II. Lambda 20: a faithful old spectrometer for a new friend

Last week I moved from a desk covered in articles to a little bench clearing in the lab. I reveled in the opportunity to finally observe the chemistry I've been reading about. Already I've spent many days becoming well acquainted with the lab's spectophotometer, Lambda 20. I can tell already I will have a particularly special friendship with this instrument for the summer.

The most common method of determining persulfate concentration is in a reaction with iodide ions (ususally from KI). This produces iodine (I2) in solution, forming a light yellow to rusty brown color in aqueous solution, depending on the concentration. Though this reaction was originally used in titrations, it has been converted to a spectrophotometric method (where the absorption of light is directly proportional to the concentration of the chemical).

However, measuring any change in color tells us about the iodine product (I2), rather than the persulfate reagent. Since there are many oxidants that will convert iodide to iodine, we cannot rightfully assume that the amount of iodine reflects the amount of persulfate in the water. Especially when we consider that some KI will spontaneously oxidize with exposure to light.

Therefore, last week I jumped right into the lab to become comfortable with the persulfate-iodide reaction. I tested the stability of aqueous solution of each reagent, observing that each had dramatic changes in absorption measurements over a period of 12 hours. Thus, each day requires new solutions of KI and persulfate. I examined the recommendations to use buffers and bicarbonate (supposedly to prevent oxidation from other oxygen species in solution), finding evidence that they made no difference in the iodine product concentrations. I also explored the possibility of using less iodine (thereby extending the time for equilibrium), but quickly eliminated that option due to the. Further, I tested time dependence of absorption measurements to find the time necessary for the persulfate-iodide reaction to equilibrate for stable absorption readings. Unfortunately, my attempts at calibration curve at the end of the week were foiled by a miscalculation of persulfate concentration. Starting over again today has already been fruitful and encouraging.

After becoming acquinted with is persulfate and KI reaction, I question the practicality of this method: it is not selective to persulfate as an oxidant, it requires an extreme excess of rather expensive KI as well as incredibly dilute persulfate, and the time of measurement has a strong sway on the absorption readings. Most of these variables we can control, and I recently uncovered an elaborate method that makes the measurements more selective to persulfate, but it can be a downright hassle. So while maintaining optimism, I am curious to see if we can revive another method.