Measuring Water Ph With Ph Meter.

[spudfarmerboy]

I have a cheap pH meter I bought of Ebay. When I calibrate it, I use Labserv 4 and 9 buffer solutions. Using the 4 buffer solution, the pH meter indicates 4, when using the 9 buffer solution, the pH meter indicates 8.9. Close enough for me.
But when I measure the pH of my tap water at home, the meter will sometimes show 5,4 6, 7. All over the place. It will usually indicate close to the last pH reading that the meter measured.
My tap water is rain water collected off a colourbond roof, onto colourbond guttering, into PVC piping, into a polyethylene ( I think) 40,000 litre water tank.
After trying to measure my tap water pH, if I put the pH probe back into the buffer solutions, it will measure them correctly. So from this, I conclude that the probe works fine. I have not tried measuring other tap water, such as from a reticulated supply.
Are there any science/chemistry people on AHB that may know why this is the case? Is it because it is rain water and doesn't contain many ions or minerals (?) or whatever?
Looking forward to learning what is happening.
Cheers

 

[Steve@PMF82]

Sounds like your water is pretty clean and stored well. So you are looking at almost distilled water, which like your rainwater is very ready to absorb Co2 from exposure to air. This can happen just as you taking your measurements.

From my experience the cheaper PH meters i found them quite inaccurate.
Try measuring some other liquids with it that have a known PH, like milk, white vinegar, a quick google search will bring up the PH for lots of different liquids you prob have around your house.

 

[davewaldo]

Also, are you leaving the meter in the liquid for a minute or so to settle? I've heard this gives the best results. Also, how are you storing the meter when not in use? The probe needs to stay wet, but not with normal water or calibration fluid. It needs to be maintenance fluid otherwise the probe will leach its contents into the storage fluid and over a bit of time will become useless.

 

[ianh]

But when I measure the pH of my tap water at home, the meter will sometimes show 5,4 6, 7. All over the place. It will usually indicate close to the last pH reading that the meter measured.

Are you rinsing the electrodes between pH measurements. It sounds as though you measure a buffer then put the electrodes straight into your tap water.

 

[sim]

Anyone got a good model they can recommend?

cheers,
sim

 

[MHB]

The pH scale is a Log scale, at pH 7 there are (very loosely) an equal number of Acid and Base ions, water H₂O can be thought of as H⁺ (Acid) and OH^- (Base) living in balance. It takes a very small change in either one to move the ph from 6-7-8, but 10 times a much either acid or base to move 5-7-9, and 100 times a much... well you get the picture.

The problem you have is that your water is so neutral that trace amounts of things like dissolved CO₂ are going to make a big difference.

Just treat the water as having zero minerals and build from there, as soon as you add some Malt or salts the pH will settle down and give you consistent and sensible readings assuming your pH meter is working, which the response to the buffer solutions suggests it is.

MHB