nimgoldman
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Moisture-preserving desiccant for storing borax primary standard
I am purifying some borax decahydrate to be used as a primary standard for acid-base titrations.
I have a problem with removing excess water while preserving the hydrate.
I understand that using desiccator with some common drying agent doesn't work since the compound may lose some molecules of hydration, making it
unusable as a standard. We need just the decahydrate, but with no excess water.
I found some sources (1,2) recommending putting the crystals in a desiccator over saturated sodium chloride and sucrose
instead.
So far I understand that the saturated salt solution will turn the desiccator into a hygrostat, keeping constant humidity. Any excess water will be
absorbed in the salt solution yet the humidity is kept high enough to preserve the hydrate.
I don't understand why the use of salt and sucrose together?
I've only found that saturated sucrose gives about 85% r.h. (at 25 °C), which is higher than with sat. NaCl (75% r.h.). One explanation might be
that since sucrose is extremely soluble in water (approx. 202 g/100 mL), we saturate the water with salt first and then with sucrose to avoid using
excess sugar.
I assume considerably less sugar will dissolve in sat. NaCl so it might be economic reasons.
Is there any other reason for using NaCl and sucrose together?
I found a patent application (3) where they use the NaCl-sucrose solution but the ratio of NaCl to sucrose is 4:1, which won't lead
to saturated solution of neither substance. Maybe they means 1:4 ? I calculated that for a given amount of salt, about four times the weight of sugar
is needed to saturate solution already saturated with the salt but I haven't checked it experimentally.
I've looked whether there is a relationship between the level of hydration and relative humidity, but also haven't found much.
One source (4) recommended drying borax decahydrate over NaBr.2H2O. Does it mean saturated NaBr? If so, this will give considerably
lower r.h. (58% @ 15 °C). It might also mean using solid sodium bromide dihydrate, which does not make much sense as the salt is not hygroscopic and
easily gives off its water of hydration (there is a transition to anhydrous NaBr at about 36 °C).
Can someone please shed some light on this equilibration process?
[1] Armarego, Wilfred LF. Purification of laboratory chemicals. Butterworth-Heinemann. 2017.
[2] Vogel, Arthur Israel, and George Harold Jeffery. Vogel's textbook of quantitative chemical analysis. Wiley, 1989.
[3] Chinese patent CN101885492B
[4] titrations.info
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unionised
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A solution saturated with both salt and sugar will have even less water in that one saturated with one or the other.
So it will maintain a humidity lower than 75%
I presume someone once checked and found that it was "the right humidity".
Both ingredients (all 3 if you count water) are cheap and easy to find.
I'd try that one first.
Having said that, for titrations, I'd use sodium carbonate made from food grade bicarbonate by heating it.
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nimgoldman
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I see.
Using NaBr as an alternative then makes sense as the saturated solution gives 58% r.h.
NaCl and sucrose are definitely cheaper.
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Carbon8
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I agree with unionized. Why not make sodium carbonate from sodium bicarbonate and use that as your primary standard? The process is easy and produces
a very pure product. Here are some links. And you only need to heat the bicarbonate to 300F to produce the carbonate.
https://archive.org/details/dli.ernet.212199/page/79/mode/2u...
https://pubs.acs.org/doi/pdf/10.1021/ed010p507
https://www.youtube.com/watch?v=cpGEc-pLXN4
Attachment: sodium carbonate as standard.pdf (240kB)
This file has been downloaded 242 times
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Bedlasky
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NaHCO3 is pretty much pure, it can be used directly as standard.
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