scientician
Harmless
Posts: 19
Registered: 12-12-2010
Member Is Offline
Mood: No Mood
|
|
Prep. of dilute sodium isopropoxide from NaOH
An experiment I'd like to perform calls for a very dilute solution (0.07M) of sodium isopropoxide in iPrOH. Ideally I'd use plain ol' sodium
hydroxide in absolute isopropanol which translates to slightly less than 0.28g NaOH per 100ml iPrOH.
iPrOH + NaOH <-----> [iPrO-] + [Na+] + H2O(g)
The water produced is distilled off with the isopropanol, driving the reaction to the right.
I've managed to get this "working" as evidenced by the fact that the fairly small amount of NaOH does indeed dissolve in boiling isopropanol (and
doesn't precipitate out when the solution cools.) Any thoughts on how I can confirm this is what's happening? My tools are all "kitchen scale,"
though I do have a set of 19/22 glass with only a few parts broken.
Thanks in advance.
|
|
|
Nicodem
|
Thread Moved
25-2-2012 at 13:47 |
Nicodem
Super Moderator
Posts: 4230
Registered: 28-12-2004
Member Is Offline
Mood: No Mood
|
|
Quote: Originally posted by scientician  | | Any thoughts on how I can confirm this is what's happening? My tools are all "kitchen scale," though I do have a set of 19/22 glass with only a few
parts broken. |
It is not really possible to measure the amount of H2O in such a solution using chemical methods (you are dealing with a dynamic equilibrium here, so
for every molecule of H2O consumed, another one forms by the reaction of hydroxide with isopropanol). Spectroscopic and perhaps some voltametric
methods can give you the answer to such a difficult question, but these are not "kitchen scale" tools. Have you checked the literature if the pKa
values of isopropanol and H2O in isopropanol are already known? It would be stupid to reinvent the wheel, just because you don't like to search the
literature.
What can be measured by the chemical methods is the combined amounts of H2O and hydroxide. This value is in any case much more useful for some
applications of such solutions where the amount of H2O is actually irrelevant, but the combined hydroxide and water is important. For example, a
chemical mean would be to react your solution with a known amount of isopropyl benzoate and measure the ratio of sodium benzoate vs. isopropyl
benzoate. From this, the amount of hydroxide and H2O can be calculated (provided that the isopropyl benzoate was dry). To measure the extent of
hydrolysis you would need to develop some analytical method which would either require an acidic quench and HPLC analysis or some other more or less
reliable method. Again, not really kitchen friendly.
The only kitchen friendly method that I can think of, is a conductometric measurement. This does not require any particularly expensive equipment
(nowadays, fairly reliable multimeters can be bought for a few dozens euros). I'm not sure if it would give reliable results (this depends on the
measurement error and the influence of the solution composition on the conductance), but another major problem is that you would need a standard
solution of sodium isopropoxide in isopropanol to create a calibration curve. This standard solution could be made by careful distillation of the
water-isopropanol azeotrope from a solution of NaOH in isopropanol, but you would need some special equipment like a very efficient distillation
column and a precise thermometer to find out when there is no more water in the distillate (constant T). You would also need to determine the exact
volume of the remaining isopropoxide solution to get a reliable concentration. The calibration curve is then done by measuring the conductance in
dependence to the amount of added water. In any case, you would need an analytical balance, precise volumetry, a reliable multimeter and a fixed
conductometric cell (at least this can be easily self-made). The most expensive "non-kitchen" equipment is therefore the analytical balance (leaving
the exact composition of the starting NaOH as the major source of error would, however, make the precision of the balance pretty obsolete).
(can you see why physical chemistry is so unpopular?)
| Quote: | | I've managed to get this "working" as evidenced by the fact that the fairly small amount of NaOH does indeed dissolve in boiling isopropanol (and
doesn't precipitate out when the solution cools.) |
I don't understand how you come to the conclusion that is evidence for whatever. NaOH is relatively soluble in isopropanol.
|
|
|
scientician
Harmless
Posts: 19
Registered: 12-12-2010
Member Is Offline
Mood: No Mood
|
|
Vogel confirms this actually works. 
|
|
|
![[*]](images/xpblue/default_icon.gif){kind=icon}
