14D1 Bicarbonate_/SE and carbonate_/SE – potentiometric titration

The aliquot (about 5 mL) used for EC_/SE and pH_/SE can be used for determination of HCO_3/SE and CO_3/SE by titration with standard sulfuric acid (H₂SO₄). The aliquot should be at ≈23–25°C.

Reagents

2.0 M Sulfuric Acid

Prepare by adding 111.1 mL of 18 M H₂SO₄ to deionised water, cool, mix well and make to 1.0 L.

Standard 0.02 M Sulfuric Acid

Prepare from a standardised solution or dilute 10.0 mL of 2 M H₂SO₄ with deionised water and make to 1.0 L when cool. Standardise against a known weight (≈0.3 g for titre of ≈40 mL) of sodium tetraborate (Na₂B₄O₇.10H₂O) with methyl red as indicator in a similar manner to that described in Method 7A1. Note that:

Methyl Red Indicator

Dissolve 100 mg methyl red (C₁₅H₁₅N₃O₂] in 60 mL ethyl alcohol (CH₃CH₂OH) and dilute with deionised water to 100 mL.

Procedure

Pipette a 5.0 mL portion of the SE, to which no sodium hexametaphosphate has been added, while keeping the pipette tip near the bottom of the flask. Titrate using the standard 0.02 M H₂SO₄, firstly to pH 8.3 for CO_3/SE and then to pH 4.5 for HCO_3/SE. Titrate a blank of deionised water in each batch. When conducting the titration (the sample should not be diluted, concentrated or filtered), add the standard acid in increments of ≤0.5 mL, such that a change of <0.2 pH unit occurs per increment. Mix gently/thoroughly after each addition and complete the titration without undue delay. As end point pH values are approached, make smaller additions of acid and be sure that pH equilibrium is reached before adding more titrant. Record mL of titrant added to achieve the required pH values.

Calculation of carbonate_/SE (CO_3/SE) concentration

where

P = mL of standard 0.02 M H₂SO₄ required to reach the pH 8.3 end-point.

Report CO_3/SE (cmol_c/L), or multiply by 30.005 to convert to mg CO_3/SE/L. Also record method codes for preparing and extracting the SE.

Calculation of bicarbonate_/SE (HCO_3/SE) concentration

where

T = total volume (mL) of standard 0.02 M H₂SO₄ used to reach pH 4.5; and

P = mL of standard 0.02 M H₂SO₄ to reach pH 8.3.

Report HCO_3/SE (cmol_c/L), or multiply by 61.017 to convert to mg HCO_3/SE/L. Also record method codes for preparing and extracting the SE.