Upcoming article: DIY pH buffers

[taricha]

My sense is that most pH probes are a calibrate 2-point and go. But shouldn't it be normal practice to have 3 standards? 2 points of calibration (say, 7, and 10) and a 3rd in between (8 or 9 or whatever) to check if the probe is functioning correctly.
The reasoning is that I have taken all kinds of neglected and non-functional probes - and you can make them do a 2 point calibration. You don't realize they aren't working until you check a 3rd standard.

 

[SDchris]

How consistant would the pH be from the left over of a Salifert Alk test?

 

[bakbay]

I almost jumped off my chair when I read this thread thinking of a magical DIY “pH buffer” that will peg pH to 8.3 w/o altering alk! It’s like a dirt cheap soda lime, CO2 scrubber media, that will last forever…now, that would change this reefing hobby forever! lol

Now that I’m sitting down, I’m game for a cheap DIY 4,7,10 calibration fluid as well. Thanks for leading this effort!

 

[Randy Holmes-Farley]

How consistant would the pH be from the left over of a Salifert Alk

I would not use it. The pH is around 4-4.5, but it has almost no buffering and will move around easily.

 

[Randy Holmes-Farley]

My sense is that most pH probes are a calibrate 2-point and go. But shouldn't it be normal practice to have 3 standards? 2 points of calibration (say, 7, and 10) and a 3rd in between (8 or 9 or whatever) to check if the probe is functioning correctly.
The reasoning is that I have taken all kinds of neglected and non-functional probes - and you can make them do a 2 point calibration. You don't realize they aren't working until you check a 3rd standard.

That’s a good point. A buffer in between can be useful.

 

[Smarkow]

Yes, that is correct.

So one can use it two ways (or something in between):

1. Use whatever standard you have been, and make more calibration standard fluid at close to its accuracy, whatever it is, at a much reduced cost.

or

2. Use a very high quality standard, such as the ones I linked, and then have inexpensive secondary standards that are close, but obviously not quite as accurate.

My recommendation in either case is to make multiple bottles of standards at once and seal them up and use them for a long time with some bottles maybe not opened for a year or more before first use, rather than repeatedly making new standards off the old standards. The standards will certainly drift over time with each generation.

In the past, this is what I actually did:

1. I had multiple 500 mL bottles of expired standards my old lab got rid of.

2. Every once in a while, I'd get new packets of pH standard, calibrate, then measure all of those old bottles for ph, and I relabeled them to the new level. Some had many pH revisions written on them, along with a date, and the higher pH fluids would tend to drift down over time. Since my pH meter allowed any value for calibration, the pH 10 ones, for example, were entered during calibration as 9.92 or 9.86 or whatever they might actually be.

The neurotic part of me (probably 60%?) would need to graph that degradation over time just to see the pretty curve on an excel sheet

I’m sure you’ve never done that yourself!

 

[Randy Holmes-Farley]

A Comparison of pH Calibration Buffers by Randy Holmes-Farley - Reefkeeping.com

FWIW, I showed in this article a while back (20 years) that some commercial buffers that i bought online and from an lfs were seriously off. Best to use reliable brands for pH calibration.

One pH 10 was only 9.01, and some pH 7 were in the pH 7.5 range. One pH 4 buffer was pH 6.1.

 

[Malcontent]

There are powder packets and even tablet forms of pH buffer with extended shelf lives.

I use Lovibond tablets which create 20 mL of buffer each at a cost of $0.20 or less if you buy 250 qty. I'm sure there are random brands on Amazon that are cheaper though.

As a semi-DIY option these might be a good compromise between liquid and DIY buffers for some.

 

[kisember]

Isn't it possible to dilute a saturated solution with a known pH to a desired pH with simple volumetric measure?
I mean 'add 10 ml saturated kalkwasser to 22 ml of ro/di to get 7/8/9... pH'. Maybe 2 different saturated solution, if the water is wrong behaving in such scenario due to unreliable pH.

I almost jumped off my chair when I read this thread thinking of a magical DIY “pH buffer” that will peg pH to 8.3 w/o altering alk! It’s like a dirt cheap soda lime, CO2 scrubber media, that will last forever…now, that would change this reefing hobby forever! lol

Like the COF-999?

 

[Randy Holmes-Farley]

Isn't it possible to dilute a saturated solution with a known pH to a desired pH with simple volumetric measure?
I mean 'add 10 ml saturated kalkwasser to 22 ml of ro/di to get 7/8/9... pH'. Maybe 2 different saturated solution, if the water is wrong behaving in such scenario due to unreliable pH.


