Chemical reactions in printing

Photography (photography) plays an important role in modern people's daily lives. In addition to the entertainment photography that everyone is familiar with, there are also architectural design blueprints, movies, X-ray films, and many other fields such as military reconnaissance, metallographic analysis, space satellites, map surveying, expeditions, and microfilm reproduction. Take photos.

1. Photochemistry of silver salts

To understand photographic chemistry, we must first consider the photochemistry of silver salts. Typical photographic film contains tiny crystals of an extremely insoluble silver salt (silver bromide, AgBr). These particles are suspended in gelatin, and the resulting gelatin emulsion is melted and applied to a glass plate or plastic base for use. When light of the appropriate wavelength strikes a particle, a series of chemical reactions begins that leave a small amount of free silver within the particle. Initially, bromine ions absorb photons to produce a bromine atom:

Ag＋Br－→Ag＋＋Br+e

Silver ions can combine with electrons to produce silver atoms (Ag＋＋e→Ag), and the association inside the particles produces Ag2+, Ag20, Ag3+, Ag30, Ag4+ and Ag40. This free silver present in the silver bromide grains after exposure provides the latent image, which can later be revealed by the developer. Aggregates need to have at least four silver atoms, namely Ag40, for exposed AgBr particles to be developed. The latent image is an "invisible but visible image" stored in silver halide particles. The particles containing free silver in the form of Ag40 are reduced by the developer to generate a large amount of free silver, so a black area appears at that point on the film. Under the same critical conditions, unexposed grains cannot be reduced by the developer.

The sensitivity of the film is related to the grain size and halide composition. As the grain size in the emulsion increases, the effective sensitivity of the film also increases. Increase. The reason is that regardless of the particle size, the number of silver atoms required to cause the entire particle to be reduced by the developer is the same. The greater the sensitivity value, the more sensitive the film is to light.

2. Development

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Silver halides are not the most sensitive photosensitive materials known. So why do they produce the most effective images? The answer lies in the fact that a single photon hitting a silver halide particle produces 1 at least 4 A core of silver atoms. This effect is amplified 1 billion times by an appropriate reducing agent (developer).

When the exposed film is placed in the developer, the particles containing the core of silver atoms are formed. The more cores there are in a given particle, the faster the reaction will be. Factors such as temperature, developer concentration, pH and the total number of cores in each particle determine the extent of development. The density (blackness) of free silver deposited in the film emulsion. The blackening of the film is caused by free silver atoms (Ag0).

The developer not only reduces the silver ions to free silver. It is selective enough that it cannot reduce unexposed particles to avoid so-called "fogging". The main compounds commonly used as developers are gallic acid (gallic acid), o-aminophenol, hydroquinone, and paramethyl. Aminophenol (Metol), 1-phenyl-3-pyrazolidinone (Phenidone), etc.

Most developers used in black and white photography are composed of hydroquinone and rice. A typical developer should contain 1 to 2 developing agents, a protective agent to prevent air oxidation, and an alkaline buffer to prevent the reduction reaction from being inhibited.

For example, the formula of a typical developer for black and white film is: take 750 ml of 50°C water and dissolve the following substances in it: 2.0 g of meture, 5.0 g of hydroquinone, 100.0 g of sodium sulfite, borax ( Na2B4O7·10H2O) 2.0 g, add cold water to 1000 ml.

When hydroquinone is used as the developer, quinone will be produced for every 2 silver atoms produced:

Hydroquinone + 2Ag + → Quinone + 2Ag0 + 2H +

Since the above reaction is reversible, the increase in hydrogen ions or quinone will hinder the development process. Sodium sulfite can react with the quinone and destroy it to form a pair. The ability of benzene. At the same time, hydrogen ions are effectively neutralized by hydroxide ions (OH-)

H++OH-==H2O

If the development time is too long or the temperature is too high. If it is higher than the specified value, thick fogging will occur and the film will be scrapped. Since the development reaction speed increases as the temperature increases, photographers usually have to carefully control the temperature of the developer solution.

The development process can be terminated by placing the film into the stop-development bath. Stop baths usually contain a weak acid that lowers the pH, such as acetic acid. The function of the stop bath is to increase the number of hydrogen ions, which will effectively terminate the reaction of converting hydroquinone to quinone.

3. Fixing

If development only produces free silver where the light intensity is greatest, and the film is not further processed, take it out of the darkroom and remove the undeveloped halogenated silver. The silver will be exposed immediately. After this point, almost any reducing agent will completely fog the film. To overcome this problem, a suitable substance must be found to remove unreduced silver halide. The most commonly used fixer in black and white photography is sodium thiosulfate solution.

The thiosulfate ions (S2O32-) and silver ions form a stable complex soluble in water, thereby achieving the purpose of "fixing" the film.

