A simple colorimetric method for determining seawater alkalinity using bromophenol blue

The development of small portable USB-spectrophotometer systems makes monitoring alkalinity and pH possible in the field and remote locations. Here, we present a method utilizing purified bromophenol blue (BPB) as an end-point indicator for making simple one-point alkalinity measurements with spectrophotometric detection. The approach utilizes purified BPB dye whose absorbance characteristics have been determined over a range of temperatures and salinities. The end-point pH for titrated samples was determined using the BPB absorbance ratio (R_{(t) = 25} A₅₉₀/A₄₃₆) for the acid and base forms via the following equation: pH+pK_a + log[(R_[25]-e₁)/(e₁-R_[25].e₃)], where, e₁ = 0.00533, e₂ = 2.232, e₃ = 0.0319. A pK_a of 3.513 was determined for the dissociation of the second proton from the BPB dye. The temperature (t) dependence of R can be expressed using the following relationship: R₍₂₅₎=R_(t)[1+0.006774±0.000009(25-t)]. The dependence of the pK_a on salinity (S) was weak and can be expressed as pK_a(S)5=pK_a(35)+[0.001746±0.00008)(35-S)]. Application of the method for determining the alkalinity of in-house and certified standards typically produced an uncertainty of ± 1.5 μmol kg⁻¹ for purified BPB dye. When the impure BPB dye was used as an end-point indicator the uncertainty for alkalinity measured was slightly higher at approximately ± 3–4 μmol kg⁻¹. Hence, if high-precision alkalinity measurements are not required (≥ 4 μmol kg⁻¹) then utilization of the unpurified BPB maybe suitable. We also compared the use of BPB to two other dyes: bromocresol purple (BCP) and bromocresol green (BCG). The utilization of all three dyes for end-point determination produced comparable results with an overall precision of ± 4 μmol kg⁻¹. The one-point titration method using BPB was utilized at a remote field location, One Tree Island, Australia and was found to be suitable for producing accurate and precise alkalinity data in a timely manner; ∼ 10–15 samples can be determined per hour. When combined with seawater pH measurements, the one-point titration method allows the full marine carbonate system to be fully constrained without the need for high-tech spectrophotometric equipment and comprehensive laboratory facilities.