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PIGMENTS

 

Pigments few decades ago were usually called mineral paints, stressing their origins: many natural pigments were obtained by grinding coloured minerals. And now do sometimes as well, especially if it is necessary to prepare bright, vibrant, long lasting colours for painting. But more often these days use synthetic pigments of various oxides and metal salts. If the pigments are organic in nature, they are often called colorants; perhaps the main area of application - staining of tissues.

Let's preparation of mineral pigments of different colors. Let's start with white.

In the role of white pigments usually are lead compounds, zinc and titanium: the last self-production is the least available. And the best source of lead in the home laboratory is the lead lotion, 20%solution of basic acetate of lead, with whom you have worked. As white lead is a basic carbonate of lead Pb(OH)2CO3, to receive them must pass through the acetate solution (i.e., through a lotion) carbon dioxide. White lead will precipitate. Filter it, rinse with water and dry. The filtrate will be a solution of lead acetate. Try to work so that the lead compounds do not fall on the hands and face, and in any case - in the mouth. Paints prepared on the basis of such compounds (including lead desiccant) it is forbidden to paint the dishes and any items that come into contact with food.

Zinc paint, to which we will now move, too, not intended for coloring utensils; all warnings caution, just made apply to them.

The starting material for zinc pigments will serve as zinc chloride ZnCl2. The solution is to get it down in a little hydrochloric acid zinc, optionally granulated. Suitable, for example, glass from old batteries - it is made of almost pure zinc. To the resulting solution was carefully added dropwise, add a solution of washing soda. First, it neutralizes excess acid (you will notice this in foaming), and then will react with chloride of zinc carbonate is produced ZnCO3 . It should be filtered, washed and calcined at a temperature of not lower than 280°C. Above this temperature the carbonate decomposes into zinc white zinc oxide ZnO and CO2.

There is another white zinc pigment zinc sulphide ZnS . For its preparation it is necessary to first obtain the sodium sulfide Na2S . The easiest way to do so: hot sodium sulfite Na2SO3 (note - this substance is sold in photoshop), then formed two substances - sulfate Na2SO4 and sulfide Na2S. We need only the second substance. The mixture after cooling, dissolve in water and added slowly a solution of zinc chloride, the receipt of which is described in the previous experience. Avoid excess acid: produced sulfide in it dissolves. After filtration and drying, you will receive an insoluble sulfide white powder ZnS.

Go to color pigments. Get the first blue-green jar-Copperhead - a mixture of basic acetates of copper.

To a solution of copper sulphate, add the soda solution, and the precipitate will fall carbonate basic copper C2(OH)2WITH3. Filter it and carefully, drop by drop, add acetic essence to dissolve the precipitate. ApARTe solution on low heat almost to dryness, preventing excessive heat and splashing liquid, then refrigerate. Fallen blue-green crystals, filter and dry between sheets of filter paper.

In the circle of this experience it is better to put under traction, and the houses do not forget after you experience how to ventilate the room to disappear smell of vinegar.

Let us turn to the pigments based on iron oxide Fe2O3. Minium, mummy, ochre, umber, colcothar, Venetian red, English red - this is not a complete list of such pigments. Depending on the method of obtaining the color can be various shades, from red to brown, and when overheated iron oxide black.

This substance is easily obtained by calcination of ferrous sulfate FeSO44*7H2O (as ferrous sulfate). Take a small portion of sulphate, then the decomposition will go faster. Nakalyvaet sulphate as long as he from green becomes black. When cooled will turn red iron oxide Fe2O3.

If you cannot buy ready vitriol (and it is sold in hardware stores), ferrous sulfate, it is easy to prepare from the more common copper sulphate. Lower in his solution of iron filings, nestrogannye file and washed in gasoline. As soon as the blue solution turns green, drain it with a sediment filter and uprite dry. You'll get pure vitriol, because iron is partially oxidized by the oxygen in the air, but the experience is not affected.

Brown iron hydroxide Fe(OH)3 you will receive from a solution of iron sulphate, which added a sodium hydroxide solution prepared from washing soda and slaked lime, as described in Chapter "Tin and lead. Working with any alkali, do not forget about safety! In the reaction in the sediment will fall out of the iron hydroxide Fe (OH)2. To the required us hydroxide Fe(OH)3 it is easily oxidized by peroxide (hydrogen peroxide) hydrogen, and if there's no hurry, just oxygen when stored in an open flask. The precipitated brown remove and dry at room temperature.

Widely known blue pigment based on iron - Prussian blue. For its preparation needed salt of trivalent iron. Here is how you can get it: svezheosazhdennoi iron hydroxide, prepared in the previous experience, dissolve in hydrochloric acid (diluted, pharmacy) or, what is worse, vinegar, and mix with a solution of potassium ferrocyanide (called yellow blood salt this substance is sold in photoshop). Instantly forms a blue precipitate familiar Prussian blue: Fe4[Fe( CN)6]3. This reaction is very sensitive, it is often used for detection in a solution of ions of trivalent iron.

Yellow oxide of lead, litharge, on the basis of which you cooked the desiccant can be introduced into the paint and as a pigment. And to get bright red, minium, a mixed oxide of lead Pb3O4, it is enough to heat the air previously prepared litharge. Subtlety, however, that the oxidation reaction is reversible, and at temperatures above 500°C minium again converted into litharge. So we need a temperature slightly below 500°C, but not by much, otherwise the reaction will not go. It is unlikely that you have a suitable thermometer. So put next to proclivity the litharge pieces of lead and zinc. Their melting temperature, respectively, 327 and 420°C, this interval is quite acceptable to get minium. It is clear that during the experience lead must be melted, and zinc solid.

The black pigment is of conventional carbon black. Here's one way of getting a good carbon black, suitable for the manufacture of paints. Aim the flame paraffin candles on a cold massive subject, of course, non-flammable. The resulting black residue from time to time suscribite. In such conditions, the wax is burned incompletely, and along with the carbon dioxide WITH2 forms of elemental carbon - soot.

In conclusion, get the green pigments. First, dark green chromium oxide CR 2 O 3 . As you remember from experiments with oxidation - restoration, many chromium compounds are brightly painted, so they are often used as pigments, but only for those colors that are not in contact with food.

We will again proceed from the bichromate of potassium K2CR2O7 - the most affordable of chromium compounds. Mix it with active charcoal or gray and carefully RUB the mixture in a mortar, and then, taking no more than 2 grams of a mixture, much of its heat in a porcelain or metal bowl (when substances taken many, the reaction proceeds too rapidly). Cool the mixture, rinse it several times with water and then filter the Remaining on the filter dark green chromium oxide dry.

There are other ways of getting this pigment, for example, by heating ammonium bichromate or a mixture of bichromate of potash with ammonium chloride (Sal ammoniac). Note that the chromium oxide obtained in such reactions, is used not only as a green pigment, but also as a fine abrasive, one of the best. He is a member of many very fine polishing pastes, for example, to adjust the lenses and mirrors in optical instruments.

Finally, a bright green pigment emerald green, chromium hydroxide. From the usual grey hydroxide of the same composition it differs in that it consists of larger particles.

Alloy bichromate of potash with pharmacy boric acid in an iron spoon. Heat it to red-hot iron, the spoon must be kept forceps. After the cooling process, the melt water and strain. You make sure that the color of the substance really is emerald green.

Having pigment in sufficient quantity, test them as described in the previous Chapter, in the composition of oil paints. Or any other purchase, adding improvised pigment in white paint or synthetic enamel.

O. Holguín. "Experiments without explosions"
M., "Chemistry", 1986

 

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