Search: permanganate index

Potassium permanganate oxidizes many organic and certain inorganic substances more or less completely in acidic, neutral or alkaline solutions. The volume of potassium permanganate required in the analysis is determined potentiometrically. Since oxidation depends on the type of solution, on its temperature and on the reaction time, the procedure described below must be followed precisely.

In acidic solutions, permanganate ions are typically reduced to manganese(II) ions:

MnO₄^- + 5 e- + 8 H₃O⁺ → Mn²⁺ + 12 H₂O

In alkaline solutions, the reduction results in tetravalent manganese only:

MnO₄^- + 3 e- + 4 H₃O⁺ → MnO₂ + 6 H₂O

Since in both cases the titration takes place in an acidic solution, this is irrelevant for the calculation. By adding oxalic acid, both the excess permanganate ions as well as the tetravalent manganese are reduced to manganese(II) ions:

2 MnO₄^- + 5 C₂O₄^2- + 16 H₃O⁺ → 2 Mn²⁺ + 24 H₂O + 10 CO₂

MnO₂ + C₂O₄²- + 4 H₃O⁺ → Mn²⁺ + 6 H₂O + 2 CO₂  

This method is used to determine the percentage of germination that will occur under normal malting conditions. The germination index provides a good indication of the germination capacity of a barley lot which is no longer dormant [1−3].

The method is based on the BRF/4 ml test with 4 × 100 kernels.

The oxidative degradation of the α-acids and β-acids in hops and conventional hop products is determined using a spectrophotometer. The HSI can be referenced in addition to or together with the spectrophotometric method for determining the α-acids and β-acids in hops outlined under R-300.04.110 α- und β-Säuren in Doldenhopfen und Hopfenpellets – Spektralphotometrische Methode.

The Kolbach index is used to express the quantity of the nitrogenous substances found in malt, which go into solution under the conditions present during the Congress mash method. It is a measure of the degree of proteolytic modification of the malt and also provides an indication for the quantity of proteolytic enzymes contained in the malt. The Kolbach index is less conclusive than other methods due to its dependence on the total nitrogen content and the provenance of the barley. Therefore, it must always be considered together with the total nitrogen content.

The analysis sample (e.g., Congress wort or wort from above the grain bed during lautering) undergoes a chemical reaction with an acetic acid/thiobarbituric acid solution; the resulting product is yellow in color and is measured spectrophotometrically. 

Manganese ions are catalytically oxidized into purple-colored permanganate ions. The purple color of the solution is determined using a comparator (or photometrically):

Mn²⁺ + 12  H₂O → MnO₄^- + 8 H₃O⁺ + 5 e^-