Chemical Forums
Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: Ramz on December 11, 2008, 05:07:53 AM
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Hello. Sorry if my English is not good.
In microcircuitry the mixture of potassium hydroxide, water and isopropyl alcohol is known to etch silica. It is also known that the system separates into two phases with the presense of a lot of KOH in the bottom phase and a lot of IPA in the upper one. Please, help me with the explaining of stratification. And what concentrations of IPA, water and KOH I have in these phases. To prepare solution I use 9 ml of the 12% aqueous solution of KOH and 35 ml of the isopropyl alcohol.
Thanks.
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Have you determined whether or not there are any reactions between species?
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It seems to me there is no reaction between species. Also, from chemistry it is known that monohydric alcohols do not react with aqueous solutions of alkali.
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If IPA and KOH separate, then you have 12% KOH in lower solution and 100% IPA in upper, organic phase. I don't know what is solubility of IPA in water. Certainly, some extent of IPA in the presence of KOH exists in alcoholate form.
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However, the alcohol reacts with the aqueous solution of alkali? What is the volume of this alcoholate form and what is it like?
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Impossible to say, very small amount of IPA react with KOH. pKa of IPA is 16,5!
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Let's say both solutions mix together, in this case you have 10,4 M IPA and 0,44 M KOH (calculate by yourself). Since pKa is 16,5, the concentration of alcoholate form is:
10-16,5 = (alcoholate)*(0,44)/(10,4) = 7,47*10-16
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Sorry, wrong. Insert conc. of H+, not OH- and you obtain that the conc. of alcoholate is 0,013 M. That is circa 0,1% of IPA exists in alcoholate form.
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The phenomena you observed, phase separation of your etchant, is not too rare -- sometimes, an ionic salt, water, and miscible organic "oil out", and phase separate like you observed. This will happen to my mobile phase for HPLC -- TEAA, water, and acetonitrile are all miscible with each other in a bottle, but sometimes separate, if allowed to sit inside a system, without a sizeable volume for natural convection to mix them. Your micro circits may even have more problems -- the hydrophobicity of the etched channel may draw the components apart ('tho that is just a guess on my part.) You may have to investigate surfactants compatible with your application.
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A while back, on these forums, someone mentioned how their phenolpthalein solution was gelling in their silica microchannels. As I recall, (because I can't find it in search), they were using a usual concentration of phenophalein, in ethanol, for a typical visual acid base titration, but it was too much ethanol for the hydroxide concentration, in a microchannel.
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Woo-Hoo. The search function is back, so I found my old post:
http://www.chemicalforums.com/index.php?topic=22851.0
See if the explanation helps for your particular application.