Here is an extremely sensitive method for measuring nitrite (NO2-) down to 1 nM in natural waters. The water sample is treated with sulfanilamide and N-(1 naphthylethylenediamine) in acidsolution to produce a colored product with a molar absorptivity of 4.5 × 104 M-1 cm-1 at 540 nm. The colored solution is pumped into a 4.5-meter-long, coiled Teflon tube whose fluorocarbon wall has a refractive index of 1.29. The aqueous solution inside the tube has a refractive index near 1.33. The colored solution is pumped through the coiled tube. An optical fiber delivers white light into one end of the tube, and an optical fiber at the other end leads to a polychromator and detector.


(a) Explain the purpose of the coiled Teflon tube and explain how it functions.
(b) What is the critical angle of incidence for total internal reflection at the Teflon/water interface?
(c) What is the predicted absorbance of a 1.0-nM solution of colored reagent?