Calculating pH from number of drops of acid demand reagent?

[JoyfulNoise]

So, just to add a little detail, I went ahead and measured all my calibrating solutions (4,7, & 10) after having calibrated last week and 4 & 7 were only off by 0.03 pH units and the 10 standard was reading 9.88. Those are within the accuracy of the unit as Apera guarantees a +/- 0.10 variation when a 3-point calibration is used. I’m not surprised at the drift at the high pH range as pH probes are notoriously difficult to get accurate measurements at high pH. Even with laboratory designed probes, you need to do special calibrations if you want a pH meter to perform accurate measurements above a pH of 10. And if you truly needed accurate pH measures in the high range, the standard Ag/AgCl electrode is not the best choice.

 

[SoDel]

So, just to add a little detail, I went ahead and measured all my calibrating solutions (4,7, & 10) after having calibrated last week and 4 & 7 were only off by 0.03 pH units and the 10 standard was reading 9.88. Those are within the accuracy of the unit as Apera guarantees a +/- 0.10 variation when a 3-point calibration is used. I’m not surprised at the drift at the high pH range as pH probes are notoriously difficult to get accurate measurements at high pH. Even with laboratory designed probes, you need to do special calibrations if you want a pH meter to perform accurate measurements above a pH of 10. And if you truly needed accurate pH measures in the high range, the standard Ag/AgCl electrode is not the best choice.

Thanks for all the info. Very helpful. I think I’m going to buy one. Plan is to use it for pool. If I need accuracy around 10 plus, I’m in big trouble .

 

[JoyfulNoise]

Thanks for all the info. Very helpful. I think I’m going to buy one. Plan is to use it for pool. If I need accuracy around 10 plus, I’m in big trouble .

Just so you’re aware, you do need the 10.01 calibrating solution. The kits typically come with 7.01 and 4 which is great for hydroponics but not so great for pools. You can do a single point calibration around 7 but that’s not going to be super accurate up near 8. If you have 7 and 10.01 then the probe will be very accurate all the way through the pool range.

Apera sells a very nice kit of calibrating standards and a 4 pocket testing cell for holding the different solutions. Since you shouldn’t use a calibrating solution for more that 5 or so calibrations, it’s a nice way of separating and holding the test solutions from their larger containers. And it makes calibrating much faster. Consider it like a SpeedStir - you don’t NEED it but once you have it you couldn’t imagine not having one.

 

[SoDel]

Just so you’re aware, you do need the 10.01 calibrating solution. The kits typically come with 7.01 and 4 which is great for hydroponics but not so great for pools. You can do a single point calibration around 7 but that’s not going to be super accurate up near 8. If you have 7 and 10.01 then the probe will be very accurate all the way through the pool range.

Apera sells a very nice kit of calibrating standards and a 4 pocket testing cell for holding the different solutions. Since you shouldn’t use a calibrating solution for more that 5 or so calibrations, it’s a nice way of separating and holding the test solutions from their larger containers. And it makes calibrating much faster. Consider it like a SpeedStir - you don’t NEED it but once you have it you couldn’t imagine not having one.

The convenient testing cell you mention, this one?

CalBox for Organizing Calibration Solutions-Apera Instruments

The Apera Instruments' unique CalBox helps users organize all the solutions in one place and create no more mess in the calibration process. It is a great accessory for pH meters and all other kinds of meters that require regular calibration.

aperainst.com

The PC 60 pen looks like it comes with pH 4.00 and 7.00 so I would also get some 10.01 separately and have what’s needed?

Thanks!

 

[JoyfulNoise]

The convenient testing cell you mention, this one?

CalBox for Organizing Calibration Solutions-Apera Instruments

The Apera Instruments' unique CalBox helps users organize all the solutions in one place and create no more mess in the calibration process. It is a great accessory for pH meters and all other kinds of meters that require regular calibration.

aperainst.com

The PC 60 pen looks like it comes with pH 4.00 and 7.00 so I would also get some 10.01 separately and have what’s needed?

Thanks!

Exactly that but the one I nabbed off Amazon didn’t have any stickers … . No worries, little Post It notes works well enough .

The kit comes with a minuscule amount of calibrating fluids, it’s enough to get you started and going for a few weeks. Best to see if you can find a deal on a bundle of calibrating solutions and just get that.

 

[rick32086]

 

[rick32086]

You can also try holding the block sideways or upside down. Often, that helps me discern colors better.

You can also try holding the block sideways or upside down. Often, that helps me discern colors bett

You can also try holding the block sideways or upside down. Often, that helps me discern colors better.

Thank you, will give it a try

 

[jmastron]

You can also try holding the block sideways or upside down. Often, that helps me discern colors better.

I'm a bit red-green colorblind (I can see all color ranges but fail those "find the number in the dots" tests and distinguishing very close colors), and initially had trouble with the pH test. I do 3 things that make it a lot easier:

1) I use a white screen app (just displays a bright white screen) on my phone and hold it right behind the block. That way I get plenty of lighting and consistency.

