Neutral litmus testing

Place a few drops of the neutral (litmus) test solution upon potassium hexa-cyanocobaltate(III) (or cobalticyanide, Rinmann s green) test paper. Dry the paper over a flame and ignite in a small crucible. Observe the colour of the ash against a white background part of it will be green. 

The neutral salt of a base as strong as ammonium hydroxide is practically not hydrolyzed at all in cold water, certainly not enough to show by the litmus test. Water at 100° is ionized about ten times as much as at ordinary temperature and consequently the hydrolysis of the ammonium sulphate is increased ... 

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. 

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. 

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 ... 

Methyl p-toluenesulphonate. This, and other alkyl esters, may be prepared in a somewhat similar manner to the n-butyl ester with good results. Use 500 g. (632 ml.) of methyl alcohol contained in a 1 litre three-necked or bolt-head flask. Add 500 g. of powdered pure p-toluene-sulphonyl chloride with mechanical stirring. Add from a separatory funnel 420 g. of 25 per cent, sodium hydroxide solution drop by drop maintain the temperature of the mixture at 23-27°. When all the alkali has been introduced, test the mixture with litmus if it is not alkaline, add more alkali until the mixture is neutral. Allow to stand for several hours the lower layer is the eater and the upper one consists of alcohol. Separate the ester, wash it with water, then with 4 per cent, sodium carbonate solution and finally with water. Dry over a little anhydrous magnesium sulphate, and distil under reduced pressure. Collect the methyl p-toluenesulphonate at 161°/10 mm. this solidifies on cooling and melts at 28°. The yield is 440 g. 

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. 

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.)... 

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. 

Isocarboxazid Boil 0.5 g with 5 ml H202 soln. (30%) and 10 ml NaOH soln. (20% w/v in DW) for 2 minutes. Cool, neutralize to litmus with HN03 and add sufficient DW to produce 40 ml. Test with the resulting solution. Complies with limit test for chlorides (25 ppm). 

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. 

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