科研之友
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摘要
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) A(590)/A(436)) for the acid and base forms via the following equation: pH = pK(a) +log[(R-[25]-e(1))/(e(2)-R-[25].e(3))], where, e(1) = 0.00533, e(2) = 2.232, e(3) = 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)) =pK(a(35)) +[(0.00174 +/- 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 mu mol 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 mu mol kg(-1). Hence, if high-precision alkalinity measurements are not required (>= 4 mu mol 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 mu mol 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; similar to 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.
- 出版日期2018-7
