Estimation of the Alkalinity in Seawater, Brines, and Brackish Water as per ASTM D3875

In determination of the capacity of water to neutralize acidic pollution, measurement of alkalinity is of paramount importance. Because of human activities, the alkalinity of rivers and streams may be affected in the long-term.

Estimation of the alkalinity in brines, seawater, and brackish water using ASTM D3875 compliant automatic potentiometric titration method is outlined in this application note. The method uses hydrochloride acid as the titrant.


Seawater, brackish water and brine

Sample Preparation

No sample preparation is required.


Approximately 80 mL of brine, seawater, or brackish water sample is taken in a beaker using a pipette. Employing the Unitrode, titration is then carried out using HCl with 0.1 mol/L concentration to achieve a pH-value of 3.0. Two endpoints, one at pH 8.1 and another at pH 4.5 are determined and used for the estimation.


. .
905 Titrando 2.905.0010
800 Dosino, 2x 2.800.0010
814 USB Sample Processor (1T/1P) 2.814.0010
772 Pump Unit 2.772.0110
Dosing unit 50 mL, (sample) 6.3032.250
Dosing unit 10 mL, (titrant) 6.3032.210
802 Rod stirrer 2.802.0020
Stirring propeller 6.1909.050
Sample rack, 16 x 150 mL 6.2041.320
Titration head, 6x NS 14 and 3x NS 9 openings 6.1458.010
Sample beakers, glass, 16 x 150 mL -
Unitrode with Pt 1000 6.0258.600
Electrode cable for plug in head U/plug F, 2x2 mm B, 1m 6.2104.600
tiamoTM 2.5 6.6056.252


. .
Titrant Hydrochloride acid, c(HCl) = 0.1 mol/L


. .
Mode DET U
Stirring rate 10
Signal drift 50 mV/min
Min. waiting time 0 seconds
Max. waiting time 26 seconds
Meas. point density 4
Dosing rate Maximum
Min. increment 10 μL
Max. increment Off
EP criterion 5
EP recognition All
Stop volume 5 mL
Stop pH 3.0
Fixed endpoint 1 pH = 8.1
Fixed endpoint 2 pH = 4.5


Sample Alkalinity
HCO3- / mg/L s(rel) / % CO32- / mg/L s(rel) / %
Seawater 136 0.2 2.4 5.7
Brackish water 8.5 0.8 0 -
Brine 743 0.1 0 -


About Metrohm

At Metrohm is one of the world’s most trusted manufacturers of high-precision instruments for chemical analysis. Metrohm was founded in 1943 by engineer Bertold Suhner in Herisau, Switzerland. Today, Metrohm is represented in 120 countries by subsidiaries and exclusive distributors. The global Metrohm Group also includes the Dutch companies Metrohm Applikon and Metrohm Autolab, manufacturers of online analyzers and instruments for electrochemical research, respectively. Recently, the Metrohm Group was joined by Metrohm Raman, a leading manufacturer of handheld Raman spectrometers.

Metrohm is the global market leader in analytical instruments for titration. Instruments for ion chromatography, voltammetry, conductivity, and stability measurement make the Metrohm portfolio for ion analysis complete. Instruments for Near-infrared and Raman spectroscopy are another, strongly growing segment of the Metrohm portfolio.

Metrohm is a problem solver, both in the laboratory and within the industrial process. To this end, the company offers their customers complete solutions, including dedicated analytical instrumentation as well as comprehensive application know-how. More than 30% of the company’s employees at the Metrohm international headquarters in Herisau work in R&D.

Metrohm has been owned 100% by the non-profit Metrohm Foundation since 1982. The Metrohm Foundation, which does not exert any influence on the company’s business operations, sponsors gifted students in the natural sciences, supports charitable and philanthropic purposes and, above all, ensures the independence of the company.

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Last updated: May 15, 2020 at 11:34 AM


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