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- Posts: 9
- Joined: Fri Oct 22, 2010 6:17 pm
- Location: MA
I am wondering if others out there periodically confirm the normality of solutions that have been purchased as traceable to NIST.
Off-topic conversations and chit-chat.
DSP007 wrote:
In our laboratory made the following rule.
If we prepare the titrant from the vial - the day we have no doubt that he is 0.1000 M.
HW Mueller wrote:
Actually, I am getting tired of commenting on regulatory measures, but, CPG, some might not understand your statement "It was fine to use April 30, but could not be used the next day, that's "science".
I understand this to mean that you sarcastically point out that this is indeed NOT science.
mckrause wrote:
Normalized standards cannot, by definition, be primary standards, and thus you must verify the normality. Primary standards have to be able to be traced to a gravimetric determination, as weight, and volume by default through weight, are your primary measures. A normality can change, due to absorption, evaporation, chemical reaction, etc.so can the amount of substance (or the mass fraction) in a primary standard - hence the choice of compounds that are resistant to most environmental influences While a CoA for a primary standard is sufficient to validate its purity (it is in solid form, after all), a CoA for a titrant or other normalized solution is not sufficient to validate the normality Because both primary and secondary standards can deteriorate in storage and in use, in both cases the CoA will have, in addition to a statement of amount of substance content, whether as mass, mass fraction or concentration, a stated requirement for storage conditions, and an expiry date.
That said, most labs that I have been in do not have issues with titrant normality drifting, and present the same arguments that have been presented here. However, any well written titrimetric method should include verification of the titrant normality.
On the other hand, use of a 1N NaOH solution in, for example, a colorimetric method or extraction method certainly does not require standardization - it is not serving a role as a secondary standard in the method but in normal use an NAOH solution will turn into sodium carbonate solution, and sooner or later will no longer be fit for purpose - just like an expired standard.
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