Titrations, while primarily known as the chemical rite of passage for fledgling science students, are still widely used for chemical analysis. With its many years of existence and improvement, the method would seem an unlikely candidate for innovation, yet it is desirable, in this age of autonomous sensing where analyzers may be sent into space or to the bottom of the ocean, to have a simplified titrimetric method that does not rely upon volumetric or gravimetric measurement of sample and titrant. In previous work on the measurement of seawater alkalinity, we found that use of a tracer in the titrant eliminates the need to measure mass or volume. Here, we show the versatility of the method for diverse types of titrations and tracers. The results suggest that tracers may be employed in all types of titrations, opening the door for greatly simplified laboratory and field-based chemical analysis.

This study examines the utility of combining pH measurements with other inorganic carbon parameters for autonomous mooring-based carbon cycle research. Determination of the full suite of inorganic carbon species in the oceans has previously been restricted to ship-based studies. Now with the availability of autonomous sensors for pH and the partial pressure of CO2 (pCO2), it is possible to characterize the inorganic carbon system on moorings and other unmanned platforms. The indicator-based pH instrument, SAMI-pH, was deployed with an autonomous equilibrator-infrared pCO2 system in Monterey Bay, California USA from June to August 2007. The two-month time-series show a high degree of short-term variability, with pH and pCO2 changing by as much as 0.32 pH units and 240 μatm, respectively, during upwelling periods. The pH and salinity-derived alkalinity (ATsalin) were used to calculate the other inorganic carbon parameters, including pCO2, total dissolved inorganic carbon (DIC) and CaCO3 saturation states. The calculated pCO2 was within 2 μatm of the measured pCO2 during the first day of the deployment and within 8 μatm over the first month. The DIC calculated from pH–ATsalin and pCO2–ATsalin were within 5 μmol kg−1 of each other during the first month. However, DIC calculated from pH–pCO2 differed by ~ 50 μmol kg−1 from the other estimates over the same period, reflecting the sensitivity of the pH–pCO2 calculation to measurement error. The data continued to diverge during the final month and this difference was likely driven by extensive biofouling. Because of the relative insensitivity of CO32− concentration to these errors, aragonite saturation calculated from the pH–pCO2 pair was within 0.15 of the pH–ATsalin values over the entire deployment. These results show that in situ pH, when combined with other CO2 parameters, can provide valuable insights into both data quality and inorganic carbon cycling.Research Highlights► In situ pH and pCO2 data are used to quantify marine inorganic carbon species. ► The pCO2 from pH and salinity-derived alkalinity closely matches the measured pCO2. ► Dissolved inorganic carbon is also accurately quantified. ► CaCO3 saturation states are also quantified for ocean acidification studies.

Two different sulfonephthalein indicators, cresol red (CR) with a pKa of ∼8.3 and bromothymol blue (BTB) with pKa of ∼7.4, were tested for an analysis of freshwater over a broad range of pH and total alkalinity values. Measurements from an autonomous sensor system using a 1 cm optical path length were compared to those using a 10 cm path length on a benchtop spectrophotometer. The indicator pH perturbation was quantified with a thermodynamic model and nonlinear least-squares analysis. The laboratory study found that the perturbation-corrected pH differed between the 1 cm (large indicator perturbation) and 10 cm (small indicator perturbation) optical path length measurements from −0.017 to +0.15 with a median of +0.0041 pH units for CR and from −0.015 to +0.026 with a median of −0.0008 pH units for BTB. Precision was ±0.0005−0.013 and ±0.0001−0.0027 pH units for the 1 and 10-cm-path-length measurements, respectively. The autonomous sensor was deployed for 14 days in a local creek. Simultaneous glass pH electrode measurements had a large negative and drifting offset (−0.15 to −0.40 pH units) compared to the indicator-based measurements. This study is the first in situ comparison between potentiometric and spectrophotometric pH methods in a freshwater system.

Indicator-based spectrophotometric pH methods are now proven and commonly used for analysis of ocean samples; however, no autonomous system for long-term in situ applications has been developed based on this method. We describe herein an autonomous indicator-based pH sensor for seawater applications adapted from a design originally developed for freshwater pH measurements (SAMI-pH). The new SAMI-pH uses a different pH indicator, flow cell design, detection system, and mixing configuration to improve upon the freshwater performance. A new method was also tested that utilizes an indicator concentration gradient in the sample to correct for the pH perturbation caused by the indicator. With these design changes, laboratory tests found the precision improved from ± 0.004 to ± 0.0007 and the accuracy improved from − 0.0030 to + 0.0017 based on comparisons with benchtop UV/Vis measurements. In situ testing of two SAMI-pH instruments was completed off the pier at Scripps Institution of Oceanography. The average pH offset between the two instruments over the 22 d deployment period was 0.0042 ± 0.0126 (n = 883), with the precision primarily regulated by large spatial and temporal variability at the site. The results demonstrate that the SAMI-pH can provide drift-free and precise pH measurements in adverse measurement conditions (extensive fouling and large tidal variability). With the current battery power (18 alkaline D-cells), the system can be deployed for periods up to ∼ 2 months with a 0.5 h measurement frequency.

The optical properties of a novel fused silica fiber-optic capillary (FOCap) waveguide for fluorescence chemical sensing are evaluated. Evanescent fluorescence from samples contained in the FOCap is measured by coupling the FOCap to a light source and a fluorescence spectrophotometer. The FOCap has negligible excitation light loss over long lengths (tested to 20 m). Evanescent fluorescence is linear in FOCaps up to 20 m long when filled with a fluorophore solution and up to 15 m when a fluorophore is covalently attached to the inner surface. Evanescent fluorescence measurements in 50, 150, and 250 μm inner diameter FOCaps indicate that greater sensitivity is achieved with thinner-walled capillaries which have more internal reflections. The ability of the waveguide to function as an indirect chemical sensor for nitroaromatic compounds is demonstrated via fluorescence quenching of a pyrene-labeled FOCap.