Titration of ammonia

EJLSwift,AnalChem 28,440(1956) (Coulo-metric titration of ammonia with hypo-bromite)... 

Chart giving densities of aq ammonia at various temps) 14)Manufacturers Chemists Association of the US, Table-Aqua Ammonia, Manual Sheet T—1 (Densities of aq ammonia at various temps), Washington, DC 15)Joint Army-Navy Specification, JAN-A-182(Synthetic Anhydrous Ammonia for use in explosives) l6)Spencer Chemical Co Standard Procedure FP-1, for analysis of Anhydrous Liquid Ammonia, is essentially the same as Ref 15 17)G.M.Arcand EJLSwift,AnalChem 28,440(1956) (Coulo-metric titration of ammonia with hypo-bromite)... 

What would be a suitable indicator for the titration of ammonia with hydrochloric acid Of acetic acid with sodium hydroxide ... 

Fig. 4.6 Tecator KjelTec Auto 1030 automatic analyser including distillation and photometric acid-base titration of ammonia. (1) Waste flask (2) water (inlet) valve (3) water aspirator (4) expansion vessel (5) feed valve (6) steam generator (7) vacuum valve (if tube drainage) (8) steam valve (9) water pump (10) digestion tube (11) alkali pump (12) distillation head (13) condenser (14) level pin (15) motor (stirrer) (16) lamp (titration vessel) (17) titration vessel (18) drain valve (titration vessel) (19) receiver solution pump (20) motor (burette) (21) burette cylinder (22) three-port valve (23) titrant storage flask. (Courtesy of Tecator).

Comparing the two titration curves one can recognize that the steepness of the titration curve at the equivalence point is much larger in case of the titration of ammonia with hydrochloric acid, than for the case of titration of ammonium ions with sodium hydroxide. This means that the random errors will be much smaller when ammonia is titrated with hydrochloric acid, and this titration is strongly to be preferred. 

Fig. 85 Reading of the relevant concentration data from the pH-logCj diagram for estimation of the systematic titration errors in case of the titration of ammonia =0.1 mol L ) with...

Figure 85 illustrates how the relevant data can be read from a pH-logCj diagram using the example of a titration of ammonia with hydrochloric acid. 

The weights of sample required for the determination of nitrogen in compounds containing between 1% and 90% of nitrogen are shown in Table 2.6. Also shown is the normality of the hydrochloric acid required for the titration of ammonia at the end of the determination. 

Nitrogen content of sample (%) Weight of sample for digestion (g) Strength of hydrochloric add to be used in titration of ammonia (normality, N)... 

From a practical standpoint, the neutralization indicators to be used must have their color-change interval located in the acid range. For example, for the above-mentioned titration of ammonia, methyl red and helianthin are suitable. This is also the case of bromophenol blue and bromocresol green. 

By the evolution of ammonia with Devarda s alloy in alkaline solution in absence of ammonium ions this is used quantitatively, the ammonia being absorbed in excess standard acid and the excess acid back-titrated. 

Titration. The solution of ammonia absorbed in saturated boric acid may now be titrated as an alkali directly with 0 025 A.HCl (best obtained by dilution of commercially available standard A.HCl in a graduated flask). Three drops of indicator (mixed methyl-red/methyl-ene-blue being most satisfactory) are added to the liquid in the receiver and the 0 025 A.HCl run in from an accurate burette. 

The method is based on the conversion of urea to amnionium carbonate and the estimation of the latter by titration with standard acid. For this purpose, two equal quantities of urea (or urine) are measured out into two flasks A and B. A is treated with 10 ml. of a strong urease preparation and some phenol-phthalein, warm water is added and the mixture is adjusted by the addition of V/io HCl from a burette A until the red colour is just discharged. This brings the mixture to about pH 8 (the optimum for urease) and also prevents loss of ammonia. 

