First, the principle of latent image formation
Whether on a camera, an imagesetter, or a scanner, when exposed to the correct amount of exposure, some desired image is always formed on the photosensitive sheet. Because it is not easily noticeable by human eyes, this image is called a latent image.
How is the latent image formed? In the process of producing a photosensitive sheet, in addition to the halogen-silver crystals, the sensitized emulsion is evenly distributed with silver atoms and silver sulfide molecules, which form many photosensitive centers on the surface of the photosensitive sheet.
When exposed, the light energy acts on the silver halide (AgX) crystals. The halide (X-) in the halogen silver loses one electron and becomes a halogen atom (X). As shown in figure B, the reaction formula is as follows:
Ag+X + light = X + e + Ag+
Halogen Silver Halide Electronic Silver Ion
The generated free electrons form electrons that move in the crystal. When they meet the photosensitive center, they are attracted by the center, causing the center to bear a negative charge. When it encounters a negative electric field, it concentrates on the photosensitive center. It is neutralized into silver atoms. Just like:
Ag+ + e = Ag
Silver ion electron silver atom
When the exposure amount of the photosensitive sheet reaches a certain level, the photosensitive center expands into a developing center due to the increase of silver, which is a basis of the latent image, so the halogen element is generated and is absorbed by the surrounding colloid.
Second, change the latent image to form a visible image
To change the latent image into a visible image, the silver atoms at the development center must continue to increase until the human eye can clearly observe it. This is accomplished through the action of development.
The purpose of the development is to reduce the silver ions of the silver salt molecules in the photosensitive layer emulsion layer to the silver atoms, which is the same as the exposure, but it is done by drugs. Therefore, the development can be regarded as the continuation of the exposure.
There are many drugs that can reduce halogen silver in development, and they are all easily oxidizable chemicals. The principle of development is described with respect to hydroquinone as an example.
The hydroquinone is ionized in the developer as follows:
The anion is the imaging ion. It is easily oxidized to helium and emits two electrons.
Halide silver gets electrons reduced to silver.
Combining halide ion with hydrogen ion to generate halogen acid
Their comprehensive reaction is as follows:
The exposed part has the light receiving part and the unexposed part, and the developing solution acts on the surface of the entire photosensitive layer. However, why only the halogen silver at the developing center is reduced to silver? This is because potassium bromide is added to the developing solution. The Br- is ionized in the solution. The development center is a silver atom, it is impossible to adsorb Br-, only Ag+ will adsorb it. Therefore, in addition to the development center, it is surrounded by Br-, and a negative electric barrier is formed. In this way, the developing ions enter the crystal from the gap in the developing center, and the silver halide reduces the silver.
Third, stable visible image
The developed film can clearly see the image, but its stability is poor, and it will disappear after light is seen. If it is stored in the developer, the film will not have any image. Therefore, it is necessary to use chemical drugs to take necessary treatment. To meet the requirements, this process is called fixing.
The function of the fixing is to dissolve the unreacted and undeveloped halogen silver on the photosensitive sheet, leaving only the reduced silver atoms. Fixers therefore require the ability to dissolve halogen silver without destroying the reduced silver. There are many agents belonging to this class, but some are toxic, and some can stain the film, so sodium thiosulfate (Na2S2O3) is usually used. In special cases, ammonium thiosulfate [(NH4)2S2O3] can also be used. The chemical reaction is as follows:
Therefore, both Na3[Ag(S2O3)2] and NaX are dissolved in water. Thus, the purpose of development is achieved.
In the developer and fixer, there are not only the developer and the fixer, but also other chemical agents, and each has its own function, so we only have to understand their role and they are justified in dealing with various phenomena.
Fourth, the composition of the developer and its role
1. Developer. Developers are drugs that reduce silver halides to silver, the most commonly used being Metol and Hydex. However, it is easily oxidized in water and should be used in conjunction with other drugs.
Mitoul is also known as Yilun, whose scientific name is methyl-p-aminophenol sulfate. The molecular structure is as follows:
It is a white or slightly grayish acicular or powdered crystal that dissolves at 4.8 grams per 100 ml of water at 10°C. It is hardly soluble in an aqueous solution of sulfurous acid, it is a strong developing agent with strong visualization. When the image is taken out very quickly, when the developer is strong, it can be imaged in the developer for 2 to 3 seconds. Its main feature is that it can make the dark part and the strong light part of the image appear at the same time, but the film The degree of difference between light and shade or black and white is small, that is, the contrast is small. Compatible developer and sulfite coexist with good storage performance and low temperature response to potassium bromide. It is comparable to Hydrangeol to the same extent, making it difficult to contaminate negative film.
Haidolinji is also known as dinitrone. Its scientific name is called hydroquinone and its molecular structure is:
Its color is grey or white, usually fine needle crystals. It dissolves 6 grams per 100 milliliters of water at 20°C. Its aqueous solution tends to turn yellow because it is an easily oxidized material. In order to prevent oxidation, it has good developing performance, and generally it is necessary to dissolve sodium sulfite first in water, and then dissolve Haidolin. Hydergine development is slow. Especially for those parts where the exposure is particularly low, the effect is slight, but once the image appears, the density will increase rapidly, so the film is very different in brightness or darkness. With the development of high contrast, in the use of Haidigoji developer solution, the PH value of the solution and temperature changes have a great impact on the development, so the use of developer should be developed at a certain temperature in order to achieve a stable purpose. It must be developed in an alkaline developer. It is preferably used in a pH range of 9.5 to 11, and the greater the pH, the more alkaline it is, and the greater the density and contrast of the negative. If the temperature of the developer is lower than 10°C, the development is very slow. When the temperature is low, the developing effect is hardly developed. However, if the temperature is higher than 20°C, the development is quick, but the negative film is liable to be flawed. Therefore, when used, the temperature should be in a relatively stable range. It is often used in combination with Methoures to achieve the correct development purpose. When used in combination with Methouren, they are called M-Q developers.
