pH buffers - solutions, tablets or sachets?

[RosalieM]

Hi Everyone,
I've just done a search on pH buffers here and I was just wondering what the sachets and tablets are like in comparison to the ready-made solutions. Someone said they are cheaper, but after looking at the serrata price list I'm not convinced. So, a few questions for those of you who use the sachets or tablets: How much solution do you make with one sachet/tablet? How long does the solution last? I just used up the last of our pH 7 buffer solution on Friday calibrating our pH metres and thought rather than just buying more of the same, I'd see what you guys had to say about it. It doesn't get used very often and this is the first time I've had to order some. I'm thinking the tablets/sachets would take up less space in the cupboard if nothing else, but I don't want to be wasting the science money...
Thanks
Rosalie

[dime]

I have bought the solutions thinking they might be more accurate. But by the time I used them on the meters, the meter said the solution wasn't fresh.!! So I didn't want to buy more to have the same thing happen. I hate those pH meters anyway and pretend they don't work anymore. They prefer the data logger one anyway, which doesn't seem to need calibrating. (of course I haven't checked, as I don't want to know, BUT they haven't complained either)

[Labbie]

Tablets in foil, the new type. or they break etc and go out of date. I believe the tablets in foil, have a good few years span. I only use the buffer tablets about once a year.

[sunray18]

I use the solutions - yes they do go 'off' after a while, but are easy to use and because they are colour-coded are easy for the teachers to use..

[RosalieM]

Thanks for your feedback. We mostly use the data logger pH probes now too, but the other ones still come out now and then, especially for field trips.

Do the tablets dissolve into a coloured solution when you make them up or are they all colourless?

[MariaC]

Hi I just bought pH buffer capsules (Merck) as I hope they last longer in storage, after taking the capsule out of the bottle I resealed it with lab film and sticky tape! Each capsule makes 100mls buffer solution but no they are not coloured. Hope that helps...Maria

[Ocker]

For pH7
Prepare 0.2M Sodium diHydrogen Phosphate (31.2g of NaH2PO4.2H2O / Litre )
and 0.2MSodium Hydrogen (ortho) Phospate (28.39g Na2HPO4.12 H2O / L)
use 19.5mls NaH2PO4 and 30.5mls Na2HPO4 and make up to 100mls with distilled water
From "The Laboratory" page 52 with tables for all other values

[Ocker]

Sorry forgot to make the numbers subscript but you can work it out

[Labbie]

BUMP

[Ocker]

And well might you Snigger!