2) I ended up getting a K-2005 test block (K-2006 is the same on the pH side) a few years ago, with 6 pH steps from 7.0-8.0 (instead of the TF-100 one with 5 steps from 6.8-8.2), and I find it a lot easier to compare the points I care about (7.6-8.0) at a glance. I suspect the bigger sample size (44ml vs I think 5 ml) helps too. Be sure to use the correct reagent (R-0004 vs R-0014).

3) I realized that it really doesn't have to be that accurate. The only real decision I'm making is to add a quart of acid when the pH gets close to 8.0 (which I find pretty easy to tell with the block above), which will lower pH to somewhere between 7.4-7.6. Other than that, is it 7.3, 7.4, 7.5, 7.6? Doesn't really matter (although I've gotten pretty good at determining those too). There's no need to chase an exact pH number in most cases.

Nothing against pH meters, just some thoughts on honing the manual process!

 

[SoDel]

I'm a bit red-green colorblind (I can see all color ranges but fail those "find the number in the dots" tests and distinguishing very close colors), and initially had trouble with the pH test. I do 3 things that make it a lot easier:

1) I use a white screen app (just displays a bright white screen) on my phone and hold it right behind the block. That way I get plenty of lighting and consistency.

2) I ended up getting a K-2005 test block (K-2006 is the same on the pH side) a few years ago, with 6 pH steps from 7.0-8.0 (instead of the TF-100 one with 5 steps from 6.8-8.2), and I find it a lot easier to compare the points I care about (7.6-8.0) at a glance. I suspect the bigger sample size (44ml vs I think 5 ml) helps too. Be sure to use the correct reagent (R-0004 vs R-0014).

3) I realized that it really doesn't have to be that accurate. The only real decision I'm making is to add a quart of acid when the pH gets close to 8.0 (which I find pretty easy to tell with the block above), which will lower pH to somewhere between 7.4-7.6. Other than that, is it 7.3, 7.4, 7.5, 7.6? Doesn't really matter (although I've gotten pretty good at determining those too). There's no need to chase an exact pH number in most cases.

Nothing against pH meters, just some thoughts on honing the manual process!

+1 . The main reason I’m interested in “exact” (or more exact at least) pH is I’m working on finding the “easy point” of the pH ceiling at my TA which gives me the CSI I want. This means a pH slightly above 8.0, which gives me fits with the K-2006 test block. I have a seat of the pants idea of pH rise per day, and a proper tongue angle / moon phase interpretation of trace of orange / redish / trace of purple of the 7.9 to 8.0 range, but it’s just too subjective for my comfort level.

The whole endeavor likely won’t work (if it sounds too good to be true . . .) but it’s something to try and what else do I have to do lol.

 

[JoyfulNoise]

+1 . The main reason I’m interested in “exact” (or more exact at least) pH is I’m working on finding the “easy point” of the pH ceiling at my TA which gives me the CSI I want. This means a pH slightly above 8.0, which gives me fits with the K-2006 test block. I have a seat of the pants idea of pH rise per day, and a proper tongue angle / moon phase interpretation of trace of orange / redish / trace of purple of the 7.9 to 8.0 range, but it’s just too subjective for my comfort level.

The whole endeavor likely won’t work (if it sounds too good to be true . . .) but it’s something to try and what else do I have to do lol.

So, funny you should mention purple. I used my R-0004 to measure my RO tap water and, when I do, it’s a shade of purple I almost never see. It’s more purple than the 8.0 color swatch. At first I was thinking the RO was off somehow. But, when I used the acid demand drops, it only took one drop to make the 8.0+ drop all the way below yellow (6.8), so I knew the alkalinity of the water was next to nothing. When I measure with my Apera, the reading is 8.15 which conforms to very purple in the phenol red color swatches. And the EC of my RO water is 60 uS/cm which indicates good RO functioning.

So yeah, if you want to hang with the high pH club, then you’ll need a good meter to make sure you can differentiate between 8.0 and 8.2

 

[SoDel]

So, funny you should mention purple. I used my R-0004 to measure my RO tap water and, when I do, it’s a shade of purple I almost never see. It’s more purple than the 8.0 color swatch. At first I was thinking the RO was off somehow. But, when I used the acid demand drops, it only took one drop to make the 8.0+ drop all the way below yellow (6.8), so I knew the alkalinity of the water was next to nothing. When I measure with my Apera, the reading is 8.15 which conforms to very purple in the phenol red color swatches. And the EC of my RO water is 60 uS/cm which indicates good RO functioning.

So yeah, if you want to hang with the high pH club, then you’ll need a good meter to make sure you can differentiate between 8.0 and 8.2

Yes! Very much exactly where I’m at with the color. Like is it somewhere between 8.0 and 8.1 or somewhere between 8.0 and 9.0 lol. I couldn’t tell you by looking at the color. What I’ve been doing is keeping it below 8.0 so I can tell what it is, but not optimal for what I’m trying to do.

