Chemical color printing

Photography plays an important role in modern people's daily life. In addition to the familiar entertainment photography, there are many fields such as architectural design blueprint, film, X-ray film, military reconnaissance, metallographic analysis, space satellite, map mapping, exploration, micro-film reproduction and so on, all of which need photography. First, the photochemistry of silver salts To understand photographic chemistry, we must first consider the photochemistry of silver salts. A typical photographic film contains tiny crystals composed of extremely insoluble silver salt (silver bromide AgBr). These particles are suspended in gelatin, and the obtained gelatin emulsion is melted and coated on a glass plate or a plastic substrate for use. When the light with appropriate wavelength is irradiated on the particles, a series of chemical reactions begin to occur, leaving a small amount of free silver in the particles. At first, bromine ions absorb photons to produce a bromine atom: Ag+Br -→ Ag+Br+E silver ions can combine with electrons to produce a silver atom (Ag++E → Ag), and the internal association of particles produces Ag2+, Ag20, Ag3+, Ag30, Ag4+ and Ag40. The free silver in the exposed silver bromide particles provides a latent image, which can then be developed by a developer. The aggregate needs at least four silver atoms, namely Ag40, before the exposed AgBr particles can be developed. The latent image is an "invisible but visible image" preserved in silver halide particles. Particles containing Ag40 free silver are reduced by the developer to generate a large amount of free silver, so a black area appears at this point of the film. Under the same critical conditions, unexposed particles cannot be reduced by the developer. The sensitivity of film is related to particle size and halide composition. With the increase of particles in emulsion, the effective sensitivity of film also increases. The reason is that the number of silver atoms required for the developer to initiate the reduction of the whole particle is the same regardless of the particle size. The greater the sensitivity value, the more sensitive the film is to light. Secondly, developing silver halide is not the most sensitive photosensitive material known. So, why can they produce the most effective images? The answer lies in that a single photon hits a silver halide particle to produce 1 nucleus with at least 4 silver atoms, and this effect is amplified by a proper reducing agent (developer) by 1 100 million times. When the exposed film is put into the developer, the particles with silver nucleus will be reduced faster than those without silver nucleus. The more cores in a given particle, the faster the reaction. Factors such as temperature, developer concentration, pH and the total number of cores in each particle determine the degree of development and the density (blackness) of free silver deposited in film emulsion. The blackening of negative film is caused by free silver atoms (Ag0). The developer can not only reduce silver ions to free silver, but also has sufficient selectivity. It cannot reduce unexposed particles to avoid so-called "ash atomization". Compounds commonly used as chromogenic agents include gallic acid, o-aminophenol, hydroquinone, p-methylaminophenol (Mi Tuer), 1- phenyl -3- pyrazolidone (fenidone) and so on. Most developers used in black-and-white photography are composed of hydroquinone and Mi Tuer or hydroquinone and fenidone. Typical developer should contain 1 ~ 2 kinds of developer, protective agent to prevent air oxidation and alkaline buffer to prevent reduction reaction from being inhibited. For example, the formula of a typical black-and-white film developer is: take 750ml of water at 50℃ and dissolve the following substances in it: 2.0g of Mi Tuer, 5.0g of hydroquinone, 0.0g of sodium sulfite100, 2.0g of borax (Na2B4 O710h2o), and add cold water to1000. When hydroquinone is used as a developer, quinone is produced. Every two silver atoms have two hydrogen ions: hydroquinone +2ag +→ quinone +2ag0+2h+. Because the above reaction is reversible, the increase of hydrogen ions or quinones will hinder the development process. Sodium sulfite can react with quinone and destroy its ability to return hydroquinone. At the same time, hydrogen ions are effectively neutralized by hydroxyl ions (OH-). H++OH-= = H2O If the development time is too long or the temperature is higher than the specified value, dense ash atomization will occur and the negative will be scrapped. Because the development reaction speed increases with the increase of temperature, photographers usually have to control the temperature of the developer very carefully. The development process can be terminated by putting the film into the stop slot. The bath usually contains weak acid, which can lower the pH value, such as acetic acid. The function of stopping the bath is to increase the number of hydrogen ions, which will effectively prevent the reaction of hydroquinone transforming into quinone. 3. Fixing solution If the development only produces free silver at the place with the maximum light intensity, and the negative film is not further processed, the undeveloped silver halide will be exposed as soon as it is taken out of the darkroom. After that, almost any reducing agent will make the negative film completely foggy. In order to overcome this problem, it is necessary to find suitable substances to remove unreduced silver halide. The most commonly used fixing solution for black-and-white photography is sodium thiosulfate solution. Among them, thiosulfate ion (S2O32-) and silver ion form a stable complex and dissolve in water, thus achieving the purpose of "fixing" the negative electrode. Agbr (s)+2S2O3 2-= = Ag (S2O3) 23-+br-Adding some acid to the fixing solution is to neutralize the alkalinity of the developing solution and stop the development. However, it is not advisable to add too much acid in the developer. If the pH value is lower than 4, the following reaction will occur, leading to the decomposition of the fixing agent: S2O32-+H+= = HSO3-+S ↓ Na2SO3 in the fixing solution will combine with H+ to form HSO3- ions. Due to the increase of HSO3= ion concentration, the decomposition of fixing agent moves to the opposite process, which inhibits the decomposition of Na2SO3. Therefore, when preparing fixing solution, it is necessary to dissolve a part of Na2SO3 first, and then add acid protection fixing agent. In order to prevent the emulsion layer of photosensitive material from swelling due to excessive water absorption during washing, it may fall off or be easily damaged. Some hardeners, such as alum, are often added to the fixing solution. A common formula of acid fixing solution is: fixing agent sodium thiosulfate 250g, protecting agent anhydrous sodium sulfite 25g, acetic acid (28%)48ml, curing agent alum 15g, and water to 1000ml.