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I recently bought a digital PH meter off ebay. The insturctions say to calibrate it with a sodium phosphate solution of PH 6.86 however this did not come with the meter.

So could i buy a different buffer solution and use this to calibrate it with (calibration is achieved with a small screw at the side)?

i think you should be able to calibrate it with distilled water at 7.0 ...

Distilled water should have a pH of less than 7 due to it's lack of mineral salts and buffers, so probably not a good idea, as the end result will be a miscalibrated meter.

Your best bet would be to contact the manufacturer or a local college/university/lab to see if you can get a sample.

" lack of mineral salts and buffers" is what actually makes distilled water neutral (i.e. pH=7.0)
the only reason for its pH being less than 7 should be dissolved carbon dioxide from air resulting in a very very dilute carbonic acid having pH lil less than 7. But again, carbonic acid is a very week acid....
anyways,yes a lab should be the most accurate way

Completely pure water has a PH of 7 (that's the considering pure water, where there is nothing other than pure h20)

A substance with a ph of 7 is what that has exactly the same number of OH- and H+ ions, therefore water with nothing else in it is PH7. However to get completely pure water is extremely difficult.

I have found calibration fluids for other PH meters that have a set PH so i may just use them.

I've got it set up in my tank at the moment showing a PH of about 7.6 (it seems to fluctuate between 7.55 and 8.01). If it's not been calibrated could this account for the large(ish) PH swings, or would a lack of calibration simply show incorect values but with the same differential between values.

If the meter is "linear" (you should be able to find that in its documentation) then a miscalibrated meter would only show a fixed deviation from actual values, like actual value +/- a fixed number. It shouldnt cause a fluctuation though...

Ok, well i used a liquid PH test (nutrafin wide range) on my tank and it showed a PH that was roughly the same (about 0.4 off but then the liquid test is nowhere near as accurate as the meter)

I presume it came from the factory pre-calibrated but i will get some calibrating fluid none-the less.

good luck with your new meter

Water with absolutely no buffers will not stay at a perfect 7.0. It is affect by the air and cannot be kept 100% pure unless it is made pure in a completely enclosed system and stored in an enclosed system. As soon as you open it it will end up changing ph. Distilled and RO water almost always tests 6.8 not 7 probably due to the CO2 it is exposed to.

You should be able to get a calibration solution from the manufacturer or possibly a lab might have that ph of solution even if it's used to calibrate something else.

Calibration solutions should be available from better pet stores, or pool stores. Pool people have to do PH tests on their pools also. The solution should say the pH on the bottle. Or just try a scientific supply store, like edmundscientific.com. I think that's the website.

Well my dad's gonna try and get me some from his work (glass container manufacturer) as i think they use PH meters there, but if that fails I have found the fluid on the internet so i shall get it off there.

My dad didn't have any at his work, so as soon as i get paid again (31st) i'm going to order some off the internet.

I seem to be getting quite large PH swings though, when i do a water change the PH goes down to about 7.5, then it slowly works it's way back up to between 8.00 and 8.50 and then at night it goes higher still to around about 9.

Now even though i have mainly black water fish they don't seem to be affected by this and my angels are still willing to breed.