My simple diy alk test

[Randy Holmes-Farley]

I have been surprised that more people do not use this diy alk test, which is far less expensive and likely more accurate than most any home alk kits.

So I thought I’d show here how easy it is.

I had previously bought 2.0N sulfuric acid from Amazon, but any acid with well established concentration is fine.

I diluted some of it with 19 parts of ro/di water to 1 part acid to make a 0.1 N acid standard. That was done once and will last a long time.

Procedure:

Collect a water sample from the tank in a clean yogurt container. Weigh it. Today’s sample was:

605 g

I measured it’s pH to be 7.95. That number is not needed for the test, but it tells me the tank pH.

I then took a small yogurt cup, added about 25 mL of the acid, and zeroed it out on a scale to read 0.00 g.

I then used a small plastic dropper to take some of the 0.1 N acid to the water sample in the large yogurt cup while continuing to measure the pH. Keep adding acid until you get to pH 4.2 or so. The drop speeds up when you get below 5 so go slow then.

I then add any excess acid in the dropper back to the yogurt cup on the scale and take a reading of the acid added. It reads negative 17.53 g, so I added 17.5 g.

Calculating the alk:

17.5 g/ 605 g x 100 x 2.8 = 8.1 dKH

The 100 converts the 0.1 N into meq/L and the 2.8 converts the meq/L into dKH.

The whole process takes just a few minutes. The vary large tank water and acid volume coupled with the scale accuracy and
Lack of strong dependence on the final pH attained ensures an accurate result.

This article has more details:

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

 

[robanister]

Ty was wondering about this!

 

[JonoH]

What are you using to measure the PH?

 

[Randy Holmes-Farley]

What are you using to measure the PH?

A pH meter. Nothing special. Both meter and probe are more than 20 years old.

 

[Miami Reef]

A pH meter. Nothing special. Both meter and probe are more than 20 years old.

How long does it take your meter to display pH changes? My Apex probe takes about 30 seconds to update readings.

 

[Red_Beard]

How hard are regular probes to keep calibrated? I have a hanna ph pen (it is pretty old...) that feels like it drifts all over to the point I dont feel confident in any reading it gives.

 

[Randy Holmes-Farley]

How long does it take your meter to display pH changes? My Apex probe takes about 30 seconds to update readings.

I don’t see any lag, but I am not certain how fast it updates. Much more frequently than 30 seconds.

 

[Randy Holmes-Farley]

How hard are regular probes to keep calibrated? I have a hanna ph pen (it is pretty old...) that feels like it drifts all over to the point I dont feel confident in any reading it gives.

I calibrated it once, ran some tests, then again calibrated after a week it and it was accurate. I have not since recalibrated it again since the tank pH remains about where it always is.

 

[Miami Reef]

I calibrated it once, ran some tests, then again calibrated after a week it and it was accurate. I have not since recalibrated it again since the tank pH remains about where it always is.

Hmm. I remember reading an older article of yours from 2005, and you mentioned the pH probe’s calibration shifted on the same day after rechecking the same standards used to calibrate it. Maybe it was a different probe?

 

[Randy Holmes-Farley]

Hmm. I remember reading an older article of yours from 2005, and you mentioned the pH probe’s calibration shifted on the same day after rechecking the same standards used to calibrate it. Maybe it was a different probe?

I don’t recall that, but it might have been. I may also have said the “could”. In those days I left the probe in the tank 24/7, so I would have had some confidence the values were useful and think that it only generally needed recalibration after power failure.

I had multiple meters and probes then, so I cannot be sure that I previously used this probe and meter.

I presently store it in the zero alk water left from a tank water alk titration.

 

[Miami Reef]

I don’t recall that, but it might have been. I may also have said the “could”. In those days I left the probe in the tank 24/7, so I would have had some confidence the values were useful and think that it only generally needed recalibration after power failure.

I had multiple meters and probes then, so I cannot be sure that I previously used this probe and meter.

I presently store it in the zero alk water left from a tank water alk titration.

You're right, you said it might have. I apologize.

I'm going to pull up the article, not to try and "prove" you wrong. I'm only bringing it up because I was citing it and using it to mention how pH probes might easily shift within a few months, and maybe even sooner.

It was written in 2005. I can't imagine the probe drifting that soon. I personally think the calibration fluids may have drifted. In any case:

http://reefkeeping.com/issues/2005-02/rhf/index.htm

 

[Red_Beard]

Sounds like i need to order new calibration fluid and try again. The probe has dried out anyhow and will definitely need recalibrated. Thank you Randy for posting this

 

[KrisReef]

Help, for the lame. Why do you add acid until pH is 4.2?
Or
Why 4.2?

Also, how good (precision and mass range) of a balance do we need?
I have a pH pen that isn't worth much but if it is calibrated to 4.2, i presume it would do the trick?

Thanks, in advance.

 

[RobertK]

Gosh Randy, maybe this seems simple to you because you are a chemist. To me it sounds pretty complicated, I'm sticking with Salifert and Hanna. Sure do appreciate you though!

 

[gbroadbridge]

Gosh Randy, maybe this seems simple to you because you are a chemist. To me it sounds pretty complicated, I'm sticking with Salifert and Hanna. Sure do appreciate you though!

If ever I have a doubt about my Salifert Test or my kH Keeper, I do pretty much the exact test Randy uses.

It's the gold standard test for me.

 

[RobertK]

If ever I have a doubt about my Salifert Test or my kH Keeper, I do pretty much the exact test Randy uses.

It's the gold standard test for me.

Thanks, I will keep that in mind if I am ever in doubt about my test kit results.

 

[Randy Holmes-Farley]

Help, for the lame. Why do you add acid until pH is 4.2?
Or
Why 4.2?

Also, how good (precision and mass range) of a balance do we need?
I have a pH pen that isn't worth much but if it is calibrated to 4.2, i presume it would do the trick?

Thanks, in advance.

The pH endpoint of about 4.2 is the pH at which all carbonate and bicarbonate is converted into CO2/carbonic. Hence the titration is a measure of those things (and a few other lesser things such as borate).

The accuracy of the scale will determine the accuracy of the result. If it has 1% error (say, 100 +/- 1 gram) then the result will have error of about 2% (since there are two measurements. Say 10 +/- 0.2 dKH. Most electronic scales are at least that good, maybe much better.

 

[Randy Holmes-Farley]

Gosh Randy, maybe this seems simple to you because you are a chemist. To me it sounds pretty complicated, I'm sticking with Salifert and Hanna. Sure do appreciate you though!

Once you have made (or bought) the titration fluid, it is essentially identical to the Salifert test, except the pH is measured by a meter instead of a color changing pH indicator dye in Salifert. :)

 

[Garf]

I'm going back to this method. Currently getting the bits together for a power backup which involves making a bigger cabinet. I'll incorporate the scales and a spare pH meter and probe somehow, maybe even a magnetic stirrer. I'll probably have to leave space for a potted plant as camouflage however.

 

[vahegan]

I'll incorporate the scales and a spare pH meter and probe somehow, maybe even a magnetic stirrer.

I was going to say that! Add a magnetic stirrer, and you'll be doing what KH Director (or other automatic meters) does, but manually!