Ammonia titration with strong acid

Numerous inorganic species can be determined by titration with strong acids or bases. For example, ammonium salts are determined by conversion to ammonia with strong base and distillation in the Kjeldahl apparatus. Also, inorganic nitrates and nitrites can be determined using Kjeldahl method by reducing these species to ammonium ion. 

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. 

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. The hydroxides of sodium, potassium, and barium are generally employed for the preparation of solutions of standard alkalis they are water-soluble strong bases. Solutions made from aqueous ammonia are undesirable, because they tend to lose ammonia, especially if the concentration exceeds 0.5M moreover, it is a weak base, and difficulties arise in titrations with weak acids (compare Section 10.15). Sodium hydroxide is most commonly used because of its cheapness. None of these solid hydroxides can be obtained pure, so that a standard solution cannot be prepared by dissolving a known weight in a definite volume of water. Both sodium hydroxide and potassium hydroxide are extremely hygroscopic a certain amount of alkali carbonate and water are always present. Exact results cannot be obtained in the presence of carbonate with some indicators, and it is therefore necessary to discuss methods for the preparation of carbonate-free alkali solutions. For many purposes sodium hydroxide (which contains 1-2 per cent of sodium carbonate) is sufficiently pure. 

Ref 3,pp 256 7) b)Direct Method in which a soln of sample is heated with coned soln of strong base(such as NaOH) in a distillation flask and the expelled ammonia is collected in a flask contg an excess of standard acid. Then the excess of acid is titrated with std NaOH or KOH soln(Ref l,p 637 Ref 2,p 493 Ref 3,pp 254-5) c)Indirect Method, in which the sample is boiled in a flask with a known excess of NaOH soln until all NH3 is expelled with the steam. Then the flask is cooled and the excess of NaOH is titrated with std acid(Ref 3, P 255)... 

The titration curves for weak bases and strong acids are similar to those for weak acids and strong bases except that they are inverted (recall that strong is added to weak). Figure 19-5 displays the titration curve for 100.0 mL of 0.100 M aqueous ammonia titrated with 0.100 MHCl solution. 

The titration of a weak base with a strong acid is completely analogous to the above case, but the titration curves are the reverse of those for a weak acid versus a strong base. The titration curve for 100 mL of 0.1 M ammonia titrated with 0.1 M hydrochloric acid is shown in Figure 8.8. The neutralization reaction is... 

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. 

The Kjeldahl conversion of amino and amide groups into ammonia, which is achieved by treating the sample with strong acids at high temperatures in the presence of a catalyst. Following alkalinization of the digest, the ammonia is steam-distilled into a nonvolatile acid and titrated. 

It may be noted that very weak acids, such as boric acid and phenol, which cannot be titrated potentiometrically in aqueous solution, can be titrated conductimetrically with relative ease. Mixtures of certain acids can be titrated more accurately by conductimetric than by potentiometric (pH) methods. Thus mixtures of hydrochloric acid (or any other strong acid) and acetic (ethanoic) acid (or any other weak acid of comparable strength) can be titrated with a weak base (e.g. aqueous ammonia) or with a strong base (e.g. sodium hydroxide) reasonably satisfactory end points are obtained. 

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. 

In many titrations, one solution—either the analyte or the titrant—contains a weak acid or base and the other solution contains a strong base or acid. For example, if we want to know the concentration of formic acid, the weak acid found in ant venom (1), we can titrate it with sodium hydroxide, a strong base. Alternatively, to find the concentration of ammonia, a weak base, in a soil sample, titrate it with hydrochloric acid, a strong acid. Weak acids are not normally titrated with weak bases, because the stoichiometric point is too difficult to locate. 

Because the salt has a basic anion, we can expect the pH to be greater than 7 at the stoichiometric point. At the stoichiometric point of the titration of aqueous ammonia with hydrochloric acid, the solute is ammonium chloride. Because NH4+ is an acid, we expect the solution to be acidic with a pH of less than 7. The same is true for the stoichiometric point of the titration of any weak base and strong acid. 

The progress of the reaction was followed by monitoring the decrease in formaldehyde concentration with time. Previous studies used the hydroxylamine hydrochloride method of analysis (5 -55), but this was avoided in the current study as it requires tedious pH titrations. Instead, a colorimetric method was used that was first developed by Nash (55), involving formation of 3,5-diacetyl-1,4-dihydrolutidine, by reaction of formaldehyde with ammonia and acetyl acetone at neutral pH. The cyclic product absorbs at 412 nm with a molar extinction coefficient of 8,000 (55). Other colorimetric methods cannot be used as they all involve very strongly acidic or basic media (55), which would force the phenol-formaldehyde reaction to completion. 

If the strong acid is titrated with a weak base, e.g., an aqueous solution of ammonia, the fii st part of the conductance-titration curve, representing the neutralization of the acid and its replacement by a salt, will be very similar to the first part of Fig. 24, since both salts are strong electrolytes. When. the equivalence-point is passed, however, the conductance will remain almost constant since the free base is a weak electrolyte and consequently has d very small conductance compared with that of the acid or salt. 

An often used method for the detn of either aminoid or nitrate nitrogen is the decompn-volumetric Kjeldahl (or Chenel) procedure in which the sample is decompd with coned sulfuric acid and suitable catalyzers to yield amm sulfate quant. After release with strong alkali, the ammonia is quant distd into an excess of standard add and back-titrated with standard alkah (See Vol 1, A616-R Vol 2, C46-L ... 

An acid may, rather arbitrarily, be called a strong acid in glacial acetic acid if HAjp - 1- Thus perchloric acid is a strong acid, and yet the pAT for the overall dissociation constant is only 4.87 because it exists largely as ion pairs. Hydrochloric acid has an overall pAT value of 8.55 ammonia, 6.40 pyridine, 6.10 sodium acetate, 6.68 potassium chloride, 6.88 and sodium perchlorate, 5.48. Perchloric acid is the strongest acid and the one used for titration of bases that may be too weak to be titrated in water as solvent. At first it appears that an attempt to titrate a base such as pyridine with perchloric acid would fail, since both have small overall dissociation constants. Critical to the success of such titrations is the small dissociation constant of the salt formed, which results in a large favorable equilibrium constant for the reaction. 

If the titrant is a weak electrolyte (such as ammonia), the curve is essentially horizontal past the equivalence point, which causes less uncertainty to the extrapolation of a curve. In titration of a weak base, such as acetate ion, with a strong acid, a salt and undissociated acetic acid are formed. After the endpoint is passed, a sharp rise in conductance attends the addition of excess hydronium ions. Salts whose acidic or basic character is too weak to give satisfactory endpoints with indicator are conveniently titrated with the conductometric method. The conductometric titration of a mixture of two acids that differ in degree of dissociation is frequently more accurate than a potentiometric titration. 

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