Direct Titrations. The most convenient and simplest manner is the measured addition of a standard chelon solution to the sample solution (brought to the proper conditions of pH, buffer, etc.) until the metal ion is stoichiometrically chelated. Auxiliary complexing agents such as citrate, tartrate, or triethanolamine are added, if necessary, to prevent the precipitation of metal hydroxides or basic salts at the optimum pH for titration. Eor example, tartrate is added in the direct titration of lead. If a pH range of 9 to 10 is suitable, a buffer of ammonia and ammonium chloride is often added in relatively concentrated form, both to adjust the pH and to supply ammonia as an auxiliary complexing agent for those metal ions which form ammine complexes. A few metals, notably iron(III), bismuth, and thorium, are titrated in acid solution. 

Analytical Procedures. Standard methods for analysis of food-grade adipic acid are described ia the Food Chemicals Codex (see Refs, ia Table 8). Classical methods are used for assay (titration), trace metals (As, heavy metals as Pb), and total ash. Water is determined by Kad-Fisher titration of a methanol solution of the acid. Determination of color ia methanol solution (APHA, Hazen equivalent, max. 10), as well as iron and other metals, are also described elsewhere (175). Other analyses frequendy are required for resia-grade acid. For example, hydrolyzable nitrogen (NH, amides, nitriles, etc) is determined by distillation of ammonia from an alkaline solution. Reducible nitrogen (nitrates and nitroorganics) may then be determined by adding DeVarda s alloy and continuing the distillation. Hydrocarbon oil contaminants may be determined by ir analysis of halocarbon extracts of alkaline solutions of the acid. 

Dry air is blown through the solution to remove the excess of ammonia, and the solution is then dissolved in its own volume of absolute alcohol. A sample of this solution is titrated with standard oxalic acid, litmus being used as an outside indicator (Note 3). The amount of oxalic acid (Note 4) necessary to form the acid salt is placed in a large evaporating dish and dissolved in 4 1. of 95 per cent alcohol. The amine solution is then slowly run into the acid with constant stirring. During the addition of the last half of the amine solution, the container must be cooled in order to avoid the formation of the neutral oxalate,... 

A slrang acid-weak base titration. The curve represents Ihe titration of 50.00 mL of 1.000 M ammonia, a weak base, with 1.000 M HCI. The solution at the equivalence point is acidic because of the NfV ion. Methyl red is a suitable indicator phenolphthalein would change color much too early. 

At 25°C and 1.00 atm pressure, one liter of ammonia is bubbled into 725 mL of water. Assume that all the ammonia dissolves and the volume of the solution is the volume of the water. A 50.0-mL portion of the prepared solution is titrated with 0.2193 M HN03. Calculate the pH of the solution... 

A solution of dry ammonia gas in absolute ethyl alcohol is prepared and titrated against standard hydrochloric acid, using methyl orange as the indicator. The solution should contain at least 9 per cent of ammonia by weight (Note 5). 

It is clear that neither thymolphthalein nor phenolphthalein can be employed in the titration of 0.1 M aqueous ammonia. The equivalence point is at pH 5.3, and it is necessary to use an indicator with a pH range on the slightly acid side (3-6.5), such as methyl orange, methyl red, bromophenol blue, or bromocresol green. The last-named indicators may be utilised for the titration of all weak bases (Kb> 5 x 10-6) with strong acids. 

Discussion. When a solution of an orthophosphate is treated with a large excess of ammonium molybdate solution in the presence of nitric acid at a temperature of 20-45 °C, a precipitate is obtained, which after washing is converted into ammonium molybdophosphate with the composition (NH4)3[P04,12Mo03]. This may be titrated with standard sodium hydroxide solution using phenolph-thalein as indicator, but the end point is rather poor due to the liberation of ammonia. If, however, the ammonium molybdate is replaced by a reagent containing sodium molybdate and quinoline, then quinoline molybdophosphate is precipitated which can be isolated and titrated with standard sodium hydroxide ... 

This colour change can be observed with the ions of Mg, Mn, Zn, Cd, Hg, Pb, Cu, Al, Fe, Ti, Co, Ni, and the Pt metals. To maintain the pH constant (ca 10) a buffer mixture is added, and most of the above metals must be kept in solution with the aid of a weak complexing reagent such as ammonia or tartrate. The cations of Cu, Co, Ni, Al, Fe(III), Ti(IV), and certain of the Pt metals form such stable indicator complexes that the dyestuff can no longer be liberated by adding EDTA direct titration of these ions using solochrome black as indicator is therefore impracticable, and the metallic ions are said to block the indicator. However, with Cu, Co, Ni, and Al a back-titration can be carried out, for the rate of reaction of their EDTA complexes with the indicator is extremely slow and it is possible to titrate the excess of EDTA with standard zinc or magnesium ion solution. 

