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Fulmen
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Standardisation from scratch?
Couldn't find any thread on this, if there is one please slap me around and point me in the right direction.
Question is how one can establish reasonable standards from scratch, without any reagent grade chems as a starting point. Of course one could just
take the listed content (say 98% tecnical grade) at face value, but a lot of chems degrade over time making such assumptions inaccurate at best.
Of course non-analytical grades might contain impurities that can interfere with different reactions, but is it possible to get reasonable
standards with a home setup?
One possible method could be gravimetric determination by precipitation, for instance determining silver by precipitating it as silver chloride.
Challenge would be a source of silver nitrate (or elemental silver) free from contaminates that would precipitate chlorides as well.
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j_sum1
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Crystallisation is a good way of purifying many substances. Repeat for even higher purity.
Once you have a pure, known substance you have a standard. Work from there.
I am however glad I can read a label and trust it most of the time rather than have to extract acids from lemons or rhubarb leaves.
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DJF90
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Inorganic Syntheses provides a purification method for sulfamic acid, which is then suitable for use as a primary acid standard.
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Fulmen
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Thanks DJF, that's good info.
Techniques like crystallization and distillation is of course a good method for purification, but without a reliable method it still involves a bit of
faith. If you could assay the product without relying on other primary standards that would be grand.
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Chemosynthesis
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In practice, the easiest way to verify reagent purity is highly dependent on the reagent. Usually I check in a book, such as Purification of
Laboratory Chemicals by Perrin et al. (version dependent). There are other books, but knowing which to use can be problematic. This covers most basic
reagents you're likely to use at home.
And then there is the chemical literature. Often in a literature search, you will find papers on purification, or synthesis and purification, of
specific compounds.
Then purify before use if you feel it's needed and just trust the authors and your technique (clean glassware, etc.). Not very rigorous in that you're
not using NMR, HPLC and GC/MS on everything prior to use, but less faith than crystallization and distillation if you believe the included spectra!
And of course, if you run your own spectra, you're also having to take it in faith that maintenance is current, some slob didn't dump sample into a
detector, etc.
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Zyklon-A
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http://library.sciencemadness.org/library/books/chemical_rea...
[Edited on 18-2-2015 by Zyklon-A]
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blogfast25
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Quote: Originally posted by Fulmen  | Of course non-analytical grades might contain impurities that can interfere with different reactions, but is it possible to get reasonable
standards with a home setup?
|
Materials I use as Primary Standards in a home set up:
Thrice recrystallized sodium carbonate, dehydrated. Acid/base standard.
Potassium hydrogenphthalate ('KHP'):
Not very expensive, easy to recrystallize. Acid/base standard.
Copper conductor:
Highly pure copper for primary standard in iodometry.
Ebay 99.99 % Zinc:
Primary standard for EDTA titrations.
Home synthesised Mohr's Salt, recrystallized:
Fe(II) redox primary standard.
[Edited on 18-2-2015 by blogfast25]
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Fulmen
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I checked out the sulfamic acid, but it's a bit too complex just yet.
Working in labs I've always taken simple chems and titrations for granted, now I'm starting to miss the possibility of doing accurate chemistry. I
need to acquire analytical chems and equipment, acid/base titrations seems like a good place to start. Where do I start? Buying reagent grade would be
the most sensible, but they are expensive and hard to get. Pharmaceutical grades are easier to get hold of, and I assume they would be pure enough for
my uses, but over the years I've become increasingly paranoid about flagging my chemistry unless I really have to.
I think I will attempt to assay sodium carbonate. It should be simple enough to recrystallize, and could perhaps be tested by precipitation with
calcium. Any better suggestions?
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blogfast25
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| Quote: Originally posted by Fulmen | I checked out the sulfamic acid, but it's a bit too complex just yet.
It should be simple enough to recrystallize, and could perhaps be tested by precipitation with calcium. Any better suggestions?
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Problem is that CaCO3 precipitates suffer from occlusion. Not a great thing for gravimetry.
Preparing pure Na2CO3 from a 'good' grade must lead to high purity by recrystallization(s).
[Edited on 18-2-2015 by blogfast25]
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DJF90
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How do you mean? It is readily available OTC as a toilet cleaner. The purification IIRC is just several recrystallisations.
Also, potassium dichromate is a suitable primary standard for redox titrations, as is thiourea. Availability may vary on location.
[Edited on 18-2-2015 by DJF90]
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blogfast25
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I've never heard of sulfamic acid as a PM, but I don't mean anything by that.
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Fulmen
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Never seen OTC sulfamic acid around here, but I must confess I've never looked for it either. I'll keep an eye out for it.
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Sulaiman
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In my teens I enjoyed chemistry as a hobby,
40+ years later I've just re-started less than a year ago,
and my first 'standard' was Sodium Carbonate.
Stated as 99.3% pure by the vendor.
The sodium Carbonate lost 0.56% weight on de-hydrating.
eBay scales, calibration weights, pH meter, and used volumetric flask and burette.
