This chapter is a critical part of the new Guide to Best Practices in Ocean Alkalinity Enhancement Research. Appropriately, this chapter builds on the wealth of resources developed over the last 2 decades of OA research, but focuses on new measurement considerations in light of OAE.

This chapter provides clear guidance on sampling (with the caveat that many of these approaches do need to be more fully tested), measurements, calculations in the context of OAE. While acknowledging the many strengths of this chapter, I do have minor suggestions that the authors may choose to incorporate to further strengthen this work:

• This may be a more appropriate comment for the “Climate targets, carbon dioxide removal and the potential role of Ocean Alkalinity Enhancement” chapter (which I assume is the GBP’s introduction), but there is inconsistent use of “marine CDR” (Climate targets...) vs. the shortened “mCDR” here.
• Line 41: this should read “with Ksp describing the temperature, salinity AND PRESSURE dependent...”
• Line 46 and throughout: it is unclear what is meant by “amount contents”
• The description of the Deffeyes diagrams should be made clearer and additional colour should be considered for those figures. The light grey and dark grey lines are difficult to distinguish from the black.
• Line 315: missing a reference
• Line 338: given the earlier focus on saturation state (and its importance to OAE), the brief intro to ‘weather’ vs ‘climate’ definitions in this paragraph would benefit from a short sentence clarifying that these definitions are based on achieving a desired uncertainty in [carbonate] (or for ‘climate’ a change in [carbonate])
• Figure 4: link the caption directly back to how this is used to check for linearity of response.
• Throughout: some issues with missing italics for “f” in fCO2, and occasionally italicised chemical symbols.
• Line 390: delete or clarify “(compare Figure 4)”
• Section 2.3.3: Although the authors are correct to caution (at length) about the use of unpurified dye, someone new to the field may be unaware of the potential downfalls of potentiometric measurements. It would be helpful to include a reference to Bockmon’s 2015 “An inter-laboratory comparison assessing the quality of seawater carbon dioxide measurements” paper and highlight how quickly the uncertainty of a potentiometric can exceed the limits for climate/weather measurements.

The authors describe seawater carbonate chemistry in the context of the proposed marine carbon dioxide removal via alkalinity enhancement (OAE).

Section 2.1.1: The paper begins with a useful, if brief, review of the acid-base equilibrium chemistry of the carbonate system in sweater. While these definitions are not new, and the descriptions not exhaustive, I found these sections useful.

Section 2.1.2: I found the description of Figure 1 (Deffeyes diagrams) confusing. The figures themselves would benefit from revision that makes the different sources of alkalinity (NaOH, NaHCO3, etc) more clear (coloured and not grey lines, perhaps)? The subsection would also benefit from a worked example using the figure, in addition to the rather vague statement that it is ‘straightforward to assess the potential of 1 kg of a particular modified seawater to remove CO2 from the atmosphere’. Since it is straightforward, please add it, for 1 kg of each of the chosen sources of TA. 

Section 2.1.3: while this section is called ‘OAE impacts on seawater’ it was dominated by descriptions of CaCO3 precipitation, and a disproportionate amount of detail is given to inorganic precipitation relative to the other sections.

Section 2.1.4: would remove the (strangely casual) ‘So, how much TA can be added, then?’, and rather refocus on recommendations, though this seems to be a bit out of context with the goals of the chapter (as stated in the abstract).

Section 2.1.4: the focus of this section on the global (or latitudinal) distributions of salinity, temperature and various CO2 system parameters seems rather detailed compared to other sections, including those about sample collection, preservation, and analysis, which were listed as key to the chapter. This doesn’t lead to recommendations about where to do the TA additions, but rather ends with a statement about how the uptake factor is minor, and the potential  is driven by the amount of added TA (keeping away from critical thresholds in Omega)….

Section 2.2: it is a pity that there are no references given for any of the suggested modifications to the SOPs for sampling, storage and analysis – very nice to the see recommendations for dealing with elevated concentrations.

The manuscript seems to be missing a (short) conclusion section. In general, while relatively well-written, the text leaves the impression that it was done in a rush, and not with as much care as you might expect from the team of authors.