Aliphatic amines, titration

Generally, the results of the measurements indicated that, where dissociation constants were determined by conductometry and also potentiometric titration, they were in agreement with each other further, KHX is low, e.g., about 10 4-10 6moll 1 for aromatic sulphonic acids and 10 13-10 16 moll-1 for carboxylic acids, Xhx2 is high, e.g., 102-104, and KBis low again, e.g., 10 5-10 6 for aliphatic amines and 10 10 for aromatic amines. 

Carboxylic and sulfonic acid groups import acidity to organic compounds. Most carboxylic acids and sulfuric acids are readily dissolved in water, and their titration with a base is straightforward. If solubility in water is not sufficient, the acid can be dissolved in ethanol and titrated with aqueous base. Aliphatic amines and many saturated cyclic amines can be titrated directly with a solution of a strong acid. Esters are determined by saponification with a measured quantity of standard base. The excess base is titrated with standard acid. 

Aliphatic amines generally have base dissociation constants on the order of 10 and can thus be titrated directly with a solution of a strong acid. In contrast, aromatic amines such as aniline and its derivatives are usually too weak for titration in aqueous medium ( Ib 10 °). The same is true for cyclic amines with aromatic character, such as pyridine and its derivatives. Many saturated cyclic amines, such as piperidine, tend to resemble aliphatic amines in their acid/base behavior and thus can be titrated in aqueous media. 

Ethylenediamine, (I I,NCH,), is a strong organic base miscible with water and alcohol. It is a colourless and viscous liquid with a density of 0.898 and a melting point of 8 °C. The pH of a 25% aqueous solution is 11.5. Like triethanolamine, it is an aliphatic amine soluble in water and, therefore, can be determined by the acid-base titration with methyl orange as an indicator. 

In addition to the determination of total base, it is also possible to titrate mixtures. This can be done in two ways. One is to titrate mixtures based on the type of amines present. For example, one can distinguish between primary, secondary, and tertiary amines. This is done simply. Acetylate the primary and secondary amines in the mixture with acetic anhydride. They are converted to amides which are only weakly basic. Tertiary amines are not affected and titrate very well. A further differentiation can be made, however. The primary amine can be reacted with salicyl aldehyde to form a Schiff base. The secondary and tertiary amines are unaffected as far as basic strength is concerned, so that one can titrate the sum of secondary and tertiary amines. By these two titrations plus a determination of total amine, one can resolve the mixture. This approach works well for aliphatic amines, but not for aromatic amines. 

Another method of analyzing a mixture of bases is to utilize the difference in the basicity of its components. As an example, let us take a mixture of aromatic and aliphatic amines. Since aliphatic amines are more strongly basic, one would expect to get a titration curve with two breaks, one for the aliphatic amine and one for the aromatic amine. However, you must not use glacial acetic acid for this titration because you will get a curve similar to curve B in Fig. 1. In other words, you get one potentiometric end point for the sum of the two. The reason is that glacial acetic acid reacts with aliphatic amines to form the acetate ion, which has about the same basic strength as the aromatic amine. Glacial acetic acid levels these two amines to the same strength. What you have to do is employ a nonaqueous solvent like acetonitrile and titrate with perchloric acid dissolved in dioxane. If you do this. 

The very weak value explains why it is not possible to titrate in water an amino acid by a strong base nor is it possible to titrate, in the same conditions, protonated aliphatic amines. This is the same problem as that encountered for the titration of the third acidity of phosphoric acid by sodium hydroxide (see Chap. 9). 

The main part of this group contains derivatives from aliphatic amines primary, secondary, and tertiary. Their pK values are, statistically, located in the range 8.5-10.5. Their titration with sodium hydroxide in water offers two major difficulties ... 

Primary, secondary, and tertiary aliphatic amines exhibit pK values located statistically in the range 8.5 < pH < 10.5. Without any surprises, they can be titrated with a hydrochloric or sulfuric acid solution. The reaction is sufficiently complete. At the equivalence point, we are in the presence of a weak acid the conjugate acid of the amine. The pH is located around 4.6. The mixed indicator is convenient for detecting it. Unfortunately, the great insolubility in water of the amines to be titrated is a very serious limit to these titrations. 

The Ka value is about 10 . By applying this relation and from thepK values of normal protonated aliphatic amines (8.5-10.5) and of normal carboxylic acids (3.6-5.0), it is clear that the microscopic constant 12 imposes its value quasi-exclusively to the macroscopic constant Ka. It is clear also that if the amine function disappears after a chemical reaction, the only acid that remains is the carboxylic function. The macroscopic constant Ka value must tend then to that of the microscopic constant k22 and its titration by a sodium hydroxide solution must become possible. 

A more specific titration method than either of the above is based upon formation of a dithiocarbamic acid. The method given below follows that of Critchfield and Johnson for the titration of secondary aliphatic amines, but results may vary within a range of about 0-5 per cent. The precipitate which forms on addition of the carbon disulphide dissolves as the titration proceeds ... 

Used in the spectrophotometric titration of aliphatic amines. Needles (QH ). Mp 136°. 

The determination of primary amines on the macro scale is most conveniently carried out by titration in non-aqueous solution (Section 10.41), but for small quantities of amines spectroscopic methods of determination are very valuable. In some cases the procedure is applicable to aromatic amines only, and the diazotisation method described for determination of nitrite (Section 17.38) can be adapted as a method for the determination of aromatic primary amines. On the other hand, the naphthaquinone method can be applied to both aliphatic and aromatic primary amines. 

The first three tertiary amines in the aliphatic series were studied by Strecker and Baltes. The authors studied tri-n-butylamine, tri-n-hexylamine, and tri-n-heptyl-amine. Each amine was ozonated at dry ice temperature and treated with picric acid. The tri-n-butylamine oxide picrate readily deposited as crystals on standing. Although the latter two amine oxides formed oily products at first, on long standing in the refrigerator they gave crystals of the amine oxide picrates. The purified amine oxide picrates were analyzed by direct titration with perchloric acid. 

Aliphatic nitriles in aqueous solution were converted to the corresponding amines by electrolytic reduction with a Pd/Ni cathode. The amines were treated with reagent 11, separated on Lichosorb RP-8 with aqueous methanol, and determined using a fluorescence detector, with detection limits of 20 mg/L of aliphatic nitriles. The following procedure was used for screening rape products in search for nitriles rape seed meal was incubated with myrosinase in the presence of Fe(II) sulphate, lyophilized, extracted with chloroform, the extract hydrolysed with concentrated alkali and the liberated ammonia was titrated. ... 

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