Annex B-15. Technical note on the measurement of total alkalinity in seawater 

1. Introduction

This technical note is intended to support the analytical quality of marine measurements in the Baltic area. Total alkalinity is a measure of the acid-neutralizing capacity of a sample, and is one of the parameters that characterize the marine acid-base equilibrium. It determines the sum of alkaline components in a sample (hydroxide, carbonate, hydrogen carbonate, and other buffering components). In the COMBINE programme, total alkalinity is used as a co-factor in primary production measurements.

2. Methods

The method described here is the direct titration method, and it is intended and recommended for seawater analysis. Methods for the determination of the alkalinity of sediments and methods based on back-titration methods are not included in this note. The recommended methods that can be applied are potentiometric or colorimetric (indicator) titration. The result is dependent on the endpoint pH of the titration, where the approximate endpoint is 4.5. The method is described in detail by Anderson et al. (1999), APHA (1995), and IS0 9963 (1994).

3. Sampling

Samples are collected and stored in gas-tight polyethylene or glass bottles that are completely filled and closed tightly. The sample volume necessary for titration depends on the total alkalinity, and can be up to 200 ml for samples of low alkalinity. Normally, seawater samples are stable for at least two weeks if stored cool and dark. However, samples with significant biological activity cannot be expected to be stable.

4. Analytical procedure

The procedure involves titration down to approximately pH 4.5 with hydrochloric acid. The titre of the hydrochloric acid needs to be determined correctly; where possible, the use of commercially available hydrochloric acid with a known concentration is recommended. Reagents shall be prepared using CO2-free water, which can be prepared by boiling purified water for 10–15 minutes followed by cooling, or by bubbling with nitrogen for 10–15 minutes. Alkalinity can be determined by colorimetric titration using an indicator. The exact endpoint depends on the alkalinity of the sample and can be determined on the basis of table values, from Gran titration graphs, or be determined from the inflection point of a titration curve. The most accurate results are normally achieved by the potentiometric titration method with an exact endpoint determination.

Equipment maintenance

For maintenance of the pH-meter, see ‘Technical Notes on pH Measurement in Sea Water’.

Calibration

Regarding pH-meter calibration, see the ‘Technical Notes on pH Measurement in Sea Water’, but use instead buffer 4 and buffer 10. Regarding the control of the electrode, Hansson buffer can be applied.

Check that the potential measurement of the electrode in Hansson buffer is between 350 mV and 450 mV. If not, repair or discard the electrode. Equilibrate the buffer to room temperature before use. ^[1]

Preparation of Hansson buffer

Add HCl as a 9973 Titrisol ampoule (Merck) containing 0.1 mole HCl. Dissolve 0.606 g Tris (2-amino-2-hydroxymethylpropane-1,3-diol) in 25.00 ml solution A. Dilute to 50.00 ml with solution B.

Stability:

• Solution A: 3 months;
• Solution B: 1 month;
• Hansson buffer: 1 week in refrigerator;
• Titrisol ampoule: 1 year.

5. Quality assurance

Check the volumetric equipment for both colorimetric and potentiometric methods. Check the accuracy of titrant addition in automatic titration systems.

Check the pH-meter pH, e.g., with a certified buffer of, e.g., pH 6.

Quality control with the use of X-charts and, if possible, certified reference material, e.g., VKI reference material QC DW¹.

Alternatively, if not used for standardization of the hydrochloric acid concentration, a reference solution of Na₂CO₃ of known concentration, where Na₂CO₃ has been dried to constant weight at 270–300 ^oC for at least 2 hours, can be used in quality control. Check the accuracy of this sample by comparing with measurements of a sample with a known and documented alkalinity, e.g., a certified reference material. A stock Na₂CO₃ solution of 40 mM is stable for 3 weeks, whereas the working solution (e.g., 2000 µM) should be prepared freshly every day.

6. Reporting

Results are normally reported in mmol [HCO3 2–]/dm3 (equals meq/dm3) or in mg CaCO₃/dm₃. Formulas for calculation are (APHA, 1995):

Total Alkalinity (meq/dm₃) = 1000 ´ v_HCl ´ t_HCl/v_b

Total Alkalinity (mg CaCO₃/dm³) = 50 000 ´

vHCl ´ tHCl/vb, vHCl is the volume of HCl in ml, tHCl is the concentration of HCl in mol/dm³ , and v_b is the volume of the sample in ml.

7. Precision

The precision obtained in a proficiency test (APHA, 1995) was 5 mg l^-1 between laboratories in samples with a total alkalinity of 120 mg l^-1.

8. References

Anderson, L.G. et al. 1999. In Methods of seawater analysis. Ed. by K. Grasshoff et al. Wiley-VCH, Germany.

ISO. 1994. Water Quality—Determination of alkalinity, Part 1: Determination of total and composite alkalinity. ISO 9963-1.

APHA (American Public Health Association) et al. 1995. Standard methods for the examination of water and wastewater.  
 

[1] 1 Information regarding the nearest dealer of VKI Reference Materials can be obtained from ProLab, DK (tel no.: (45) 45 76 79 76, fax no.: (45) 45 76 26 02)

Last updated: 29.10.2012 (Annex number changed from Annex B 16 to Annex B 15)