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ENTERTAINING EXPERIENCES AT HOME
Pigments. Chemical experiments
Entertaining experiences at home / Chemistry experiments for children A few decades ago, pigments were usually called mineral paints, emphasizing their origin: many natural pigments were obtained by grinding colored minerals. And now they sometimes do the same, especially if you need to prepare bright, juicy, resistant paints for painting. But much more often today synthetic pigments are used - all kinds of metal oxides and salts. If the pigments are of an organic nature, then they are more often called dyes; perhaps their main area of application is the dyeing of fabrics. Let's start preparing mineral pigments of different colors. Let's start with white. Compounds of lead, zinc and titanium usually act as white pigments: the latter are the least available for self-production. And the best source of lead in the home lab is a lead lotion, a 20% solution of basic lead acetate that you have worked with before. Since lead white is the main lead carbonate Pb (OH)2CO3, then to obtain them, it is necessary to pass carbon dioxide through a solution of acetate (i.e., through a lotion). Lead white will precipitate. Filter it, rinse with water and dry. The lead acetate solution will remain in the filtrate. Try to work in such a way that lead compounds do not get on the hands and face, and in no case - in the mouth. It is forbidden to paint dishes and any objects that come into contact with food with paints prepared on the basis of such compounds (including those with lead desiccant). Zinc paints, which we will now turn to, are also not intended for painting dishes; all the cautionary warnings just given apply to them. The starting material for zinc pigments will be zinc chloride ZnCl2. Its solution can be obtained by dropping a little zinc, optionally granular, into hydrochloric acid. For example, a glass from an old battery is suitable - it is made of almost pure zinc. To the resulting solution, carefully, drop by drop, add a solution of washing soda. First, it will neutralize the excess acid (you will notice this by foaming), and then it will react with zinc chloride, forming ZnCO carbonate3. It must be filtered, washed with water and calcined at a temperature not lower than 280 ° C. Above this temperature, zinc carbonate decomposes into white zinc oxide ZnO and CO2. There is another white zinc pigment - zinc sulfide ZnS. To prepare it, you must first obtain sodium sulfide Na2S. The easiest way to do this is to strongly heat sodium sulfite Na2SO3 (we remind you again - this substance is sold in photo stores), then two substances are formed - Na sulfate2SO4 and Na sulfide2S. We only need the second substance. After cooling, dissolve the mixture in water and add a little zinc chloride solution, the preparation of which is described in the previous experiment. Avoid excess acid: the resulting sulfide dissolves in it. After filtering and drying, you will get an insoluble white sulfide - ZnS powder. Let's move on to color pigments. First we get blue-green verdigris - a mixture of basic copper acetates. Add a solution of soda to a solution of copper sulphate, and basic copper carbonate Cu will precipitate.2(HE)2WITH3. Filter it, and carefully, drop by drop, add vinegar essence until the precipitate is completely dissolved. Evaporate the solution over low heat almost to dryness, avoiding strong overheating and splashing of the liquid, and then cool. Filter the precipitated blue-green crystals and dry between sheets of filter paper. In a circle, it is better to put such an experiment under draft, and at home, after the experiment, do not forget to properly ventilate the room so that the smell of vinegar disappears. Let's move on to pigments based on iron oxide Fe2O3. Minium, mummy, ocher, umber, kolhotar, Venetian red, English red - this is not a complete list of such pigments. Depending on the method of preparation, the paint can have various shades, from red to brown, and with strong heating, iron oxide turns black. This substance is easily obtained by calcining ferrous sulfate FeSO4* 7H2O (iron sulfate heptahydrate). Take small portions of vitriol, then decomposition will go faster. Heat the vitriol until it turns from green to black. When cooled, red oxide Fe is obtained.2О3. If it is not possible to buy ready-made iron sulfate (and it is sold in hardware stores), then ferrous sulfate is not difficult to prepare from the more common copper sulfate; dip into its solution iron filings, filed and washed in gasoline. As