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I just bought a digital ph meter. It came with

6.86 powder and
4.00 powder

I mixed two solutions. One with 1/4 cup of tap water and 6.86 solution and one with 1/4 cup 4.00 solution.

When I place my meter in the 6.86 ph water, I calibrated it so it's around 6.85

But when I place it into the 4.00 ph solution, it reads 4.3. I'm stuck on how to calibrate it.

I tested it In my fish tank with 7 ph tap water buffered by Amazonia new and it reads less than 6 ph. Something's wrong. I know the ph should be around 6.5 that's what my liquid ph tester reads. So the meter is calibrated low. But eve if I crank it up, the 4.0 solution and 6.86 will be too high. What do I do?
 

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idealy you should use DI water to mix the solution, which has most chemicals removed and is usually 7.0 pH (neutral) because you are using tap water it is changing the value of the pH solution. you could use a pH buffer to help with using tap water

at work I have a Hach pH meter and do a 3 point pH calibration using pH standard solution of 7, 4 and 10.
 

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It's the one probe in the back. It's a yellow one I got off eBay
I'm not sure which model/make this is....

I suppose if it was a cheap pH probe, it'd only be a one point calibration :confused:
 

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Well if it is the same cheap one I got from Ebay then you need to buy a different brand of calibration fluid/powder. The one i received where PH 7.01 (was actually 7.72 with RO water). And yes it is single point calibration.
 

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Using the powder it came with, did you mix these powders with RO or distilled water?

You ought to be able to use RO or distilled as a 3rd reference, but if either of these are exposed to the air they can pick up CO2 and the pH of this will be lower. No way of telling.
 
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