Working Group: Seawater Carbonate Chemistry Common themes, considerations and issues: • existing established best practices for methods (Dickson et al. , 2007) including standard operating procedures (SOP’s); • history of method documentation and “metadata” information built on JGOFS, IOCCP; IOC-SCOR; SOLAS-IMBER, OA best practices manuals • available reference materials (CRM’s) • mix of long-established and emergent methodology/instrumentation. Seawater Carbonate Chemistry parameters Directly measured 1. Dissolved Inorganic Carbon (i. e. , DIC, CT, or TCO 2) 2. Total alkalinity (TA or AT) 3. p. CO 2 (partial pressure of CO 2) or f. CO 2 (fugacity) 4. p. H 5. [CO 3]26. 13 DIC Computed parameters 7. [HCO 3]-; Ca. CO 3 saturation state (i. e. , Wcalcite, Waragonite
Working Group: Seawater Carbonate Chemistry DIC Best/optimal • Coulometer based detector system for measurement of evolved CO 2 from known volume of acidified seawater. • 0. 05% precision; 0. 1% accuracy (CRM’s as reference material) Good • IR based detector system based for measurement of evolved CO 2 from known volume of acidified seawater. Can be small volume. • 0. 05 -0. 2% precision; 0. 1 -0. 2% accuracy (CRM’s) Acceptable • Potentiometric based system based on titration of known volume of seawater with dilute HCl at ionic strength of 0. 7. Low cost; closed cell titration yields TA as well as DIC. • 0. 05 -0. 2% precision; 0. 5 -1% accuracy. Emerging technologies 1. Spectrophotometrically based systems 2. CRDS (cavity ring down system, e. g. PICARRO), good for p. CO 2 but discrete DIC needs work!
Working Group: Seawater Carbonate Chemistry DIC Recommendations • Certified reference materials of different DIC content prepared and distributed to bracket typical range of seawater values. Synergistic to this is the routine use of secondary sets of reference materials that are tied to CRM’s expensive, and representative of North Pacific water only Some merit for use of CO 2 gas calibration discussed. Some merit for use of Na. CO 3 standards of different DIC content; (both above less easy to implement broadly) • Follow best practices guides Synergistic to this is updating of SOP’s to be more inclusive of coastal ocean • Capability testing of newer generation coulometric detectors Optimal coulometric method is based on old detectors (i. e. , UIC 5011) with rigorous testing of newer detectors (both UIC 5014/5015 and other coulometer systems in development. Use of aging detectors with end in sight for their maintenance is a major issue for capacity building and expansion of high quality measurements produced by broader community. • Better quantification of offset of DIC with potentiometric titration Lower cost option can provide both DIC and TA; can be done with caveat that DIC has, at present, undetermined offset (1 -2%). Bracketed reference materials
Working Group: Seawater Carbonate Chemistry Best/optimal TA • Potentiometric based system based on titration of known volume of seawater with dilute HCl at ionic strength of 0. 7. • 0. 05% precision; 0. 05 -0. 2% accuracy (CRM’s as reference materials) • spectrophotometric based systems (similar precision and accuracy but much less broadly used). More expensive but more stable than potentiometric systems. Recommendations • Certified reference materials of different TA content prepared and distributed to bracket typical range of seawater values. Some merit for use of Na. CO 3 standards of different TA content
Working Group: Seawater Carbonate Chemistry Best/optimal Good Acceptable p. CO 2 (discrete) • GC based or IR detector system based on measurement of evolved CO 2 from known volume of seawater by equilibration. • 0. 05% precision; 0. 3% accuracy (CRM’s as reference material) • standardized with CO 2 -in-air standards • Cheaper systems based on less sensitive and lower cost IR detectors could be developed. 20 ppm precision? Good for coastal, dynamic and “metabolism” studies Recommendations • Develop lower cost systems based on lower cost IR detectors Would broaden user base for this quasi-state variable
Working Group: Seawater Carbonate Chemistry Best/optimal p. H • Spectrophotometric p. H • 0. 0002 -0. 0004 precision and accuracy (0. 002) • temperature control very important Good Acceptable • Potentiometric based p. H briefly discussed but inaccuracy and stability an issue. Significant requirements for high-quality data. Emerging technologies 1. ISFET/DURAFET sensors with milli p. H sensitivity 2. Distribution of high-purity crystallized indicator dye
Working Group: Seawater Carbonate Chemistry Computation Recommendations • Computation of all components of seawater carbonate system requires at least two measured state or quasi state variables; possible combinations in order of decreasing achievable accuracy: 1. One state (DIC or TA) and one quasi state variable (p. CO 2 or p. H) 2. Two state (DIC and TA) variables 3. Two quasi state variable (p. CO 2 and p. H) • Intercomparability of software computations of seawater carbonate chemistry For example, IOCCP, sponsor intercomparability exercise between available software programs (e. g. , CO 2 sys, CO 2 calc, R). Do they all give the same results? • Better quantification of equilibrium thermodynamics and dissociation constants outside typical range of temperature and salinity (e. g. , < 0°C and salinity close to 0.
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