How accurate should a PH meter be for DIY AlK Test

[Scottie]

Is it necessary for a super accurate ph meter for doing the DIY titration to calculate alkalinity? I notice the cheaper ones have a lower resolution which Randy, in his post about the method, explained that you don’t need a ph reading to the thousandths.

I am a little confused about the accuracy that is needed however. Even some meters with high resolution have low accuracy (+/- 0.2 ph) This seems like a lot since 8.2 is 20 times more acidic than 8. It makes you wonder why the high resolution is even there if it can’t be trusted?

Towards the end of Randy’s post he mentions that going from 5 ph to 4.5 ph is hard because a tiny drop of acid can create a sudden dropin PH. In the beginning of the post he alludes to having some room for error- when using a scale to measure the tank water, 1 gram of difference is only 0.4 % difference in the result for example.

I understand that along the way the test result is only as accurate as the steps. For example potentials in errors like diluting 10 N acid to to 0.1 N, not accounting for heavier weight of salt water per volume of pure water, not stirring the test water, and not measuring the acid correctly. The one thing that bothers me though is what is beyond my control -the accuracy of my ph meter.

How does the ph affect the accuracy? How
much does the end result get skewed from a difference of say 0.7 ph ( if accounting for +/- 0.2 ph and an inability to target down to the .5 ph with titration)? Would the resulting value still be more accurate than a Hanna checker ( +/- 3 dkh or 5%)?

This method still appeals to me because it is way cheaper than buying overpriced reagents like Hanna offers. I would hope that if all the steps are done properly and consistently that the result would be more accurate than the dismal +/- 5 % even with a calibrated ph meter with accuracy up to 0.2 ph. Or should I just bite the bullet and get one that reads to 0.05? There are other qualities that my current ph pen has like not getting an instant reading. It often takes 30 seconds to get to the stable reading - this may not work with titration. Let me know your thoughts thanks.

 

[Stigigemla]

In the reality You dont need a high precision kH test. +- 10% is good enough. A better resolution can be good as the corals are slowly adaptable to different kH but are sensitive to sudden changes.
Normally we calibrate our pH meters at 7.0 and 9.11 but for kH testing it is a little better to calibrate at 7.0 and 4.00.
If I should make a kH test of my own I wold use a 20 ml syringe and color indicator as for instance Bromphenol blue.
Citric acid is acid enough and frendlier to Your clothes than sulfuric or muriatis acid. Or if it is difficult to find use methyl orange instead. The first is yellow and blue and the second yellow and red. Both are changing color att about the same pH. A color indicator is always working. No need for calibration or keeping the sensor wet. I bought my Methyl orange around 1971 and it is still functioning as new. A lot of pH meters has gone through my hands in this time.
For kH testing I use Salifert. It is sensitive enough easy to use and very cheap. So cheap that I dont care to make my own test having the chemicals on a shelf.

 

[Scottie]

In the reality You dont need a high precision kH test. +- 10% is good enough. A better resolution can be good as the corals are slowly adaptable to different kH but are sensitive to sudden changes.
Normally we calibrate our pH meters at 7.0 and 9.11 but for kH testing it is a little better to calibrate at 7.0 and 4.00.
If I should make a kH test of my own I wold use a 20 ml syringe and color indicator as for instance Bromphenol blue.
Citric acid is acid enough and frendlier to Your clothes than sulfuric or muriatis acid. Or if it is difficult to find use methyl orange instead. The first is yellow and blue and the second yellow and red. Both are changing color att about the same pH. A color indicator is always working. No need for calibration or keeping the sensor wet. I bought my Methyl orange around 1971 and it is still functioning as new. A lot of pH meters has gone through my hands in this time.
For kH testing I use Salifert. It is sensitive enough easy to use and very cheap. So cheap that I dont care to make my own test having the chemicals on a shelf.

I guess what worries me about lower resolution tests is that it is impossible to really know if there is either a decrease or increase in consumption or if this is a test inaccuracy or an error in consistency. I understand that alk consumption varies throughout the day so it is important to test at the same times. When there actually is a problem or sudden growth I would like to catch it early to adjust dosing and not have to wait until the dkh drops or rises to dangerous levels too quickly.

