pH - who do I trust?

**dev** · 07-01-2007, 05:10 PM

Some time ago I got a Hanna pHep instrument to help me get more accurate readings from the tanks. This Hanna pen has two point calibration and is specified with +/- 0.1 accuracy.

However, this intrument would always show a higher value than my JBL test kits. I bought a new test kit, and recalibrated the Hanna instrument. Still getting the same result.

Recently I decided to get a second pH pen, to see if things would get any clearer. The Trans Instruments Eco-Pal came recomended at a decent price (only about $100, less than half of the Hanna). This one has single point calibration and is specified with +/- 0.2 accuracy.

So, now the confusion is total and complete :)

In my south-american community tank, where I want a fairly stable pH of 6.8 I'm reading:

JBL (6,0-7,6): 7,0
Hanna pHep: 7,8
TI Eco-Pal: 7,5

These are hardly the kind of numbers I would like to base my CO2 calculations on.

What test should I trust? Are there any secrets to getting an accurate reading from the electronic instruments? Does anyone have experience with the pHep or Eco-Pal instruments? Can anyone reccomend an instrument that will definately give correct and accurate readings?

Anything that can shed some light on this would be very much appreciated.

**gm72** · 07-01-2007, 05:15 PM

If I were you I'd average out your two pen readings and guess you are around 7.6 or so. Sorry you're having trouble with those readings. Makes it next to impossible to plan anything!

**dev** · 07-01-2007, 05:26 PM

Well, that was my first thought - to trust the electronic instruments. But the JBL test kits have kept my plants and fish alive for the past decade. Can it really be off by that much?

**gm72** · 07-01-2007, 05:28 PM

Is it old? I wonder if it has become off-kilter with age?

**dev** · 07-01-2007, 05:41 PM

I bought the most recent about three months ago, but there is no way of knowing how long they've been in store as there are no date stamps on the bottles or packages.

**gm72** · 07-01-2007, 05:47 PM

I know how that goes. Is there somewhere you can take your pens to have them re-calibrated so you can be sure of your readings?

**dev** · 07-01-2007, 06:09 PM

I've tried this a couple of times. The shop do it no differently than I do it myself, and the result is the same.

It's quite simple really, just put the instrument in the correct buffer and turn on a small knob until it shows the correct value. For the Hanna's two point calibration, this involves two buffers (either 7 and 4 or 7 and 10) and two little knobs.

I've even tried buffer fluids of different brands so I could be sure there wasnt anything wrong with the fluids.

**gm72** · 07-01-2007, 06:10 PM

Odd. I still think an average of your two pens is probably pretty accurate and is certainly close enough to allow you to do what you need with the tank.

**dev** · 07-01-2007, 08:50 PM

Originally Posted by gm72

Odd. I still think an average of your two pens is probably pretty accurate and is certainly close enough to allow you to do what you need with the tank.

I must beg to differ. With a KH of 4 the 0.6 variation in pH makes for a huge difference in the CO2 level.

If the electronic instruments is right, and I add CO2 until l they indicate a pH of 6.8 I will have a CO2 concentration of approx 20 ppm. Good.

If the electronic instruments are wrong, and I instead lower the actual pH to 6.2 the CO2 concentration will be insane, making the water toxic.

Come to think of it, a pH of 7.6 indicates a CO2 concentration of <3 ppm. I would be very surprised if this was true. After all the plants are pearling slightly, indicating that the CO2 is at least higher than atmospheric concentration.

**dev** · 07-01-2007, 10:51 PM

I believe I have thought of a way to test this.

Assuming the following is true:
* There is a fixed relationship between carbonate hardness, CO2 concentration and pH when carbonates are the only buffer present, as illustrated by the Akvaariomaailma co2 table.
* aerated water contains 4-5 ppm dissolved CO2 (three times the CO2 content of the air, per unit of volume)
NOTE: this assumption was wrong. While a fish tank may contain 4-5 ppm due to breathing fish and organic breakdown, aerated water will only have a .5 ppm concentration (source: thekrib.com)
* the carbonate hardness is known

I could then aerate some water from one of the tanks and see if the pH changes.

If the water cointains <5 ppm CO2 the pH should stay at 7.6 when the water is aerated, validating the pH instrument and proving that I need new CO2 systems :o

If the water contains 10-15 ppm CO2 the pH should rise slightly (at least .2) as the water is aerated down to 5 ppm validating the JBL test kit.

I'll go fill a bucket now :)