Purification and Characterization of meta-Cresol Purple for Spectrophotometric Seawater pH Measurements

Spectrophotometric procedures allow rapid and precise measurements of the pH of natural waters. However, impurities in the acid–base indicators used in these analyses can significantly affect measurement accuracy. This work describes HPLC procedures for purifying one such indicator, meta-cresol purple (mCP), and reports mCP physical–chemical characteristics (thermodynamic equilibrium constants and visible-light absorbances) over a range of temperature (T) and salinity (S). Using pure mCP, seawater pH on the total hydrogen ion concentration scale (pHT) can be expressed in terms of measured mCP absorbance ratios (R = λ2A/λ1A) as follows:where −log(K2Te2) = a + (b/T) + c ln T – dT; a = −246.64209 + 0.315971S + 2.8855 × 10–4S2; b = 7229.23864 – 7.098137S – 0.057034S2; c = 44.493382 – 0.052711S; d = 0.0781344; and mCP molar absorbance ratios (ei) are expressed as e1 = −0.007762 + 4.5174 × 10–5T and e3/e2 = −0.020813 + 2.60262 × 10–4T + 1.0436 × 10–4 (S – 35). The mCP absorbances, λ1A and λ2A, used to calculate R are measured at wavelengths (λ) of 434 and 578 nm. This characterization is appropriate for 278.15 ≤ T ≤ 308.15 and 20 ≤ S ≤ 40

Purification of Meta-cresol Purple and Cresol Red by Flash Chromatography: Procedures for Ensuring Accurate Spectrophotometric Seawater PH Measurements

Impurities in sulphonephthalein indicator salts can result in significant errors in seawater pH determinations. To ensure suitable measurement accuracy and intercomparability on a global basis, impurities must be removed from all indicators used for oceanographic CO2 system analyses. Previous work has described an effective HPLC (high-performance liquid chromatography) procedure for purification of meta-cresol purple, but the technique is labor-intensive, with each HPLC run producing only a small batch of purified indicator. This work describes the use of flash chromatography to more efficiently produce large batches of purified meta-cresol purple (mCP) and cresol red (CR), the preferred indicators for direct water column determinations of seawater pH. Several batches of unrefined mCP and CR of independent origin were prepared by flash chromatography. Indicator purity was then assessed in two ways: by (a) HPLC verification and (b) pH measurements of highly buffered solutions. HPLC chromatograms of the various flash-prepared mCPs indicated that the process did not always result in a completely pure product. In terms of performance, however – i.e., pH measurements of highly buffered solutions – no differences were observed between an HPLC-purified reference mCP and the flash-purified mCPs. HPLC examination of the flash-purified CRs indicated that every product was free of detectable impurities. No differences were seen in comparative pH measurements made with the purified CRs. The flash chromatography procedures outlined in this work are suitable for producing bulk quantities of mCP and CR for use in high-precision spectrophotometric pH measurements in seawater

Physical–chemical Characterization of Purified Cresol Red for Spectrophotometric pH Measurements in Seawater

The use of impure cresol red in spectrophotometric seawater pH measurements can introduce systematic inaccuracies greater than 0.1. Cresol red has been purified on a bulk scale to address this problem, but a characterization of the dye\u27s physical–chemical properties has not been provided to date. This work reports the physical–chemical characteristics of purified cresol red for use in spectrophotometric seawater pH measurements over a range of temperatures and salinities. Seawater pH is expressed on the total hydrogen ion concentration scale (pHT) in terms of the ratio (R) of cresol red absorbances (A) at 433 and 573 nm (RCR = 573A/433A): pHT=−logK2Te2+logRCR−e11−RCRe3e2 where − log(K2Te2) = a + b/T + c ln T − dT a=−859.326051+0.14616S+7.81164×10−4S2b=22969.9366+8.04468S−0.20512S2c=152.209523−0.0317821Sd=0.259915 and cresol red molar absorptivity ratios are expressed as: e1=−0.00413+1.814×10−5Te3/e2=−0.021683+1.8107×10−4T+3.163×10−5S−35 for 278.15 ≤ T ≤ 308.15 K and 20 ≤ S ≤ 40. We recommend using cresol red to measure the acidity of seawater that has (at 298.15 K) a pHT of 6.8–7.8. This range might be encountered in ocean areas such as oxygen minimum zones or, hydrothermal vent fields, or it might be imposed in controlled laboratory studies. Ocean acidification will make cresol red an increasingly important indicator in coming decades as waters within ever larger ocean areas shift into its optimal indicating range

Seawater PH Measurements in the Field: A DIY Photometer with 0.01 Unit PH Accuracy

