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Topic: Titration of TEA with HCl  (Read 6572 times)

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Offline GreenHobGlobulin

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Titration of TEA with HCl
« on: January 23, 2012, 10:38:41 AM »
Hello all,

I'm trying to understand a test method here and was hoping you guys would lend a hand (or brain). It's an acid/base titration of triethanolamine with hydrochloric acid.

The test solution has a target concentration of 1M TEA. The sample is prepared by adding 5mL to about 70mL of purified water along with 4-5 drops of methyl red indicator (prepared by adding 100mg of methyl red to 100mL of ethanol and filtering).

The resulting solution is then titrated using 1N HCl to a chemical endpoint. Sounds simple right? Well, here's where things get a little more complicated. Instead of methyl red going from yellow to red/orange like it's supposed to, a white precipitate is formed. There also appears to be a lot of gas being produced.

Now it gets even odder. The test method (not written by me) claims that 1 mL of 1N HCl is equivalent to 149.2mg of TEA. This is seems right considering that the MW of TEA is 149.2 g/mol or 149.2 mg/mmol. So 1mL of 1mmol/mL HCl = 1 mmol of HCL = 1mmol of TEA = 149.2mg TEA (at chemical endpoint).

Then it goes on to give an example calculation:
0.9mL of 1N HCl = 0.9 x 149.2mg = 134.28mg TEA

This is where the test method author loses me. Assuming you added 5mL of a 1M TEA solution, shouldn't the titration require 5mL of 1N HCl?

Thanks in advance and sorry for such a long post. I thought it important to provide all the background info.

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