Fast sulphon black F ( C.I.26990). This dyestuff is the sodium salt of 1-hydroxy-8-( 2-hydroxynaphthylazo) -2- (sulphonaphthylazo) -3,6-disulph onic acid. The colour reaction seems virtually specific for copper ions. In ammoniacal solution it forms complexes with only copper and nickel the presence of ammonia or pyridine is required for colour formation. In the direct titration of copper in ammoniacal solution the colour change at the end point is from magenta or [depending upon the concentration of copper(II) ions] pale blue to bright green. The indicator action with nickel is poor. Metal ions, such as those of Cd, Pb, Ni, Zn, Ca, and Ba, may be titrated using this indicator by the prior addition of a reasonable excess of standard copper(II) solution. 

Pipette 25 mL of an aluminium ion solution (approximately 0.01 M) into a conical flask and from a burette add a slight excess of 0.01 M EDTA solution adjust the pH to between 7 and 8 by the addition of ammonia solution (test drops on phenol red paper or use a pH meter). Boil the solution for a few minutes to ensure complete complexation of the aluminium cool to room temperature and adjust the pH to 7-8. Add 50 mg of solochrome black/potassium nitrate mixture [see Section 10.50(C)] and titrate rapidly with standard 0.01 M zinc sulphate solution until the colour changes from blue to wine red. 

The monoethanolamine-hydrochloric acid buffer has a buffering capacity equal to the ammonia-ammonium chloride buffer commonly employed for the titration of calcium and magnesium with EDTA and solochrome black (compare Section 10.54). The buffer has excellent keeping qualities, sharp end points are obtainable, and the strong ammonia solution is completely eliminated. 

Either the Mohr titration or the adsorption indicator method may be used for the determination of chlorides in neutral solution by titration with standard 0.1M silver nitrate. If the solution is acid, neutralisation may be effected with chloride-free calcium carbonate, sodium tetraborate, or sodium hydrogencarbonate. Mineral acid may also be removed by neutralising most ofthe acid with ammonia solution and then adding an excess of ammonium acetate. Titration of the neutral solution, prepared with calcium carbonate, by the adsorption indicator method is rendered easier by the addition of 5 mL of 2 per cent dextrin solution this offsets the coagulating effect of the calcium ion. If the solution is basic, it may be neutralised with chloride-free nitric acid, using phenolphthalein as indicator. 

There are two methods for overcoming these disadvantages. In the first the precipitation of silver cyanoargentate at the end point can be avoided by the addition of ammonia solution, in which it is readily soluble, and if a little potassium iodide solution is added before the titration is commenced, sparingly soluble silver iodide, which is insoluble in ammonia solution, will be precipitated at the end point. The precipitation is best seen by viewing against a black background. 

Strong acid with a weak base. The titration of a strong acid with a moderately weak base (K sslO-5) may be illustrated by the neutralisation of dilute sulphuric acid by dilute ammonia solution [curves 1 and 3, Fig. 13.2(a)]. The first branch of the graph reflects the disappearance of the hydrogen ions during the neutralisation, but after the end point has been reached the graph becomes almost horizontal, since the excess aqueous ammonia is not appreciably ionised in the presence of ammonium sulphate. 

Weak acids with weak bases. The titration of a weak acid and a weak base can be readily carried out, and frequently it is preferable to employ this procedure rather than use a strong base. Curve (c) in Fig. 13.2 is the titration curve of 0.003 M acetic acid with 0.0973 M aqueous ammonia solution. The neutralisation curve up to the equivalence point is similar to that obtained with sodium hydroxide solution, since both sodium and ammonium acetates are strong electrolytes after the equivalence point an excess of aqueous ammonia solution has little effect upon the conductance, as its dissociation is depressed by the ammonium salt present in the solution. The advantages over the use of strong alkali are that the end point is easier to detect, and in dilute solution the influence of carbon dioxide may be neglected. 

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