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Causes and detection methods of hexavalent chromium in leather
The reason and detection method of hexavalent chromium in leather: The raw leather itself does not contain chromium, but it is an objective fact that hexavalent chromium is detected after being processed into finished leather. There are generally two reasons for the production of hexavalent chromium in leather. One is that the tanning material contains hexavalent chromium; the other is that part of the trivalent chromium in the chrome tanned leather is oxidized into hexavalent chromium during the tanning or storage process. Since tanning is a complex and systematic project involving many kinds of materials and multiple processes, the formation of hexavalent chromium in leather products is the result of many factors, which are summarized as follows. The influence of chromium-containing materials: In the tanning process, the use of hexavalent chromium-containing chemicals is the most direct cause of excessive hexavalent chromium content in the finished leather. Chromium-containing materials include chromium powder, chromium-containing tanning agents, and Chromium salt dyes, pigment pastes, fixatives, etc. Chromium powder is the largest and most direct source of chromium in leather. Experiments have shown that once chromium powder with excessive hexavalent chromium is used, even if some remedial measures are taken during the pre-tanning or tanning process, such as adding a certain amount of soda, At the same time, controlling the pH value as low as possible will not achieve the reduction effect. It can even be said that it has no effect on the hexavalent chromium measured in mg [s]. In addition to chromium powder, the content of hexavalent chromium in some chromium-containing retanning agents, chromium-containing dyes and pigments is also quite high, which will inevitably lead to excessive hexavalent chromium content in leather products after use. The influence of the pH value of the bath liquid: The pH value of the bath liquid not only plays a vital role in the leather making process, but also has a certain influence on the cause of hexavalent chromium. According to the principle of thermodynamics, the oxidability of hexavalent chromium decreases sharply as the pH of the medium increases. Under acidic conditions, trivalent chromium is not easily oxidized to hexavalent chromium; under alkaline conditions, trivalent chromium is easily oxidized to hexavalent chromium. In theory, when the pH value of the solution is greater than 1, the oxygen in the air can oxidize trivalent chromium, but in fact, the pH value required for the oxidation of trivalent chromium by oxygen in the air is greater than 5t. Therefore, in the tanning process In the process of neutralization, retanning and fatliquoring, when the pH value is greater than 5, it is possible to oxidize the trivalent chromium in the leather to form hexavalent chromium. The influence of fatliquor (or grease): Although the fatliquor itself does not contain chromium, it has a structure to produce hexavalent chromium. Studies have found that a fatliquor or esterified fatty acid containing one or more unsaturated bonds can cause the production of hexavalent chromium in leather products. The fatliquor without unsaturated bonds is natural or Synthetic, will not lead to the production of hexavalent chromium. In addition, studies have shown that the iodine value of the fatliquor is different, the amount of hexavalent chromium produced in the leather is also different, and the general trend shows that the higher the iodine value of the fatliquor used, the conversion of trivalent chromium to hexavalent chromium The more likely it is. This is because fatliquing or fats containing unsaturated bonds are easily oxidized by oxygen in the air under external conditions. The structure of unsaturated bonds in the molecule is easily oxidized to generate peroxides and peroxide free radicals. This kind of peroxides and peroxides Material free radicals have strong oxidizing properties, which easily oxidize free trivalent chromium in leather to hexavalent chromium. The influence of heating and light: Both heating and light can increase the content of hexavalent chromium in leather. Under the conditions of heating and light, the covalent bonds in collagen, synthetic tanning agents, fatliquors or dyes will be broken and free radicals will be generated. These free radicals contact with oxygen in the air to generate strong oxidation. The oxygen-containing free radicals can oxidize trivalent chromium to hexavalent chromium. In addition, there is a dynamic balance between the conversion of trivalent chromium and hexavalent chromium, and heat is a necessary condition for kinetic conversion, and the heat provided by heating and light becomes the favorable conditions for the conversion of trivalent chromium to hexavalent chromium. . The influence of air humidity: During the storage of finished leather, the content of hexavalent chromium will change greatly. Studies have shown that the air humidity during storage is an important factor affecting the content of hexavalent chromium in leather. With the increase of air humidity, the content of hexavalent chromium in leather will decrease; on the contrary, it will increase. Storing leather under high humidity conditions is beneficial to inhibit the formation of hexavalent chromium in the leather. Because the content of organic matter (often reducible) in leather is relatively high, and it is generally acidic, even if hexavalent chromium is formed, it is easy to be reduced when the relative humidity is high (equivalent to in solution). After understanding the cause of the formation of hexavalent chromium, in the detection of hexavalent chromium, the factors that may affect the results can be avoided as much as possible, which has an important guiding role in the detection of hexavalent chromium in leather products. Detection of hexavalent chromium in leather products: The main methods of detection of hexavalent chromium at present are: spectrophotometry, atomic absorption spectrometry, electrochemical analysis, fluorescence analysis, etc. (iv). For the inspection of hexavalent chromium in leather products, the most widely used method is spectrophotometry, which is adopted in relevant domestic and foreign testing standards.
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