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From Wikipedia

Nitrate test

A nitrate test is a chemical test used to determine the presence of nitrate ion in solution. Testing for the presence of nitrate via wet chemistry is generally difficult compared with testing for other anions, as almost all nitrates are soluble in water. In contrast, many common ions give insoluble salts, e.g. halides precipitate with silver, and sulfates precipitate with barium.

The nitrate anion is an oxidizer, and many tests for the nitrate anion are based on this property. Unfortunately, other oxidants present in the analyte may interfere and give erroneous results.

Brown ring test

A common nitrate test, known as the brown ring test can be performed by adding iron(II) sulfate to a solution of a nitrate, then slowly adding concentrated sulfuric acid such that the sulfuric acid forms a layer above the aqueous solution. A brown ring will form at the juction of the two layers, indicating the presence of the nitrate ion. Note that the presence of nitrite ions will interfere with this test.

The overall reaction is the reduction of the nitrate ion by iron(II) which is oxidised to iron(III) and formation of a nitrosyl complex.

NO3- + 3Fe2+ + 4H+→ 3Fe3+ + NO + 2H2O
[Fe(H2O)6]2+ + NO → [Fe(H2O)5(NO)]2+

Devarda's test

Devarda's alloy (Cu/Al/Zn) is a reducing agent. When reacted with nitrate in sodium hydroxide solution, ammonia is liberated. The ammonia formed may be detected by its characteristic odor, and by moist red litmus — very few gases other than ammonia evolved from wet chemistry are alkaline.

3 NO|3|- + 8 Al + 5 OH|- + 18 H|2|O → 3 NH|3 + 8 [Al(OH)|4|]|-

The aluminium does the reducing in this reaction.

Diphenylamine test

Diphenylamine may be used as a wet chemical test for the presence of the nitrate ion. In this test, a solution of diphenylamine and ammonium chloride in sulfuric acid is used. In the presence of nitrates, diphenylamine is oxidized, giving a blue coloration. This reaction has been used to test for organic nitrates as well, and has found use in gunshot residue kits detecting nitroglycerine and nitrocellulose.

Other oxidants such as chlorate, bromate, etc. interfere by similarly oxidizing diphenylamine. They may be removed by reduction with sodium sulfite. Where nitrite is present, a false negative result may be observed due to sulfite reducing nitrate in the presence of nitrite.

The diphenylamine test may be selective for nitrate by reducing nitrite with sodium azide, prior to treatment with sodium sulfite. Other derivatives have been reported as well.


Silver halide

A silver halide is one of the compounds formed between silver and one of the halogens — silver bromide (AgBr), chloride (AgCl), iodide (AgI), and three forms of silver fluorides. As a group, they are often referred to as the silver halides, and are often given the pseudo-chemical notation AgX. Although most silver halides involve silver atoms with oxidation states of +1 (Ag+), silver halides in which the silver atoms have oxidation states of +2 (Ag2+) are known, of which silver(II) fluoride is the only known stable one.

The light-sensitive chemicals used in photographic film and paper are silver halides.

Applications

Light sensitivity

Silver halides are used in photographic film and photographic paper, as well as graphic art film and paper, where silver halide crystals in gelatin are coated on to a film base, glass or paper substrate. The gelatin is a vital part of the emulsion as the protective colloid of appropriate physical and chemical properties. Gelatin may also contain trace elements (such as sulfur) which increase the light sensitivity of the emulsion, although modern practice uses gelatin without such components. When absorbed by an AgX crystal, photons cause electrons to be promoted to a conduction band (de-localized electron orbital with higher energy than a valence band) which can be attracted by a sensitivity speck, which is a shallow electron trap, which may be a crystalline defect or a cluster of silver sulfide, gold, other trace elements (dopant), or combination thereof, and then combined with an interstitial silver ion to form silver metal speck.

Silver bromide and silver chloride may be used separately or combined, depending on the sensitivity and tonal qualities desired in the product. Silver iodide is always combined with silver bromide or silver chloride, except in the case of daguerreotype production where a daguerreotype (one of the oldest photographic processes) is developed with pure red light instead of mercury vapors (a method known as the Bequerelle method, named for the inventor who discovered the phenomenon). Silver fluoride is not used in photography.

Silver halides are also used to make corrective lenses darken when exposed to ultraviolet light (see photochromism).

When a silver halide crystal is exposed to light, a sensitivity speck on the surface of the crystal is turned into a small speck of metallic silver (these comprise the invisible or latent image). If the speck of silver contains approximately four or more atoms, it is rendered developable - meaning that it can undergo development which turns the entire crystal into metallic silver. Areas of the emulsion receiving larger amounts of light (reflected from a subject being photographed, for example) undergo the greatest development and therefore results in the highest optical density.

Chemistry

Silver halides, except for silver fluoride, are extremely insoluble in water. Silver nitrate can be used to precipitate halides; this application is useful in quantitative analysis of halides. The precipitation of silver halides via silver nitrate is also useful for abstracting halide leaving groups.

