Potentiometric titration

Measurements, first and second derivative in a potentiometric titration.

Potentiometric titration is a technique similar to direct titration of a redox reaction. It is a useful means of characterizing an acid. No indicator is used; instead the potential is measured across the analyte, typically an electrolyte solution. To do this, two electrodes are used, an indicator electrode (the glass electrode and metal ion indicator electrode) and a reference electrode. Reference electrodes generally used are hydrogen electrodes, calomel electrodes, and silver chloride electrodes. The indicator electrode forms an electrochemical half cell with the interested ions in the test solution. The reference electrode forms the other half cell,

The overall electric potential is calculated as E_cell = E_ind - E_ref + E_sol. E_sol is the potential drop over the test solution between the two electrodes. E_cell is recorded at intervals as the titrant is added. A graph of potential against volume added can be drawn and the end point of the reaction is half way between the jump in voltage. E_cell depends on the concentration of the interested ions with which the indicator electrode is in contact. For example, the electrode reaction may be

Mⁿ⁺+ne⁻----->M

As the concentration of Mⁿ⁺ changes,the E_cell changes correspondingly. Thus the potentiometric titration involve measurement of E_cell with the addition of titrant. types of potentiometric titration: acid-base titration (total alkalinity and total acidity), redox titration (HI/HY and cerate), precipitation titration (halides), and complexometric titration (free EDTA and Antical #5).

History

The first potentiometric titration was carried out in 1893 by Robert Behrend at Ostwald's Institute in Leipzig. He titrated mercurous solution with potassium chloride, potassium bromide, and potassium iodide. He used a mercury electrode along with a mercury/mercurous nitrate reference electrode. He found that in a cell composed of mercurous nitrate and mercurous nitrate/mercury, the initial voltage is 0. If potassium chloride is added to mercurous nitrate on one side, mercury (I) chloride is precipitated. This decreased the osmotic pressure of mercury (I) ions on the side and creates a potential difference. This potential difference increases slowly as additional potassium chloride is added, but then increases more rapidly. He found the greatest potential difference is achieved once all of the mercurous nitrate has been precipitated. This was used to discern end points of titrations.

Wilhelm Bottger then developed the tool of potentiometric titration while working at Ostwald's Institute. He used potentiometric titration to observe the differences in titration between strong and weak acids, as well as the behavior of polybasic acids. He introduced the idea of using potentiometric titrations for acids and bases that could not be titrated in conjunction with a colorimetric indicator

Potentiometric titrations were first used for redox titrations by Crotogino. He titrated halide ions using potassium permanganate using a shiny platinum electrode and a calomel electrode. He says that if an oxidizing agent is added to a reducing solution then the equilibrium between the reducing substance and reaction product will shift towards the reaction product. The changes the potential very slowly until the amount of reducing substance becomes very small. A large change in potential will occur then once a small addition of the titrating solution is added, as the final amounts of reducing agent are removed and the potential corresponds solely to the oxidizing agent. This large increase in potential difference signifies the endpoint of the reaction.

Applications

Beer Chemistry

Potentiometric titration is a useful tool for estimating the amounts of isohumulones in hops bitter substances, which are a large component of beer. Isohumulones are seen to combine with metals in stoichiometric proportions. This allows solutions of isohumulones, generally created by extracting isohumulones from hops using isooctane or petroleum and a liquid-liquid extraction, can be titrated with solutions of metal salts using changes in conductivity to determine an end point. The solutions of isohumulones can then be titrated against a metal salt such as nickel acetate or lead acetate. Readings of potential difference over time then give a titration curve in which potential will increase very slowly at first and then very rapidly. The rapid change in difference corresponds to all of the isohumulones reacting, signifying the end point of the reaction.

When this titration is done using nickel acetate, two moles of isohumulones combines with one mole of nickel acetate, and each milliliter of titrant contains 1/500 of a millimole of nickel. This allows the concentration (C) of the bitter substance to be calculated as:

C=(mL. of nickel acetate x 710 x 10)/500

See also

• Chronoamperometry
• Electroanalytical methods