Like the COF-999?

A pH calibration buffer really needs to have some buffering capacity at the pH of interest, or small amounts of acids and bases will cause it to move around. Simple dilution of a hydroxide solution can attain any pH above 7, but its not really suitable for calibration.

Just as an fyi, remember too that pH is logarithmic, so 1 mL kalk to 10 mL water will give a pH in the 11's, 1 mL to 100 mL will give a pH in the 10's, 1 mL to 1000 mL will give a pH in the 9's, etc.

 

[Miami Reef]

I’m interested in a standard that is around 8 pH so I can check my probe’s accuracy without wasting my more expensive pH calibration packets.

I can use my calibrated probe to make the standard. That would be useful for me.

 

[Miami Reef]

For any pH desired from 9.2 to 10.6. Best buffering at ~ pH 10. Start with 0.2 M sodium bicarbonate (16.8 g/L, exact value not critical) and slowly add sodium carbonate to it (could be dry solids or a solution of similar strength) until the pH reaches pH 10 (or whatever buffer pH is desired). Done, Bottle it up and enjoy.

I might try this one. I’ll report back on my results. I think the most challenging part is waiting for my probe to adjust the reading. Sitting in the solution, it takes about a minute to pop a new reading. I worry about chasing a moving target, lol.



Could someone use sodium hydroxide instead of sodium carbonate? Just curious. I have both. I think hydroxide will be harder to fine-tune because the solids are much more concentrated.



Last question. If I weigh the amount of sodium carbonate to precisely 10 pH, can I use that recipe repeatedly without needing to test it?



For example, I added 16.8g of sodium bicarbonate and found that 10g of carbonate reads to pH 10. Assuming the carbonate will be as dry as the Sahara desert, is that recipe repeatable for me and others?

 

[Randy Holmes-Farley]

Could someone use sodium hydroxide instead of sodium carbonate? Just curious. I have both. I think hydroxide will be harder to fine-tune because the solids are much more concentrated

You could, but the buffering (that is, the ability of the solution to hold pH against changes in acid or base such as CO2 entering or leaving) will be less since there is less HCO3-/CO3-- in the final solution.

I might try this one. I’ll report back on my results. I think the most challenging part is waiting for my probe to adjust the reading. Sitting in the solution, it takes about a minute to pop a new reading. I worry about chasing a moving target, lol.

The time needed to stabilize a pH reading is a frustrating aspect, and may depend on the probe as well as how strongly the solution is buffered.

Last question. If I weigh the amount of sodium carbonate to precisely 10 pH, can I use that recipe repeatedly without needing to test it?


For example, I added 16.8g of sodium bicarbonate and found that 10g of carbonate reads to pH 10. Assuming the carbonate will be as dry as the Sahara desert, is that recipe repeatable for me and others?

That would work, but buffers with only a single chemical added are typically less stable than those with two. Reason being one holds the pH from dropping and the other holds it from rising. If you use just carbonate, there will be only a little bicarbonate to do its job. The kalk buffer gets around this by being saturated and the dissolution or precipitation of calcium hydroxide holds the pH steady.

 

[Miami Reef]

That would work, but buffers with only a single chemical added are typically less stable than those with two.

Thanks for your responses. I was trying to ask if I start with 0.2M bicarbonate and weigh the amount of carbonate needed for 10 pH, would this recipe be repeatable for me and other users?

 

[Randy Holmes-Farley]

Yes, if you measure both parts of what you do once, and the materials did not change for some reason, you can do that same thing again and get the same pH.

That's the basis for making standards from fixed weights of two ingredients. I expect nearly all commercial buffers we buy are made that way, unless they claim to be NIST traceable, which may mean going back and checking them for pH against some sort of primary standards (don't know what they use).

 

[scotty333]

Can 7 and 10 bd mixed to form 8.5?

 

[Randy Holmes-Farley]

Can 7 and 10 bd mixed to form 8.5?

Nope. pH doesn't work that way. You'll get somewhere in between, and maybe have very poor buffering capacity at that final pH.

 

[N11morales]

Hydros uses these 3 for calibration

• Calibration fluids 4.01, 7.01, and 10.01 at tank temperature

My pH has been reading 8.1-8.18, so this article would be great for me since I want to increase it a bit.

 

[Randy Holmes-Farley]

Can 7 and 10 bd mixed to form 8.5?

No.
As a simple example, mix 1 M HCl (pH =0) and totally pure water at pH 7, at 1:1, and you'll get a pH around 0.3.