However, close attention is necessary for other compounds in the test solution. Some compounds can considerably increase or decrease the solubility of AgX. Examples of compounds that increase the solubility include: cyanide, thiocyanate, thiosulfate, thiourea, amines, ammonia, sulfite, thioether, crown ether. Examples of compounds that reduces the solubility include many organic thiols and nitrogen compounds that do not possess solubilizing group other than mercapto group or the nitrogen site, such as mercaptooxazoles, mercaptotetrazoles, especially 1-phenyl-5-mercaptotetrazole, benzimidazoles, especially 2-mercaptobenzimidazole, benzotriazole, and these compounds further substituted by hydrophobic groups. Compounds such as thiocyanate and thiosulfate enhance solubility when they are present in a sufficiently large quantity, due to formation of highly soluble complex ions, but they also significantly depress solubility when present in a very small quantity, due to formation of sparingly soluble complex ions.

Medical technology

Scientists from Tel Aviv University are experimenting with silver halide optical fibers for transmitting mid-infrared light from carbon dioxide lasers. The fibers allow laser welding of human tissue, as an alternative to traditional sutures.



From Yahoo Answers

Question:I have a question on halides testing. Each halide unknown solution had silver nitrate added to it and a precipitate was formed. Dilute and then conc. ammonia were added to help distinguish which halide was which. All this I understand what I don't get is why nitric acid was then added. I know it has something to do with an acid ad a base reaction but I'm not sure what? Any help?

Answers:for example when we add AgNO3 to a solution of halides we we get any AgCl, AgBr, & any AgI that can form but we might be interested if the mix contains any AgCl by adding concentrated ammonia, AgCl dissociates into solution more than any of the others do but with the others in the mix, how do we now if the ammonia was able to dissociate anything... we pour off the ammonia extract & acidify with HNO3, if the ammonia had dissociated any AgCl, the ppt will return when the ammonia is neutralized if no precipitate returns upon acidifying with HNO3, then we say that the mix did not contain AgCl

Question:Can I add this up please?? Thanks!! What is the equations involved in the silver nitrate (AgNO3) test? Thanks a LoT!!! :)

Answers:This is because a compound of silver with any halogen is definitely insoluble in water. This is the ONLY compound of silver that gives such a result, and it can be immediately concluded that halogen(s) are present in the unknown reactant. Also, the other product of the reaction is a nitrate, which is soluble in water and will not affect the results. This is why it is the specific test for the halogens. FYI, silver chloride gives white, silver bromide gives yellow-white, and silver iodide gives yellow. The colour intensity increases down the group. Taking Y to be a halide and X to be a positive ion, AgNO3 (aq) + XY (aq) --> AgY (s) + XNO3 (aq) Thus it can be seen very clearly that the precipitate formed contains a halogen.

Question:To test an unknown anion present in the solution by studying whether it will precipitate with silver nitrate, the solution is added with nitric acid first before adding silver nitrate. Why is it that way? Does it have something to do with the common-ion effect? Or is it through another concept? Thanks!

Answers:well the acid is simply added to know if sulphite ion or carbonate ion is present...usually when acid are added, required observations are to look for effervescence

Question:After we finished the simple experiment about distillation, we collected the distillate.we divide it into two sample. the first 5ml was placed in a test tube and the second 5ml was placed in another. the first sample was tasted and the second sample was added 2 drops of AgNO3. The first appearance was it is the same-clear..but we noticed it turned to a little cloudy. when we compared our result to the other group, their result turned to yellow. Others turned into orange..what does it mean? help me!our post lab will b on thursday.. maam dasalla, i hope all of us would be exempted for the written exam!

Answers:If you'll recall your solubility chart, you'll see that most salts of Ag+ are insoluble, and most of them are white (before they react with light, in which case they turn black). I would guess that the reason you're adding this is to ensure that the distillate is free of ions. If it is, you should observe no change. If there are other ions in there (that form insoluble salts with Ag+), then you will see a precipitate (which will appear as a solid if there's enough of it, or it will make the solution cloudy if there's just a little bit). I don't know what the orange and yellow are all about. Of course, I am assuming that you are distilling water. Are you certain that it is silver nitrate and not lead nitrate? The latter would make a yellow salt with chloride ions...

From Youtube

Silver nitrate :I got a 40g unit of silver nitrate from the Czech republic, and seeing that it came in a duct taped safe way bag in newspaper I was skeptical if it even was AgNO3. As usual a few quick tests were preformed and sure thing it passed with relative ease. The first test was making sure it was of a nitrate salt of silver so a pinch of Potassium Iodide crystals were added. The precipitated material was yellow-white/tan and an aqueous layer formed. The next test was seeing if it was Ag2I, heating decomposed it into silver and iodine vapors. The next was mixing it with a bit of magnesium and adding water, the two displace exothermically and spark- an indication it is silver nitrate. Playing with chemicals is a good thing. :3

Silver Nitrate Crystallization :A copper wire is placed in a test tube containing 45 ml of 0.1 M AgNO3(aq). The greater the concentration of Silver Nitrate is, the more crystalls will be formed. NOTE: This was pure silver crystal which were formed.