AgBr(s)+2S2O32-==Ag(S2O3)23-+Br-

Adding some acid to the fixer is to neutralize the alkalinity of the developer. To stop development. However, do not add too much acid to the developer. If the pH value is lower than 4, the following reaction will occur, leading to the decomposition of the fixer:

S2O32-＋H+==HSO3-＋S↓

Na2SO3 in the fixer will combine with H+ into HSO3- ions. As the concentration of HSO3= ions increases, the decomposition of the fixer moves to the reverse process and inhibits the decomposition of Na2SO3. Therefore, when preparing the fixing solution, before adding acid, a part of Na2SO3 must be dissolved to protect the fixing agent.

In order to prevent the emulsion layer of the photosensitive material from absorbing water and swelling too much during the washing process, causing it to fall off or be easily damaged, some film hardeners, such as alum, are often added to the fixer.

A commonly used acidic fixing solution formula is: 250 grams of sodium sulfate sulfate as a fixing agent, 25 grams of anhydrous sodium sulfite as a protective agent, 48 ml of acetic acid (28%), 15 grams of alum as a film hardener, Add water to 1000ml.

, Thickening

For underexposed photos, the resulting image is too thin. Chemical methods can be used to thicken the photo to increase the blackness and contrast of the photo.

The basic principle of thickening is to add some information substances such as silver or other metals or metal compounds where there is already image information on the photo. At present, the most commonly used and effective thickening method is the chromium thickening method.

The chromium thickening method uses potassium dichromate (K2Cr2O7) and hydrochloric acid (HCl) to fade (bleach) the image. As a result of the reaction, the silver shadow is oxidized to AgCl, and a layer of brown chromium oxide (CrO2) is added. This effect can be expressed by the following reaction equation: 6Ag+2K2Cr2O7+8HCl==6AgCl+3CrO2+K2CrO4+2KCl+4H2O.

After bleaching, it is washed with water and then developed to reduce AgCl to Ag again. The CrO2 formed during bleaching still remains on the silver shadow, thereby increasing image information and acting as a thickener.

2. Thinning

In the photography process, only when the exposure and development are correct can you get good quality photos. If the image is overexposed or overdeveloped, the blackness of the image will increase. This can be remedied by chemical thinning.

The basic principle of thinning is to use oxidants, such as red blood salts, potassium permanganate, ammonium persulfate, etc., to first oxidize the metallic silver in the image into silver salts, and then use some chemicals to These silver salts dissolve to achieve the purpose of thinning. There are many commonly used thinning methods, and the following two are the most effective.

(1) Red blood salt thinning method

Red blood salt is an oxidant that oxidizes excess silver on the image into silver ferrocyanide, and then uses Na2S2O3 Dissolve it to make it thinner. This thinning process can be expressed by the following chemical reaction equation:

4Ag+4K3[Fe(CN)6]==Ag4[Fe(CN)6]+3K4[Fe(CN)6]

< p>Ag4[Fe(CN)6]+8Na2S2O3==4Na3[Ag(S2O3)2]+Na4[Fe(CN)6]

(2) Potassium permanganate thinning method

This method is to place the photo (or negative) in a KMnO4 sulfuric acid solution to oxidize part of the metallic silver into water-soluble (actually slightly soluble) Ag2SO4 to achieve the purpose of thinning. The above process can be expressed by the following chemical reaction equation:

10Ag＋2KMnO4＋8H2SO4==K2SO4＋2MnSO4＋5Ag2SO4＋8H2O

When the image is appropriately thinned in the thinning fluid, take it out and wash it thoroughly with water, otherwise contamination or pollution will occur. There is a risk of uneven thinning.

3. Photo color grading

Using chemical methods, images can be converted into different colors. This operation is called photo color grading.

Color correction is usually done on the photo. It uses chemicals to convert silver shadows into colored silver compounds, or deposits colored compounds of other metals on the image to make the photo appear in a variety of monochromatic tones. For example:

1. Sodium sulfide browning method

First put the photo into a solution containing red blood salt and KBr for bleaching, so that the metallic silver is oxidized into AgBr. After washing with water, it is then immersed in Na2S solution to change the image to brown Ag2S.

4Ag+4K3[Fe(CN)6]==Ag4[Fe(CN)6]+3K4[Fe(CN)6]

Ag4[Fe(CN)6]+4KBr= =4AgBr＋K4[Fe(CN)6]

2AgBr＋Na2S==Ag2S＋2NaBr

2. Copper salt toning method

Copper salt toning method utilizes the property of copper sulfate and bleached silver shadow to produce reddish-purple copper ferrocyanide.

Ag4[Fe(CN)6]+2CuSO4==Cu2[Fe(CN)6]+2Ag2SO4

For example, the practical copper salt coloring formula is: Ⅰ Liquid: CuSO46.3g , 2.5 grams of potassium citrate, add water to 1000 ml; Liquid II: 5.2 grams of red blood salt, 2.5 grams of potassium citrate, add water to 1000 ml.

When using it, mix the two liquids Ⅰ and Ⅱ, and then immerse the fully washed photos. With the length of the toning time, you can get a color tone from deep purple to bright red. Finally, wash the photo thoroughly in running water.