Wickbold Burn

The Wickbold method is a procedure for the determination of sulfur and halogens , especially chlorine , in organic substances. It is suitable for the determination of these elements in the trace range.

history

The method was developed in 1957 by R. Wickbold in the research laboratory of Chemischen Werke Hüls AG in Marl.

principle

The sample to be examined is burned in a flame made of oxygen and hydrogen . The combustion products are condensed, producing a large amount of water. Sulfur is oxidized to sulfur dioxide and sulfur trioxide , chlorine is converted to hydrogen chloride. The oxidation products are absorbed in the condensed water and form sulfuric acid or hydrochloric acid . These products can be specifically analyzed for their content there. The halogen content is calculated as chlorine. All halogens except fluorine are recorded.

apparatus

The determination apparatus consists of a burner, a cooler as a condenser and a collecting vessel. The burner is a special construction made up of four concentric tubes. The sample to be analyzed is sucked in through the innermost tube. Oxygen in excess, hydrogen as fuel and another, larger oxygen flow in excess are introduced through the next surrounding pipes from the inside to the outside .

The mixture is completely burned. The combustion gases are cooled with a subsequent cooler and partially condensed. A large amount of water is produced as an oxidation product of hydrogen from fuel gas and sample. The condensate is collected in a storage vessel. The sulfur oxides and HCl formed dissolve in this water with the formation of the corresponding acids. As the water's absorption capacity for these substances is not very high, the method is only suitable for small amounts. The amounts to be determined should therefore be in the range from 1 to 1000 ppm.

The absorption capacity of the water is increased by adding hydrogen peroxide (approx. 5%) for the sulfur determination and sodium hydroxide solution (approx. 2 g / l) for the halogen determination. It is advisable to dye the sodium hydroxide solution with methyl orange in order to detect an overload of the absorption solution with acidic oxide gases (large amounts of halogens, but also sulfur or phosphorus). An acidic absorption solution does not bind halogens well.

After the sample has been sucked up, the sample vessel is rinsed with a pure solvent and the rinsing solution is also sucked into the apparatus for burning.

The apparatus is operated under vacuum so that the samples can be sucked in. This is achieved by connecting them to a vacuum pump . The connection is made via a vessel after the storage container, in which liquid splashes are caught and returned to the storage vessel.

Determination methods

Photometric determination of the sulfate

After the sample has been burned, the sulfur is in the solution as sulfuric acid. An alkalimetric determination is not possible due to the presence of hydrochloric or nitric acid (from small amounts of nitrogen in the combustion oxygen). A photometric turbidity titration is therefore used to determine the sulfate . For this purpose, a little sodium chloride is added to the sample in order to fix the free sulfuric acid as sodium sulfate , i. H. in a non-volatile form. The sample is then evaporated, i.e. the water is largely evaporated by heating. The sample is then taken up in methanol and barium chloride is added. In approx. 90% methanolic solution, sulphate and barium ions react with one another with immediate precipitation as barium sulphate .

${\displaystyle \mathrm {SO_{4}^{2-}+BaCl_{2}\longrightarrow BaSO_{4}\!\downarrow +\ 2\,Cl^{-}} }$

The sample is slowly titrated with a very dilute barium chloride solution and the increasing turbidity is observed photometrically. If the turbidity no longer increases, the titration is ended.

Photometric determination of the chloride

The chloride content of the solution can be determined photometrically by precipitation as silver chloride . The extinction of the precipitated solution is measured.

Individual evidence

1. ↑ Wickbold, R .; "Determination of sulfur and chlorine traces in organic substances", Angew. Chem. 69, 1957, No. 16; Pp. 530-533; doi : 10.1002 / anie.19570691604