A portable light-emitting-diode (LED) photometer has been developed to provide low-cost seawater pH measurements. The benefits of the new system include a simple “do-it-yourself” construction design, a hundredfold reduction in cost relative to benchtop spectrophotometric systems, routine calibration-free operation in the field, and precision and accuracy well suited to applications such as education, coastal zone monitoring (including citizen science programs), and aquaculture and aquarium management. The photometer uses a high-sensitivity light-to-voltage integrated circuit as a detector, two LED light sources, and an open-source Arduino microcontroller for system control and data processing. Measurements are based on observations of absorbances of a pH-sensitive indicator. With meta-cresol purple, a sulfonephthalein indicator appropriate to natural seawater, the photometer system produces pHT measurements within 0.01 units of state-of-the-art spectrophotometric measurements (7.6 ≤ pH ≤ 8.2, 30 ≤ S ≤ 36.2, and 15 °C ≤ t ≤ 30 °C) and has a pH precision of ± 0.002. Measurement accuracy is achieved with a one-time calibration that relates absorbance ratios measured by the broadband photometer (RB) to absorbance ratios measured by a high-quality (narrowband) spectrophotometer (RN). Calculation of RN from RB allows the use of published algorithms that yield seawater pH as a function of RN, temperature, and salinity

Seawater PH Measurements in the Field: A DIY Photometer with 0.01 Unit PH Accuracy

A portable light-emitting-diode (LED) photometer has been developed to provide low-cost seawater pH measurements. The benefits of the new system include a simple “do-it-yourself” construction design, a hundredfold reduction in cost relative to benchtop spectrophotometric systems, routine calibration-free operation in the field, and precision and accuracy well suited to applications such as education, coastal zone monitoring (including citizen science programs), and aquaculture and aquarium management. The photometer uses a high-sensitivity light-to-voltage integrated circuit as a detector, two LED light sources, and an open-source Arduino microcontroller for system control and data processing. Measurements are based on observations of absorbances of a pH-sensitive indicator. With meta-cresol purple, a sulfonephthalein indicator appropriate to natural seawater, the photometer system produces pHT measurements within 0.01 units of state-of-the-art spectrophotometric measurements (7.6 ≤ pH ≤ 8.2, 30 ≤ S ≤ 36.2, and 15 °C ≤ t ≤ 30 °C) and has a pH precision of ± 0.002. Measurement accuracy is achieved with a one-time calibration that relates absorbance ratios measured by the broadband photometer (RB) to absorbance ratios measured by a high-quality (narrowband) spectrophotometer (RN). Calculation of RN from RB allows the use of published algorithms that yield seawater pH as a function of RN, temperature, and salinity

Internal Consistency of Marine Carbonate System Measurements and Assessments of Aragonite Saturation State: Insights from Two U.S. Coastal Cruises

This research assesses the thermodynamic consistency of recent marine CO2 system measurements in United States coastal waters. As one means of assessment, we compared aragonite saturation states calculated using various combinations of measured parameters. We also compared directly measured and calculated values of total alkalinity and CO2 fugacity. The primary data set consists of state-of-the-art measurements of the keystone parameters of the marine CO2 system: dissolved inorganic carbon (DIC), total alkalinity (TA), CO2 fugacity (fCO2), and pH. This study is the first thermodynamic CO2 system intercomparison based on measurements obtained using purified meta cresol purple as a pH indicator. The data are from 1890 water samples collected during NOAA\u27s West Coast Ocean Acidification Cruise of 2011 (WCOA2011) and NOAA\u27s Gulf of Mexico and East Coast Carbon Cruise of 2012 (GOMECC-2). Calculations of in situ aragonite saturation states (ΩA) near the saturation horizon exhibited differences on the order of ± 10% between predictions based on the (DIC, TA) pair of measurements vs. the (pH, DIC), (fCO2, DIC), or (fCO2, pH) pairs. Differences of this magnitude, which are largely attributable to the imprecision of ΩA calculated from the (DIC, TA) pair, are roughly equivalent to the magnitude of ΩA change projected to occur over the next several decades due to ocean acidification. These observations highlight the importance of including either pH or fCO2 in saturation state calculations. Calculations of TA from (pH, DIC) and (fCO2, DIC) showed that internal consistency could be achieved if minor subtractions of TA (≤ 4 μmol kg− 1) were applied to samples of salinity \u3c 35. The extent of thermodynamic consistency is also exemplified by the small offset between TA calculated from (DIC, pH) and that calculated from (DIC, fCO2): ~ 3 μmol kg− 1, which is similar to the accuracy of the TA measurements. Systematic trends can be detected in the offsets between measured and calculated parameters, but for this high-quality data set the magnitude of methodological improvements required to achieve exact thermodynamic consistency is quite small

Procedures for Direct Spectrophotometric Determination of Carbonate Ion Concentrations: Measurements in US Gulf of Mexico and East Coast Waters