 

[SoDel]

Went ahead and did it. As my f-i-l always says, only live once. What’s a finger or toe anyway lol.

ordered this: https://a.co/d/0SdrG1F

and this: APERA INSTRUMENTS AI1116 pH Calibration Solution Kit (7.00, 4.00, 10.01), 16 oz. Each, Plus a CalPod Solution Holder for Easy Calibration, NIST traceable: Amazon.com: Industrial & Scientific

So in addition to in-season high pH attempt, approaching winter as temp drops up until closing, will add TA up to where I want pH to equilibrate (is that a word?) to keep CSI where I want it at that temp, and use the pH meter to see if it’s actually working. Then at the end, add TA for CSI ok at 32 F, circulate and cover — long winter slumber. I look forward to leaving color matching behind lol.

 

[JamesW]

Try the acid demand test method to compare to the pH tester to see if they are close so we know if the method has any value.

 

[SoDel]

Try the acid demand test method to compare to the pH tester to see if they are close so we know if the method has any value.

I finally got back to this and tried the acid demand method this am. Current #’s are:
pH 8.2 (measured)
TA 80
CH 425
CYA 60
TDS 4000

Acid demand to reduce pH to 7.5 — 1 drop
Taylor tables say 9.16 oz. HCl needed for 10k gallons to achieve desired reduction in pH

Poolmath webpage using 10k gallon pool predicts starting pH was 7.87 when converging on 9.16 oz. HCL and terminal pH of 7.5

I think the “drop” method isn’t granular enough, at least the way I’m doing it. Each drop goes a long way and accounts for a wide range of possible initial pH values. I think if the “drop” could be quantified (I’m sure Taylor has a value for the standard “drop”) and the actual exact acid demand carefully measured, the results would be more accurate, but as it stands going by drops of acid demand doesn’t seem to be a practical way to measure pH. Maybe a much larger initial sample and scaling up for the drops would help, but seems very cumbersome. I’m sure a method could be devised.

It was fun to try

 

[JoyfulNoise]

40 uL per droplet or roughly 25 drops to make 1 mL. The droplet volume will be affected by viscosity and squeezing pressure on the bottle. If you have an old bottle of reagent, you can add water to it and test out the fitments droplet size.

 

[JamesW]

Acid demand to reduce pH to 7.5 — 1 drop

If it only took 1 drop to reduce the to 7.5, the pH was probably not at 8.2 or the TA was lower than 80.

Maybe the drop sizes are off?

 

[SoDel]

If it only took 1 drop to reduce the to 7.5, the pH was probably not at 8.2 or the TA was lower than 80.

Maybe the drop sizes are off?

If the numbers don’t jibe, I think it might be my CYA measurement thus affecting the CA that poolmath calculates (?). Even using the standard, I have trouble with consistent results — angle of the sun, if my eyes aren’t focusing right that day, moon phase, butterfly effect, etc. lol. Possible that the TA measurement was off if my bottle of reagent has slightly smaller drops or something as you say (but that is an easy test to get repeatable results). The pH “should” be correct. The meter and my “color calibrated” eyes agree it was something above 8.0 lol.

I will measure 10 drops of R-0009 — just a minute here — says sulphuric acid; takes longer for gloves and glasses than measure this stuff — 10 drops measured 0.36 ml. Using Matt’s drop size data (thank you), I think we have a problem lol. Basically 10% small on the drops. Your prediction is spot on. Does that account (at least partly) for the TA anomaly?

 

[JamesW]

Everything has a tolerance and tolerances can add up or cancel out.

So, if every test result is a little off, the combined inaccuracy can be somewhat high.

 

[JamesW]

In my opinion, the drop method should be close enough to tell you if you’re way off, but not accurate enough to get to +/- 0.1.

I would think that it should get within about +/- 0.3 most of the time.

If you are testing on a regular basis and logging the results, the averages should help cancel out the errors or inaccuracies and provide more precision.

 

[mgtfp]

To get a feel for the tolerances you can play around a bit in PoolMath. See what happens if TA is actually 60, 65, 75 or 80 instead of 70, similar with CYA.

I fiddled around a while ago with the Taylor drop size. As Matt said, it is specified as 25 drops/ml with a tolerance of +/-1:

Two Ways To Imporve Drop-Test Accuracy

That means a drops size of 0.04ml, with a tolerance between 1/26=0.0385 to 1/24=0.0417 ml/drop.

A while ago, I did what Matt suggested and used an old bottle to test the drop size with distilled water. My impression was that the 0.04ml are spot on on average, but individual drop sizes can vary, certainly all within the range Taylor specifies, probably less when good care is taken about consistent pressure and keeping the bottle vertical, but this is not that easy to verify with home equipment. I tried two methods: dripping into a 1ml syringe, and dripping onto a precision(-ish) scale and weighing the drops. Both seemed to be consistent.

This is probably a main issue with the method, especially when you need only one drop, where you don't get any averaging over a number of drops.

You can try to estimate the effect of this by assuming you actually need for example 0.5 or 1.5 drops and interpolate in the acid demand table.