silver nitrate... water-soluble, colorless ionic complex. why does it stain skin?

Question

if you get silver nitrate on something, be it skin, clothes, etc, it will produce a localized brown stain after it has been exposed to sunlight. i have some of it on my skin now, can't argue with the results! i was just wondering what the reaction is... does the Ag+ and/or NO3- form some other compound? why does the brown stain remain localized, if the compound is so water-soluble (and the human body is mostly water)?

no wikipedia answers, i was totally dissatisfied by their information.

there is indeed direct contact with water on the skin (sweat as well as insensible water), and it doesn't matter if it is pure water or not, since it is not saturated with respect to any ion. many chemicals travel through the body via absorption through the skin and either active ion transport or travelling down a gradient.

Answer 1

the nitrate ion leaves the silver ion... either it washes off or is absorbed. when the silver ion is exposed to sunlight, it picks up an electron and becomes silver metal, which produces the localized stain.

it is highly likely that some silver ion also enters the body, but in very dilute quantities which may or may not be exposed to light, so you don't see staining elsewhere.

if you know you've spilled some silver nitrate on your clothes or skin, you can reduce the chance of staining by soaking the area in water, or better yet very salty water... sodium chloride (table salt), potassium chloride ("diet" salt) whatever you have available. the silver will react with the chloride to form insoluble white precipitate, and the water will help dilute everything. there are chloride ions in your body water, so if you don't add the external chloride salts, the insoluble precipitate can form on/inside you and trap the silver metal... not easy to wash away, though not particularly harmful.

after the stain appears, you can resolubilize the silver with sodium thiosulfate (works well on clothes... for skin you can just wait it out, it will come off as you shed skin cells).

Answer 2

Skin contact with silver nitrate solid or solutions is likely to leave silver stains on the skin. These develop slowly over a period of hours and are initially brown, but darken gradually to black. Once the stains become apparent, they cannot usually be removed with soap and water, but gradually disappear as new skin grows. A spill of AgNO3 on the skin results in a dark stain resulting from a combination of silver metal and silver sulfide....

Answer 3

What you see in your skin is pure silver precipitated from a reaction that occurs when aqueous silver nitrate contacts your skin. It works just like photographic film. If you mix aqueous solutions of silver nitrate and sodium chloride, the result will be an aqueous solution of sodium nitrate and a silver chloride precipitate. Silver chloride is similar to the compounds used to make a light-sensitive photographic emulsion, which is then coated on film.

When silver nitrate hits your skin, it reacts with the halide ions in your body and precipitates. Exposure to light makes the reaction product behave just like a piece of exposed film - it turns black with the formation of stable silver atoms via electron transfer within the silver halide lattice. Does that help?

Answer 4

This is the same process that is used on the black and white photographic paper. I'm not sure of the exact chemistry behind it, but you might try a search on photo-reactive chemicals...
I know before the advent of digital cameras, Kodak was the biggest users of silver, for the making of film.

Answer 5

It happens because of light reaction on silvercompound.Even if human body has plenty of water,no direct contact is there and the water is not in its pure form.