In the case of the salifert test vs the Hanna checker- I went to a fish store to have them check my water just as a back up against my numbers. It was free anyways. They used a salifert Alk test and told me it was low at 7 dkh. My Hanna checker was consistently showing me 8 dkh. Which test do you trust? My corals looked healthy to the naked eye so I did nothing. It basically was useless to me because what if my corals had begun consuming more alk and they looked healthy today but tomorrow the dkh drops more than 5 percent? My vial could be have been smudged or the color change could have been missed. Both tests also can just plainly be inaccurate with Hanna claimed as twice as accurate as salifert. If is a change in three days I could have a problem. Or if I misinterpret information I could create a problem by chasing imaginary numbers.

I guess my point is that if you are trying to notice a change in consumption that is less than 5 % with a tool that has a baseline inaccuracy of 5 % and you double check this with another tool that can be up to 10% inaccurate then what is the use anyways? If both tests also have ways in which the testing can be inconsistent beyond there baseline inaccuracies then at which point do you recognize a problem in consumption without going crazy doing several tests

I would like a test result that I can double check my instant read out with before taking any action if something seems off. Since there are many ways a Hanna Checker can fail- dirty or scratched vial, evaporated reagent, or incorrect volume relying on a meniscus line. The use of a PH meter, known acid, and scale really appeals to me because it seems more accurate than the 5% of Hanna or the 10% of salifert - while still having room for a user’s error the steps seem more controllable than determining color change or keeping a vial smudge free. The cheapness is also a secondary bonus but not my driving decision.

I guess the question remains. Does the math change significantly from 5.2 to 4.5 ph when calculating dkh with a known acid in a known volume of sample water? If My PH meter has a baseline inaccuracy of 0.2 + or - and one drop can make the difference of 0.5 ph are these leaps in ph only changing a hundredth of a dkh at the end of the day? This is the part that confuses me. I am trying to look at Randy’s graphs to compare when the dkh changes in relation to the acid added- the tables don’t have ph values next to the volume of acid added.

 

[Randy Holmes-Farley]

High precision is not needed because the pH change is very fast at the endpoint. one drop may take the pH from well above the endpoint to well below it.

They is also why rather imprecise color change kits also work.

 

[Randy Holmes-Farley]

Look at the titration curve in the article and you will see being off by 0.2 pH units is a fairly small alk difference.

 

[Scottie]

Look at the titration curve in the article and you will see being off by 0.2 pH units is a fairly small alk difference.

I took another look at the graph. I think what was throwing me off was the amount of acid added was in meq/l, but you later go on to explain that there is 0.1 meq/l in 1 ml of water so 40 ml would be 4 meq/l.

My interpretation is that from 5 to 4 ph there is a Alk difference from 3.2 to 3.6 meq/l which is roughly 8.96 dkh to 10.08 dkh. Am I getting this wrong? It is hard to see the exact dots where they line up on the x - axis without a ruler.

I also what to try this method with as little sample water as possible. Does the precision go down with less water used because there are greater steps from dkh with less acid added? I plan on using 1ml syringe for 50 ml of water. So I should expect on using 1.6 ml of acid if my DKH is 9? So every 0.05 ml change is a change of approx 0.3 dkh? I am assuming with less water it makes it more difficult to get the target ph of 4.5?

 

[Randy Holmes-Farley]

That’s about right, but your pH should be far more accurate that that.

 

[Randy Holmes-Farley]

A more realistic pH range of 0.2 or 0.3 pH range total gives a small alk error due to the steepness of the curve at pH 4.3

Also, unless you keep changing calibration buffers, the reproducibility will be much more narrow than that, allowing easy detection of alk changes.

 

[Scottie]

A more realistic pH range of 0.2 or 0.3 pH range total gives a small alk error due to the steepness of the curve at pH 4.3

Also, unless you keep changing calibration buffers, the reproducibility will be much more narrow than that, allowing easy detection of alk changes.

Would you recommend using a conductivity meter for salinity rather than a refractometer for this method of alk testing? How does a 1% error in salinity skew the calculations for determining dkh?

 

[Randy Holmes-Farley]

Would you recommend using a conductivity meter for salinity rather than a refractometer for this method of alk testing? How does a 1% error in salinity skew the calculations for determining dkh?

You do not need to know salinity for an alk titration by this method, or any other.