Search: potassium dichromate consumption

Potassium dichromate oxidizes many organic and certain inorganic substances to various extents in an acidic solution. Since the level of oxidation depends upon the kinds of substances, the concentration of potassium dichromate, the pH of the solution, and the temperature and reaction time, the procedure described below must be followed precisely. The volume of potassium dichromate required in the analysis is determined potentiometrically. In an acidic solution, the dichromate ions are reduced to chromium(III) ions:

Cr₂O₇^2- + 6 e^- + 14 H₃O⁺ → 2 Cr³⁺ + 21 H₂O

Dichromate ions in excess of those required are determined through titration with an ammonium iron(II) sulfate solution:

Cr₂O₇^2- + 6 Fe²⁺ + 14 H₃O⁺ →2 Cr³⁺ + 6 Fe³⁺ + 21 H₂O 

If a water sample is run through a strongly acidic cation exchanger, all of the cations are replaced with hydrogen ions, thus producing the corresponding free acids in equivalent quantities (total mineral acid value). Since the carbonates and bicarbonates are transformed into carbon dioxide and therefore escape determination, their content must be determined through titration with acid to a pH of 4.3 (m value).

Since analyses for determining the concentrations of sodium and potassium ions in water require considerable effort, and these ions are not very relevant for evaluating the suitability of water for the processes of brewing and malting, a simple calculation will suffice. The difference in the concentrations of anions and cations in the water is determined, allowing the calculation to be performed under the assumption that only sodium ions are present in the water.

Potassium dichromate is added to the acidified sample as an oxidizing agent along with silver sulfate as a catalyst; mercury sulfate is also added to prevent the formation of elemental chlorine from chlorides. After oxidation of the organic substances in the sample (the dichromate ion is reduced to the chromium(III) ion in an acidic solution), the chromate required to achieve this is determined through reverse titration of the excess potassium dichromate with iron(II) solution (adjusted) against ferroin as an indicator [1].

Cr₂O₇^2- + 6 e^- + 14 H₃O⁺ → 2 Cr³⁺ + 21 H₂O

Cr₂O₇^2- + 6 Fe²⁺ + 14 H₃O⁺ → 2 Cr³⁺ + 6 Fe³⁺ + 21 H₂O

Potassium in wort is measured using the AAS technique by directly aspirating the diluted sample into an acetylene oxygen flame; the measurement is made at 766.5 nm.

Inductively coupled plasma optical emission spectroscopy (ICP-OES) is a fast and reliable method for the laboratory analysis of metals. Inductively coupled plasma (ICP), a high frequency field of ionized gas, serves as a medium for atomizing and exciting the substances found in samples. Liquid, dissolved or aerosol samples are injected into the ionized gas stream. In emission spectroscopy, ICP can be used in conjunction with a number of optical and electronic systems either simultaneously or sequentially in multi-element spectrometers. In the plasma, the atoms and ions are excited to a higher energy state bringing about the emission of electromagnetic radiation (light), primarily in the ultraviolet and visible region of the spectrum. Metals ordinarily occur as ions in the temperature range typical for ICP of 6000 to 10000 K; however, non-metals and metalloids are only partially ionized.

ICP-OES operates within a very wide range. This usually encompasses six orders of magnitude in concentrations smaller than μg/l up to g/l, depending upon the element and the concentrations used for the set of analysis data. With ICP-OES, beer and wort can also be analyzed without prior processing of the samples, in contrast to AAS. Methods for determining the following in beer and wort will be described below: Al, B, Ba, Ca, Co, Cu, Fe, K, Mg, Mn, Mo, Na, P, Si, Sr, Sn and Zn.