Refined procedures were developed for directly determining carbonate ion concentrations in seawater through measurement of the ultraviolet absorbances of lead carbonate and chloride complexes after addition of divalent lead (Pb(II)) to a seawater sample. Our model algorithm is based on carbonate ion concentrations calculated from measurements of pH and dissolved inorganic carbon (DIC) obtained on a NOAA ocean acidification cruise (GOMECC-2, the second Gulf of Mexico and East Coast Carbon cruise). These calculated carbonate concentrations, in conjunction with Pb(II) absorbance measurements for the same seawater samples, were used to refine previous algorithms based on different chemical-measurement techniques and a limited range of carbonate concentrations. The precision of the spectrophotometric carbonate measurements is affected by the concentration of Pb(II) in the titrated seawater samples. Doubling the concentration of the titrant improved precision relative to previously published procedures but required formulation of a correction for changes in carbonate ion concentration caused by the titrant addition. Minor changes in the new algorithm for the spectrophotometric method produced carbonate ion values (at 25 °C) in excellent agreement with values calculated from paired pH and DIC observations over a carbonate concentration range of 73–258 μmol kg − 1. This new algorithm, tested on three subsequent research cruises in the Gulf of Mexico, showed a random scatter of residuals and an average offset between measured and calculated carbonate concentrations equal to − 0.92 ± 5.33 μmol kg− 1

Spectrophotometric Measurement of Calcium Carbonate Saturation States in Seawater

Measurements of ocean pH and carbonate ion concentrations in the North Pacific and Arctic Oceans were used to determine calcium carbonate saturation states (ΩCaCO3) from spectrophotometric methods alone. Total carbonate ion concentrations, [CO32–]T, were for the first time at sea directly measured using Pb(II) UV absorbance spectra. The basis of the method is given by the following: where CO3β1 is the PbCO30 formation constant, ei are molar absorptivity ratios, and R = 250A/234A (ratio of absorbances measured at 250 and 234 nm). On the basis of shipboard and laboratory Pb(II) data and complementary carbon-system measurements, the experimental parameters were determined to be (25 °C) the following: The resulting mean difference between the shipboard spectrophotometric and conventional determinations of [CO32–]T was ±2.03 μmol kg–1. The shipboard analytical precision of the Pb(II) method was ∼1.71 μmol kg–1 (2.28%). Spectrophotometric [CO32–]T and pHT were then combined to calculate ΩCaCO3. For the case of aragonite, 95% of the spectrophotometric aragonite saturation states (ΩAspec) were within ±0.06 of the conventionally calculated values (ΩAcalc) when 0.5 ≤ ΩA ≤ 2.0. When ΩA \u3e 2.0, 95% of the ΩAspec values were within ±0.18 of ΩAcalc. Our shipboard experience indicates that spectrophotometric determinations of [CO32–]T and ΩCaCO3 are straightforward, fast, and precise. The method yields high-quality measurements of two important, rapidly changing aspects of ocean chemistry and offers capabilities suitable for long-term automated in situ monitoring

Spectrophotometric Measurement of Calcium Carbonate Saturation States in Seawater

Measurements of ocean pH and carbonate ion concentrations in the North Pacific and Arctic Oceans were used to determine calcium carbonate saturation states (Ω_CaCO₃) from spectrophotometric methods alone. Total carbonate ion concentrations, [CO₃^2–]_T, were for the first time at sea directly measured using Pb­(II) UV absorbance spectra. The basis of the method is given by the following: where _CO₃β₁ is the PbCO₃⁰ formation constant, <i>e</i>_<i>i</i> are molar absorptivity ratios, and <i>R</i> = ₂₅₀<i>A</i>/₂₃₄<i>A</i> (ratio of absorbances measured at 250 and 234 nm). On the basis of shipboard and laboratory Pb­(II) data and complementary carbon-system measurements, the experimental parameters were determined to be (25 °C) the following: The resulting mean difference between the shipboard spectrophotometric and conventional determinations of [CO₃^2–]_T was ±2.03 μmol kg^–1. The shipboard analytical precision of the Pb­(II) method was ∼1.71 μmol kg^–1 (2.28%). Spectrophotometric [CO₃^2–]_T and pH_T were then combined to calculate Ω_CaCO₃. For the case of aragonite, 95% of the spectrophotometric aragonite saturation states (Ω_Aspec) were within ±0.06 of the conventionally calculated values (Ω_Acalc) when 0.5 ≤ Ω_A ≤ 2.0. When Ω_A > 2.0, 95% of the Ω_Aspec values were within ±0.18 of Ω_Acalc. Our shipboard experience indicates that spectrophotometric determinations of [CO₃^2–]_T and Ω_CaCO₃ are straightforward, fast, and precise. The method yields high-quality measurements of two important, rapidly changing aspects of ocean chemistry and offers capabilities suitable for long-term automated in situ monitoring

Methods used to measure carbonate system parameters on the HLY1002 (2010) and HLY1102 (2011) cruises.

<p>Methods used to measure carbonate system parameters on the HLY1002 (2010) and HLY1102 (2011) cruises.</p