soon as the blue solution turns green, drain it from the precipitate, filter and evaporate to dryness. You will not get pure iron sulfate, because the iron will be partially oxidized by atmospheric oxygen, but this will not affect the result of the experiment. Brown iron hydroxide Fe(OH)3 you will get from a solution of ferrous sulfate, to which a solution of caustic soda, prepared from washing soda and slaked lime, has been added, as described in the chapter "Tin and lead". When working with any alkalis, do not forget to be careful! As a result of the reaction, iron hydroxide Fe (OH) will precipitate2. To the required Fe (OH) hydroxide3 it is easily oxidized by hydrogen peroxide (peroxide), and if it takes time, then simply by oxygen in the air when stored in an open bottle. Separate the brown precipitate and dry at room temperature. A well-known iron-based blue pigment is Prussian blue. For its preparation, ferric salt is needed. Here's how you can get it: dissolve the freshly precipitated iron hydroxide prepared in the previous experiment in hydrochloric acid (possibly diluted, pharmacy) or, somewhat worse, in vinegar essence, and mix with a solution of potassium ferrocyanide (under the name of yellow blood salt, this substance sold in photo shops). A blue precipitate of the familiar Prussian blue is instantly formed: Fe4[Fe (CN)6]3. This reaction is very sensitive and is often used to detect ferric ions in solution. Yellow lead oxide - litharge, on the basis of which you prepared the desiccant, can be introduced into the paint and as a pigment. And to get bright red red lead, mixed lead oxide Pb3О4, it is enough to heat the previously prepared litharge in air. The subtlety, however, is that the oxidation reaction is reversible, and at temperatures above 500 ° C, red lead again turns into litharge. This means that a temperature slightly lower than 500 ° C is needed, but not much, otherwise the reaction will not go at all. It is unlikely that you have a suitable thermometer. Therefore, put pieces of lead and zinc next to the calcined litharge. Their melting points are respectively 327 and 420 0C, this interval is quite acceptable for getting minium. It is clear that during the experiment the lead must be molten and the zinc solid. The black pigment is ordinary soot. Here is one way to get good carbon black suitable for making paint. Point the flame of a paraffin candle at a cold, massive object, of course, non-flammable. Scrap off the resulting black coating from time to time. Under such conditions, the paraffin does not burn completely and, along with carbon dioxide, CO2 elemental carbon is formed - soot. In conclusion, we get green pigments. First dark green chromium oxide Cr2О3. As you remember from experiments with oxidation - reduction, many chromium compounds are brightly colored, so they are often used as pigments, but only for those colors that do not come into contact with food. We will again proceed from potassium bichromate K2Cr2O7, the most accessible of chromium compounds. Mix it with active carbon or sulfur and carefully grind the mixture in a mortar, and then, taking no more than 2 g of the mixture, heat it strongly in porcelain or metal dishes (when a lot of substances are taken, the reaction is too violent). Cool the mixture, wash it several times with water and filter. Dry the dark green chromium oxide remaining on the filter. There are other ways to obtain this pigment, for example, by heating ammonium dichromate or a mixture of potassium dichromate with ammonium chloride (ammonia). Bear in mind that chromium oxide obtained in such reactions finds use not only as a green pigment, but also as a fine abrasive material, one of the best. It is an ingredient in many particularly fine polishing pastes, for example, for finishing lenses and mirrors of optical instruments. Finally, a bright green emerald green pigment, chromium hydroxide. It differs from ordinary gray hydroxide of the same composition in that it consists of larger particles. Fuse potassium bichromate with pharmacy boric acid in an iron spoon. It is necessary to heat the iron to a red heat, the spoon must be held with tongs. After cooling, treat the melt with water and filter. You will see that the color of the substance is indeed emerald green. Having obtained pigments in sufficient quantities, test them, as written off in the previous chapter, in the composition of oil paints. Or any other, purchased, adding homemade pigment to white paint or synthetic enamel. Author: Olgin O.M.
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