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precipitate, acid, white, acids, chloride, add and filtrate

ANALYSIS.- In chemistry, the examination of bodies with a view of ascertaining of what substances they are composed and in what proportions these substances are contained. The former is termed qualitative and the latter quantitative analysis. There are various methods of analyzing or separating the particles of a chemical compound. These may be classified into the following : Blowpipe, qualitative, gravimetrical, and volumetric, and the proximate and ultimate analysis of organic bodies. Having isolated these constituents we next proceed to put them together again to reproduce the original substance. This is called synthesis, and is more clearly explained under Chemistry.

The following information is likely to be of use to photographers wishing to analyze any substance or to detect any impurities: The substance to be analyzed is dissolved in a test tube. If not soluble in water, hot or cold ; then in hydrochloric acid or in aqua regia, and if insoluble in these it is fused with sodium carbonate. The solution thus obtained is termed the original solution. To this add a few drops of hydrochloric acid. If a white precipitate is formed it is either silver chloride, mercurous chloride, or lead chloride, indicating the presence of these metals. Filter, and pass sulphuretted hydrogen gas through the filtrate. If the precipitate turns black, lead, copper, mercury, or bis muth is present. A yellow precipitate indicates cadmium arsenic or tin. A brown precipitate shows tin (in another form), and an orange color denotes the presence of antimony.

Filter again, and boil the filtrate in order to expel the sulphuretted hydrogen. Now add a few drops of nitric acid, boil to oxidize the iron, and add chloride of ammonium and ammonia. A red precipitate shows the presence of iron ; if of a bluish-green color, chronium. A white precipitate indicates aluminium, or phosphates, borates and oxalates.

Again filter, and add ammonium sulphide to the filtrate. If it turns black, cobalt or nickel ; if pink, turning brown afterward, manganese is present, or, if white, zinc.

Filter, and add to the filtrate ammonium carbonate. A white precipitate shows calcium, barium or strontium. Filter and divide the filtrate into two equal parts. To one add a small

quantity of sodium phosphate, and if a white precipitate is formed, it proves the presence of magnesium. The other portion is evaporated to a dry powder, heated to drive off the ammoniacal salts, and tested for potash and soda.

Take another portion of the original solution and test for ammoniacal salts. Add to it caustic potash. This will liberate the ammonia, which is easily recognized by its smell and its action upon red litmus or turmeric paper.

Arsenic, antimony, and tin sulphides are soluble in ammonium sulphide, and can be repre cipitated by hydrochloric acid.

To test acids, observe the following : Carbonic, hydrosulphuric and hydrocyanic acids are liberated by stronger acids with effervescence. Carbonic, arsenious arsenic, chromic, boracic, phosphoric, oxalic, hydrofluoric, and sillicic acid, give from a neutral solution a white precipitate, insoluble in acids ; tartaric and citric acids may be easily recognized by the precipitate charring when heated, and giving out fumes of a peculiar odor. If calcium chloride be added to phos phoric acid, a white precipitate is formed, which is soluble in acetic acid. Oxalic and hydro fluoric also give a white precipitate with calcium chloride, but it is insoluble in acetic acid.

Silver nitrate gives a black precipitate with hydrosulphuric acid, a red precipitate with chromic and arsenic acids, a white precipitate with boracic and oxalic acids, a yellow precipitate with arseneous, phosphoric and sillicic acid. All these precipitates can be dissolved in nitric acid. Hydrochloric, hydrocy anic and uytmouit; how ever, give a precipitate insoluble in nitric acid.

Acetic acid and sulphuric acid give with ferric chloride a red color. If ferric chloride be added to gallic and tannic acids, a black precipitate is the result.

A doubtful substance should be treated in as many different ways as possible, and it is not difficult to prove all the com ponent Darts. if each be tested for by 'the different methods given. With the description of each chemical is given, where possible, one or more methods of testing and proving existence of adulterations.

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