The common electropolish for Nitinol and other Titanium alloys is a generic Sulfuric acid/Methyl Alcohol blend. When this is made up, the addition of Sulfuric acid to the Methyl alcohol heats the Methyl Alcohol, which causes the Methyl alcohol to evaporate rapidly. Besides throwing off the mix ratio, this is dangerous, as Methyl alcohol is both flammable and poisonous. [Other than that it is a pussycat!}
In process of investigating making this blend to offer for sale, we were initially profoundly confused and frustrated because it appeared that some of the Sulfuric Acid seemed to "disappear" during the manufacturing process. This led to close examination of the raw materials to determine where the problem originated.
Much effort was expended with no initial clarification of the problem. We began to doubt both our sanity and the laws of chemistry...
One day, we had an epiphany. We realized that In fact, some of the acid was being consumed, in a reaction we initially thought would have required much more time and temperature. It turns out that the Sulfuric Acid reacts with the Methyl Alcohol to produce Methyl Sulfate ester. For the chemists reading this, the reaction is:
H2SO4 + CH3-OH --> HSO4-CH3 + H2O
[Sulfuric acid] [Methanol] [Methyl Sulfate] [Water]
A key part of this equation is that water is formed as a byproduct.
The formation of water is actually an important side reaction, as when the water level increases, the reaction forming methyl ester slows, and ultimately stops, thus limiting the amount of methyl ester which can be formed. It also significant that we have found that approximately 2% water is formed. If the sulfuric acid used to make the blend is 95% active, thus contains 5% water, this would imply that the final mix contains 3.5% water. This clearly limits the life of this mix since water is a poison, and will ultimately stop the polishing as it increases in concentration.
However, this reaction completely negates the possibility of a simple acid/base titration to determine sulfuric acid levels in the final blend. It is clear that for appropriate Quality Control a way around this problem needed to be found. Thank modern chromatography for the answer to this. Using ion chromatography, determination of methyl sulfate is easy, and the sulfate level can be determined at the same time, thus allowing calculation of total sulfuric acid added to the blend.
We hope that this clarifies the situation for those people making Methyl alcohol/ Sulfuric Acid blends, and finally allows precise QC of the blends.