pH and Conductivity Open Bottle Shelf Life

[NumberCruncher]

Hi WalkingSeed

I don't know if this thread is still 'live' so I may be a bit out of date with this contribution (ironic huh?).

Have you considered making your own pH standards?

The following is from a 1991 edition of Vogel's textbook of quantitative inorganic analysis (5th edition).

"The IUPAC definition of pH is based upon a 0.05M solution of potassium hydrogenphthalate"
"The British standard (BS 1647:1984 Parts 1 and 2) is also based upon potassium hydrogenphthalate"
"Disolve 10.21g of the solid (dried below 130 C) in water and dilute to 1kg. The pH is not affected by atmospheric carbon dioxide: the buffer capacity is rather low. The solution should be replaced after 5-6 weeks."

pH values are
3.998 at 15 C
4.001 at 20 C
4.005 at 25 C
4.011 at 30 C

As long as you have access to high purity K.H.P. and (preferably) distilled water, you can make your own primary standard and check your bought in standards against it to determine typical shelf life. (In fact you can simply use the solution as a daily working standard if you want to).

(There are several other solutions used as secondary pH standards ranging from pH3.5 to pH12).

You make up the primary standard on a regular basis (weekly for example) and check your bought in standards against it. Do this for the lifetime of several bottles of bought in standards and you will be able to give the auditor hard evidence to back up your statement:

"Bought in standards must be disposed of within X weeks of being opened"

or some similar statement.

If the auditor wants to be really picky, (s)he could as "How do you know that the primary standards that you prepare in the lab last for 1 week?"

Easy!

In week 1 you make up your primary standard and check your other standards against it.
You also measure the primary standard several times each day to work out short term repeatability of the measurements.

In week 2 you make up another primary standard and repeat the above. However, you also keep measuring your week one primary standard as well. This means that you will now have two sets of replicate values. One set from the week 1 standard, the other from the week 2 standard.

Repeat for as many weeks as necessary.

You should find that the average values you get for primary standards are statistically the same.

One way ANOVA will make the statistical analysis robust.

In retrospect, perhaps it's not quite so easy as I exclaimed at the start...

The reason the manufacturers of standards do not want to give a simple answer to the question of shelf life is simply that they have no control over what you do with their standards once you have bought them.

"it depends on temperature, humidity, the time that the bottle is exposed to atmosphere, how close the bottle was to its unopened expiry, etc..."

Yes!

Unfortunately, conductivity is not as simple. Whilst you can make your own standard KCL solutions, you need fantastically low conductivity water to make up low conductivity standards. Even dissolved CO2 will affect the result.

Hope this helps

NumberCruncher