to maintain stable levels.

www.reefedition.com

Figure 1. A pH titration of pure water and water from my reef aquarium using 0.1 N HCl.

Curious thing: could the double shoulder with the tank water arrise from the different chemicals in the water ? Or is the first one around 7 something else ?

 

[Randy Holmes-Farley]

A seawater titration consumes carbonate and borate at high pH, and bicarbonate for pH below about 7.

 

[Sral]

A seawater titration consumes carbonate and borate at high pH, and bicarbonate for pH below about 7.

Thanks s as lot ! so my residual knowledge of chemistry hasn’t failed me completely it seems

Just trying to understand this and my terminology probably isn’t on point either:
1. at very high pH (above the „desired“ pH of the bicarbonate „buffer“) the bicarbonate will act as an acid and give up its hydrogen, becoming carbonate.
2. When lowering pH below the „desired“ pH of the carbonate, it will act as a base and take up a hydrogen, counteracting the lowering of the pH (e.g. buffering), becoming bicarbonate.
3. Same for bicarbonate: when lowering pH further, below the „desired“ pH of bicarbonate, it will take up another hydrogen, counteracting the lowering of the pH (e.g. buffering) and become carbonate acid.

So each flat part results from some chemical species taking up hydrogen and counteracting the addition of the acid.

Is that about correct ?

 

[Randy Holmes-Farley]

Yes. I give a full rundown on the chemistry changes with pH during an alk test in the same article as that graph.