Bezaleel
Hazard to Others
Posts: 444
Registered: 28-2-2009
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Mood: transitional
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melting sodium bisulphate
Last week I discovered an old bottle labelled NaHSO4∙H2O. The bottle contained a solid crystalline mass, taking up about half its volume, and on top
of that was an almost equally think layer of colourless liquid. I suppose that Na2SO4 had crystallised out and the supernatant liquid was H2SO4 + H2O
1:1.
I poured off the liquid in a flask, and put some water on the crystals, and put it on a stirrer for half an hour. All crystals had dissolved or were
moving freely in the vial then. The content was added to the flask and heated in an all-glass distillation setup. A few pieces of glass were added to
reduce bumping.
For about half an hour, vapour came over at 100°C. About 1/3 of the total contents of the flask had distilled over then. The temperature increased to
110°C, and some more liquid came over. The contents of the flask then started to bubble less, and finally bubbling almost ceased. The temperature
measured in the still head dropped to below 100°C then.
Out of fear that the flask would break on cooling, I decided to pour the contents into a petri dish. Shortly after crystallisation started, the dish
broke, and some of the half solidified substance reacted with the wooden board underneath the dish. I collected the remaining white solid after cool
down.
From wiki:
MP NaHSO4∙H2O 58.5°C
MP NaHSO4 315°C (decomposes to Na2S2O7 + H2O)
(The threads here and here on SM confirm the melting point of the hydrate and the temperature where the pyrosulphate is formed.)
I wonder what the substance is I got. My guess would be NaHSO4∙H2O or NaHSO4. The low MP of the hydrate, would be in good agreement with the long
time it took before the substance had completely solidified.
What I can't find on the net is the point where NaHSO4∙H2O looses its molecule of H2O and changes to NaHSO4. The melting point of NaHSO4 is 315°C,
but that is also the point where it condensates to Na2S2O7. This makes me doubt about whether I did get the hydrate or not. Also, the strange
temperature increase to 110°C, which lasted for an estimated 15 minutes, makes me wonder what has been going on.
Of course I could do a titration neutralising the acid, and precipitating the sulphate as BaSO4, but I hope someone here can tell me what I got from
personal experience or a reference.
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JJay
International Hazard
Posts: 3440
Registered: 15-10-2015
Member Is Offline
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I've also noticed that physical properties data is not as freely available for sodium bisulfate as one would expect given how common it is and how
widely it is recommended for use in amateur chemistry. I haven't checked the major references like Lange's handbook, but in the past, I found some of
sodium bisulfate's properties by searching JACS articles.
You can check the melting point and do a titration. The supernatant liquid likely contained some dissolved sodium bisulfate. There may be impurities
such as other sodium salts... these will be a little tricky to detect.
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