The pH meter is great if you leisurely want to draw a titration curve for one acid-base titration
(e.g. Sodium Carbonate + Sulphuric Acid)
but I found it so slow that I gave up cross-titrations (to check my errors).
I'll do my titrations again with Phenolphthalein or similar.
So, in my opinion, it's not too difficult to get results with less than 1% error
with a small investment in equipment that will be generally useful,
but for small-scale acid/base titrations (hobby, cheap) don't bother with a pH meter,
acid/base indicators are good enough I believe.
For daily qualitative chemistry,
it generally seems adequate to use the concentration and purity stated by the supplier
and dilute or use accordingly.
That's my impression so far, others more experienced may differ !
[Edited on 18-2-2015 by Sulaiman]
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Fulmen
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Zyklon: Missed your post until now, looks like I have some reading to do. Thank you, I love them old books.
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papaya
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| Quote: Originally posted by blogfast25 |
Materials I use as Primary Standards in a home set up:
Thrice recrystallized sodium carbonate, dehydrated. Acid/base standard.
[Edited on 18-2-2015 by blogfast25] |
I'm sorry, but Na2CO3 can form different hydrates if I remember properly, isn't it? How do you use it as a standard then?
EDIt: Only now I noticed "dehydrated", ok then how do you dehydrate it completely, how high the temperature must be? Also what indicator is best with
it if trying to titrate strong acids?
[Edited on 18-2-2015 by papaya]
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S.C. Wack
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http://pac.iupac.org/publications/pac/18/3/0443/pdf
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papaya
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thanks!
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Fulmen
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I've looked at both, good finds. The IUPAC-paper used reagent grade bicarbonate as a source, so that doesn't help much. It does mention a silver
standard for hydrochloric acid, but again based on purity graded silver. This can of course not regress in infinity, question is whether or not it
ends at something useful to a home lab.
Looks like the only practical route is to purify something like sodium carbonate by recrystallizing a few times and do qualitative tests for "the
usual suspects" of contaminates listed in "Chemical Reagents".
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blogfast25
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| Quote: Originally posted by papaya |
I'm sorry, but Na2CO3 can form different hydrates if I remember properly, isn't it? How do you use it as a standard then?
EDIt: Only now I noticed "dehydrated", ok then how do you dehydrate it completely, how high the temperature must be? Also what indicator is best with
it if trying to titrate strong acids?
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Dehydrate at 200 C until constant weight is achieved.
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blogfast25
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| Quote: Originally posted by Sulaiman | In my teens I enjoyed chemistry as a hobby,
40+ years later I've just re-started less than a year ago,
and my first 'standard' was Sodium Carbonate.
Stated as 99.3% pure by the vendor.
The sodium Carbonate lost 0.56% weight on de-hydrating.
eBay scales, calibration weights, pH meter, and used volumetric flask and burette.
The pH meter is great if you leisurely want to draw a titration curve for one acid-base titration
(e.g. Sodium Carbonate + Sulphuric Acid)
but I found it so slow that I gave up cross-titrations (to check my errors).
I'll do my titrations again with Phenolphthalein or similar.
So, in my opinion, it's not too difficult to get results with less than 1% error
with a small investment in equipment that will be generally useful,
but for small-scale acid/base titrations (hobby, cheap) don't bother with a pH meter,
acid/base indicators are good enough I believe.
For daily qualitative chemistry,
it generally seems adequate to use the concentration and purity stated by the supplier
and dilute or use accordingly.
That's my impression so far, others more experienced may differ !
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I broadly concur with that.
Don't expect miracles at the home lab level and for the most part they aren't needed anyway.
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macckone
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Sodium carbonate is easy to purify by crystallization.
It has a really good solubility curve. It is usually standardized
with potassium hydrogen phthalate for the highest precision.
But it can be purified and dehydrated and used on its own.
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S.C. Wack
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Actually it wasn't posted *just for you*, and BTW I did not suspect that there was anything I could do to help you in any way. I don't expect that you
have a Pt dish either.
It would be an interesting experiment to see if the highest grade of bicarbonate that Sigma-Aldrich sells gives better results in your hands than the
cheapest off-brand of USP baking soda straight from the box.
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Fulmen
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You know what, I'm all out of Pt-dishes right now :-)
Interesting experiment, I agree. I wouldn't put money on me telling them apart, I'm not expecting miracles here. It's just about doing it right, so
you know instead of believe.
But everything boils down to accuracy, there are no absolutes here.
We're not banging rocks together here. We know how to put a man back together.
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Zombie
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I found a PDF version of the 6th edition "Purification of Laboratory Chemicals" here...
http://www.pyrobin.com/files/purification%20of%20laboratory%...
They tried to have me "put to sleep" so I came back to return the favor.
Zom.
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Fulmen
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Wow, lot of good info coming forth here.
There is one question that hasn't been discussed yet, how do one establish a primary standard? I'm thinking spectroscopy is the only way, chemical
analysis can only compare to other standards or detect individual contaminates. And since there is virtually an infinite number of possible
contaminates, you can't hope to eliminate all.
We're not banging rocks together here. We know how to put a man back together.
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