The error of titrations? A titration is a perfectly acceptable form of quantitative chemical analysis – but only in the right application.
The production of MEA-triazine is not the right application. In fact, titrations are the simplest, cheapest, easiest way to learn nothing about your triazine concentration. A poor quality triazine can be made to look high quality simply by adding free ethanolamine. Excess ethanolamine appears in a manual titration to be triazine. If you’re using a titration for quality control or quality assurance, I can assure you that you have neither quality nor control.
I recently visited a customer manufacturing MEA-triazine. They perform titrations for triazine analysis. Their setup is not too dissimilar from the version I used in high school chemistry–roughly around the time the Berlin Wall fell: a tall glass burette in a clamp, filled with hydrochloric acid; and a beaker containing a sample on a stir plate with a stir rod. The only difference is a pH probe instead of colorimetric pH indicators. The approach here is to add acid until the pH reaches a pH setpoint, with the volume of added acid corresponding to the triazine concentration.
This approach is deceiving.
Take a simple solution of ethanolamine (MEA) in water. Ethanolamine is a weak base with a pKa of around 9.5. Since a pH measurement is simply a measurement of hydrogen ion concentration, when you add acid to a solution, you increase the number of hydrogen ions and lower the pH.
With ethanolamine present, the amine group grabs the hydrogen ions and becomes protonated. The amine becomes a positively charged ion. As you continue adding acid, the pH changes slowly, until you approach the equivalence point–the point is when the all available amine is protonated. Near the equivalence point, the pH changed very rapidly. Once past this point, the continued addition of acid again results in a slow change in pH.
With a pH probe, one approach to amine analysis is to add acid while monitoring the pH. When the pH is at the equivalence point, you can calculate amine concentration based on how much acid was required to get there. This is all well and cool, except for a couple things. First, you need to know the equivalence point. You can do the hard work to calibrate against known standards; or, like this customer, you can just get a number from someone that has done it before and hope it applies to your samples.
Second, what happens if the mixture is more complex? What if there are two chemicals that consume hydrogen ions? MEA-triazine is simply a stoichiometrically equal mixture of formaldehyde and ethanolamine. When acid is added to an MEA-triazine solution, it hydrolyzes into ethanolamine and formaldehyde, consuming the hydrogen ion. So the pH does not change until all of the triazine is consumed. And since this hydrolysis produces ethanolamine, the pH does not change until all of the ethanolamine is protonated. If a sample contains only MEA-triazine, a well-calibrated titration can be used to calculate the triazine concentration.
But what if there is excess ethanolamine? More importantly, what if there is an unknown amount of MEA-triazine and excess ethanolamine? You have one equation and two unknowns. Simply adding acid to a pH endpoint tells you nothing about the MEA-triazine concentration. This is the error of titrations.
This customer produces a triazine using formalin and ethanolamine, with a theoretical maximum of 51%. They believe, however, that they should get a 60%, so they continue adding amine until the titration shows the concentration to be 60%. They have not made more triazine, they only adjust the equivalence point by adding additional amine. Then they dilute and ship to the customer. The “42% triaizine” the end user receives is really 35% triazine with 7% excess MEA.
Why does the end user not complain? Well, they use a titration on receipt. Everything looks perfect! I asked the manufacturer, “Why add formaldehyde?” Just titrate ethanolamine to the pH endpoint. And unfortunately their reply was not one that gave me confidence they wouldn’t try this in the future. Caveat emptor.
What’s the solution? Quantitative Raman Spectroscopy (QRS™) using an OPAL-103 Triazine Analysis System. Our instrument generates multivariate data that allows us to calculate MEA-triazine, ethanolamine, and formaldehyde concentrations with a single pipette step and two minutes of time. Avoid the error of titrations with QRS.
