Acids neutral litmus testing

Of the primary monoamines, some, such as. aniline, o-toluidine, xylidine, are colourless liquids. Others, such as p-toluidine, pseudo-cumidine and the naphthylamines, are solids. They can be distilled without decomposition and are volatile with steam. In water they are rather sparingly soluble—a 3 per cent solution of aniline can be made. The di- and polyamines are usually solids, not volatile in steam and much more soluble in water than the monoamines. The amines are basic in character, but, as a result of the negative nature of the phenyl-group, the aromatic amines are considerably weaker bases than are the aliphatic amines. Consequently aqueous solutions of the (stoicheio-metrically) neutral aniline salts are acid to litmus because of the hydrolysis which they undergo. For the same reason a small amount of the free base can be extracted with ether from an aqueous solution of an aniline salt. (Test with a solution of hydrogen chloride in ether or, after evaporation of the ether, by the reaction with bleaching powder.)... 

It is best to neutralize the test solution first by ammonia against litmus paper, then add some drops of dilute nitric acid, and then the reagent. The test is a very sensitive one. Antimony interferes and should be absent. 

Test with calcium chloride solution For tests 5-7 a neutralized soda extract is required. This is prepared as follows. Take 10 ml soda extract in a porcelain dish and render it faintly acid with dilute nitric acid (use litmus paper or other equivalent test-paper). Boil for 1-2 minutes to expel carbon dioxide, allow to cool, then add dilute ammonia solution until just alkaline and boil for 1 minute to expel the slight excess of ammonia. Divide the solution into three equal parts for tests 5, 6, and 7. 

Consider the following neutralization reaction. Hydrochloric acid, HCl, is a common household and laboratory acid. Muriatic acid is the common household name of hydrochloric acid. It is often sold in hardware stores to be used in masonry work to remove excess mortar from brick. Sodium hydroxide, NaOH, is a common household and laboratory base. The common name of sodium hydroxide is lye. It is the primary component of many drain cleaners. Figure 15.1 shows litmus tests before and after mixing these substances together. 

Neutral Compounds of Group I.—Aldehydes, ketones, and alcohols of low molecufar weight, together with a few esters, are found in Solubility Group I, since they are soluble in water and also in ether. They will usually, but not always, be met as liquids. When a substance is located in Group I, the aqueous solution of the unknown is immediately tested for acidity, so as to differentiate the neutral from the acidic substances. If the aqueous solution is acid to litmus, a portion of the unknown, about 0.2 g., is titrated with 0.1 N. alkali, using phenolphthalein as an indicator. Small amounts of acid, often inorganic, may be present as impurities and it is important therefore to know approximately tlic amount of acidity. 

Litmus is a substance that is extracted from a type of lichen and absorbed into porous paper. There cire three different types of litmus — red, blue, and neutral. Red litmus is used to test for bases, blue litmus is used to test for acids, and neutral litmus can be used to test for both. Here s how the paper reacts to acids and bases ... 

Colorations or coloured precipitates are frequently given by the reaction of ferric chloride solution with.(i) solutions of neutral salts of acids, (ii) phenols and many of their derivatives, (iii) a few amines. If a free acid is under investigation it must first be neutralised as follows Place about 01 g. of the acid in a boiling-tube and add a slight excess of ammonia solution, i,e., until the solution is just alkaline to litmus-paper. Add a piece of unglazed porcelain and boil until the odour of ammonia is completely removed, and then cool. To the solution so obtained add a few drops of the "neutralised ferric chloride solution. Perform this test with the following acids and note the result ... 

Arid .—Make a solution (if not already dissolved) and test with litmus. If the liquid is acid, a fiee (UvVf is probably present. If the liquid is neutral and a metal has been found, a metallic salt is probably present. If the liquid is alkaline, it may be the alkaline salt of a phenol or an alkaline cyanide, both of which are hydiolysed in solution. The separation and identification ot the acid is not a eiy simple niattei. If the acid is an aromatK ... 

Experiment.—Dissolve 1 c.c. of nitromethane in water and test the solution with litmus paper. Then add some phenolphthalein and, drop by drop from a burette, OliV-sodium hydroxide solution. Before a permanent pink colour develops about 2 c.c. of the alkali will be added—a sign that an acid, aci-nitromethane, H2C NOOH, has been formed from the neutral nitromethane. A small sample of this solution gives with ferric chloride a blood-red colour, characteristic of aci-nitro-compounds. The salts of the oci-compound undergo extensive hydrolysis. This is shown by further addition of 0-1 N-alkali which produces a deep red colour. If 10 c.c. of alkali were added and 5 c.c. of 0-1 JV-hydrochloric acid are now run in the solution is decolorised because the liberated oci-compound restricts the hydrolysis of its salt. But the conversion of H2C N02H into H3C.N02 proceeds so rapidly that the red colour reappears in a few moments. 

The salts of p-nitrosodimethylaniline, on the other hand, are yellow. Moreover, since they are neutral-—test this with the pure salt—and since dimethylaniline hydrochloride is add to litmus paper, they cannot have resulted from the simple addition of the acid to the tertiary dimethylamino-group. It is therefore assumed that the salts are formed by the addition of H and acid ion in the 1 7-positions, so that a quinonoid system results through a rearrangement ... 

In order now to remove proteins from the thin sludge it is vigorously stirred with 0-05 A-acetic acid in amount just sufficient to change the colour of methyl red (pH=4) (test with a sample), and is then filtered as above after shaking with a little kieselguhr if necessary. The filtrate is made neutral to litmus with dilute ammonia and in this condition, protected by a little toluene, can be kept unchanged for several days. 

The usual method of testing neutrality by means of litmus or phenolphthalein paper is not applicable when the pH of the material is around 7.0- Litmus paper does not turn red until die acidity is between approx pH 4 and 6, and phenolphthalein paper does not change color until the acidity is between approx pH 8 and 10... 

Mercury fulminate prepared by one of the methods outlined above is tested to check the acid content and the content of other mercury compounds. When tested with litmus paper, the moist product should give a neutral reaction. A 5 g test sample mixed with 2 g of sodium hydrogen carbonate should not give a black or bluish tint. 

If nitrogen and/or sulphur is present, acidify 3-4 ml of the fusion solution with dilute nitric acid and evaporate to half the original volume in order to expel any HCN and/or H2S which may be present. If nitrogen and sulphur are absent, proceed directly with 2 ml of the sodium fusion filtrate. Render the solution just neutral to litmus by the addition of dilute (5 m) aqueous ammonia solution, then add 5 drops of 5 m acetic acid and 20 mg of lanthanum chloranilate (2,5-dichloro-3,6-dihydroxy-p-benzoquinone, lanthanum salt) and shake intermittently for 10-15 minutes. Filter. A pink-violet coloration of the filtrate is a positive test for fluorine. 

A. Mix about 500 mg of sample with 10 mL of 2.7 N hydrochloric acid, filter, and neutralize the filtrate to litmus paper with 6 N ammonium hydroxide. The neutralized filtrate gives positive tests for Calcium, Appendix IIIA. 

Test Solution Dissolve about 100 mg of sample, accurately weighed, in 30 to 40 mL of water, and mix. Add 10% nitric acid dropwise until the solution is neutral to litmus. Add an additional 1 mL of 10% nitric acid, and dilute with water to a total volume of 50 mL. 

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