2. Preservation agent. The preservative is usually sodium sulfite, also known as sulphur oxide powder, and its function is to prevent the oxidation of mitel. There are two kinds of anhydrous and crystalline. Its molecular formula is Na2SO3 (anhydrous sodium sulfite) and Na2SO3 7H2O (crystalline). Colorless white powder, relatively stable in air; crystallized as white granules, easily oxidized into sodium sulphate in the air, so it should be sealed and stored, both can be used when used, but should pay attention to the amount, if not For 1 g of water, 2 g of crystalline was used because the molecular weight of anhydrous was 126 and the molecular weight of the crystalline was 252, which was exactly twice that of anhydrous. In the existing circumstances, we often use anhydrous to facilitate the preservation of the developer. Generally, when the temperature is high, the amount can be increased appropriately to increase the image contrast, but too much is not good for the quality of the negative film, and the halogen silver in the negative film is easily dissolved. Therefore, it is often required that the developer has a stable temperature.
3. Accelerators. Accelerators are all alkaline substances. Usually there are 3 types of strong bases, weak bases and soft bases. Soft bases are commonly used.
(1) Soft alkali. Soft alkali is mainly composed of sodium carbonate (Na2CO3) and potassium carbonate (K2CO3), and its aqueous solution has a pH of 10.8. Alkaline is weaker.
There are three kinds of anhydrous sodium carbonate, one water (Na2CO3·H2O) and crystal (Na2CO3·10H2O). Both can be used. The ratio of water to water is: water: crystal: 1:1.17:2.7. When the temperature is high, the amount should be appropriately reduced, and it is easy for the amount of the film to produce a blemish and affect the quality of the film.
Potassium carbonate easily absorbs water in the air and gradually deliquesces. Its solubility in water is greater than that of sodium carbonate, and it is stronger than sodium carbonate. Use caution when using. The ratio of sodium carbonate to potassium carbonate is 1:0.77.
(2) Weak base. There are usually borax (Na2B4O7·102O) and asymmetric sodium borate (NaBO2). The latter is more alkaline than the former. They ionize in the hydrolysate, but only a small fraction. When the PH value of the developer drops during use, they will again undergo ionization, so the stability of the pH is maintained under certain conditions, which facilitates development.
(3) Strong alkali. Commonly used strong alkalis include sodium hydroxide (NaOH) and potassium hydroxide (KOH), also known as caustic soda and caustic potash, which are white crystalline blocks that are easily deliquescent in air, dissolve in water, and release a large amount of heat. Corrosion of the skin, should pay attention when using. At the same time, it should also be noted that closed storage to avoid reaction with CO2 in the air reduces the strength of the alkali, and is often used in fast developing solutions that require large contrast of the negative film. However, the amount should not be too much, the liquid temperature can not be too high, or the film will produce a large ignorance. Affect film transparency.
4. Inhibitors. The commonly used inhibitor is potassium bromide (KBr). In addition, there are sodium bromide (NaBr), sodium chloride (NaCl), potassium iodide (KI), and the like. Both of them can improve the contrast on the negative image, reduce the blemishes, and can adjust the correct appearance of the strong light portion and the dark tone portion on the film. However, the use of too much, although the contrast can be enhanced, but the image level is very vulnerable. In the dots or small lines and text, the amount of potassium bromide is more than usual. However, it has a particularly large impact on Haidolin. The use amount is too much, and it appears to be too slow, because it suppresses the development characteristics of Hydex, and if it is used in excess, the developer will completely lose the developing effect.
Among the various chemical components of the developer, the developer is an easily oxidizable substance, and electrons are emitted due to its oxidation, so that the halogen silver obtains electrons to reduce the metallic silver, thereby achieving the purpose of development. Developers with strong stamina tend to ionize in the developer.
The ionization of benzophenone is as follows:
Its comprehensive reaction in the developer is:
The role of the preservative is to prevent the developer from oxidizing when the developer is formulated or during storage. Especially Mitol, oxidation is rapid. Therefore, a preservative should be added when formulating medicines to control the chemical reaction of Mitol to oxygen. Its reaction in the developer is as follows:
Oxidized cesium produced no development. The role of sulfite is to capture oxygen and prevent it from reacting with the developer, allowing the developer to maintain the developing function of cesium ions in water.
Sodium sulphite not only prevents Haidigo from being oxidized, but also oxidizes Haidigogan, which has been developed to produce weaker developer, hydroquinone sulphonate.
The role of the accelerator is to adjust the concentration of hydrogen ions in the developer to control the ionization concentration of the developer. During the development process, the hydrogen ions released from the solution, if not controlled, will cause the concentration of hydrogen ions in the solution to continue to increase, so that the PH value will be significantly reduced, and the ionization rate of the developer will be significantly slowed, thereby seriously affecting the Development speed. The promoting effect of the alkali is that it is continuously neutralized with hydrogen ions to generate water, so that the developing ions in the developer maintain a stable concentration and the development proceeds normally. The chemical reaction is as follows:
The function of the inhibitor is to make halogen-free silver undeveloped. Because the potassium bromide ionizes in the solution, the bromide ion (Br-) is absorbed in the place where the photosensitive sheet has silver ions (Ag+). We know that the development center is composed of metallic silver, so its ability to adsorb Br- is much smaller than that of the development center, so that there is no development center on the photosensitive film covered by Br-, and forms a negative charge barrier, repelling developing ions, So that the Ag+ can not be reduced to Ag. However, when the amount of potassium bromide is large, the concentration of bromide ions is large and the surrounding of the development center
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