Solvents and Titrating Agents for Quantitative Analyses of Acids and Bases
Quantitative determination of substances via titration is a useful method in laboratories. Most titration applications are performed in aqueous solutions; however, for certain substances this is not possible. When substances are insoluble or not soluble enough in water or too weakly protolysed, water as a solvent prevents titration of weak bases or separate determination of strong acids (e.g. perchloric acid/hydrochloric acid).
Advantages of non-aqueous titration:
- Enlargement of solubility range: many substances that are not soluble in water can be easily titrated in water-free media (e.g. fats and oils)
- Enlargement of application range: weak bases and acids can be easily titrated
- Substance compositions that cannot be separately determined in aqueous media can often be titrated in non-aqueous media
A high accuracy in non-aqueous titrations can be reached by special monitoring of the titration conditions; equivalent points must be clearly identifiable, and standard solutions precisely prepared. Ambient temperature is of special importance for solutions in nonaqueous titrations; organic solvents have a thermal coefficient of extension approximately 10 times higher than water, and a temperature difference of 1 °C may cause an error of 0.3 %.
Titration of bases
In aqueous solutions, only strong acids with a pKB-value up to about 7 can be titrated, e.g. aliphatic amines. Weaker bases like aromatic amines or heterocyclic compounds with pKB-values up to 12 can be titrated in organic solvents. Most common is the titration in glacial acetic acid. As a titrating agent, perchloric acid in glacial acetic acid is commonly used. Characteristic titration curves of different amines are shown in Figure 1. Acetic acid is a levelling solvent; therefore nearly all bases show similar types of titration curves.
Figure 1 Titration of different bases in acetic acid with 0.1 mol/L perchloric acid in acetic acid: 1/ pyrazole (pKB = 11.51) 2/ pyridine (8.81) 3/ imidazole (7.00) 4/ diethanolamine (5.12) 5/ iso-propylamine (3.37) 6/ tetramethylammonium hydroxide
Solvents for base titration
Choosing the suitable solvent is crucial for the titration to be performed. The acid/base-character and also the purity of the solvent must be suitable, as well as the solubility properties for all reaction products. The dielectric constant should be as high as possible to enable a clear indication of the potential. Sometimes acetylations can occur, e.g. with aromatic amines. Some of the most suitable solvents include the following:
Acetic Acid is the most commonly used solvent, with good solubility properties for most bases; it gives distinct potential curves with clear endpoints and also enables titration of weak bases with pKB values up to 12. Disadvantageous is the levelling effect on weak bases, therefore simultaneous determinations are not possible.
Acetic anhydride shows similar properties to acetic acid, but is superior for determination of weak bases, because it is free of water. Primary and secondary amines cannot be titrated; they will be rapidly acetylated.
Methanol shows good solubility properties and does not cause indication problems. Its titration behaviour is very similar to water, and only substances with pKB up to 9 can be titrated.
Isopropanol is a good solvent because of its high dielectric constant and low acidity. Simultaneous determinations are possible. Not recommended for very weak bases (pKB > 10).
Acetone is often used because it enables a very good differentiation of bases and shows sufficient solubility properties.
Titrating agents for base titration
For acidimetric titration, perchloric acid is mostly used as a titrating agent. When using glacial acetic acid as a solvent, 0.1 mol/L perchloric acid in acetic acid is used. When using differentiating solvents like acetone or isopropanol, acetic acid interferes because of the levelling effect. Instead, 0.1 mol/L perchloric acid in dioxan is recommended; it gives excellent titration results but is not stable for a long time. Once its colour turns to brown, titration curves get shifted. 0.1 mol/L perchloric acid in isopropanol can also be recommended as it gives the same results as perchloric acid in dioxan and is unlimitedly storable.
0.1 mol/L perchloric acid in glacial acetic acid
The FIXANAL ampoule of perchloric acid (Fluka 32046, see product list in Table 1) has unlimited storage and can be used for preparation of a standard solution in the necessary solvent. It can be filled up to 1L to form a solution of 0.1 mol/L or filled up to any other volume to give the desired concentration. Temperature must be taken into account when preparing the solution.
0.1 mol/L perchloric acid in dioxan
Before opening the FIXANAL ampoule, fill approx. 500 mL dioxan in a volumetric flask and cool. Then open the ampoule and swirl around the dioxan carefully, so the acid is diluted at once. Rinse the ampoule with dioxan and fill the volumetric flask up to the mark. This solution is limited in storage. It slowly takes on a brownish colour. Addition of 1–2 % water increases stability, but may interfere when titrating weak bases.
0.1 mol/L perchloric acid in isopropanol
In a volumetric flask, fill approx. 500 mL isopropanol. Open the ampoule and immediately swirl around the isopropanol to dilute the acid at once. Rinse the ampoule with isopropanol and fill the volumetric flask up to the mark.
Titration of acids
Acids with pKA values up to 7 can be titrated in aqueous solutions, if they are sufficiently soluble. Weaker acids with pKA values up to 12 can be titrated in organic solvents.
Solvents for acid titration
1,2-Ethylenediamine enables determination of all acids up to pKA 11. Stronger acids are levelled; below pKA 6 they cannot be differentiated any more. Equivalent points are very distinct. Disadvantages are the unpleasant odour and the possible absorption of carbon dioxide from ambient air. Some of the most suitable solvents include the following:
Dimethylformamide (DMF) is an excellent solvent for salts and for titration of acids. It shows strong differentiation; only strong acids with pKA 0 are levelled. Figure 2 shows that the equivalent points are indicated very clearly. Note: after standing for some time, DMF can be hydrolysed by strong acids, causing interferences in the titration curves.
Figure 2 Titration of different acids in DMF with 0.1 mol/L tetramethylammonium hydroxide in isopropanol: 1/ phenol (pKA = 9.95) 2/ 2-nitrophenol (7.23) 3/ acetic acid (4.73) 4/ 2,4-dinitrophenol (3.96) 5/ picric acid (0.71) 6/ hydrochloric acid (approx. -3) 7/ perchloric acid (approx. -9)
Isopropanol is a good solvent for acids; it differentiates down to pKA 0. For very weak acids (pKA 11), this solvent is not recommended.
Acetone is strongly differentiating; even strong acids like hydrochloric or perchloric acid can be separated. It is an ideal solvent for simultaneous determinations and separations.
Titrating agents for acid titration
As titrating agents, strong bases are required, soluble in organic solvents. Mainly tetramethylammonium hydroxide or tetrabutylammonium hydroxide are used. Tetramethylammonium hydroxide is adequately soluble in polar organic solvents; but not all solutions are stable for unlimited storage. They decompose under formation of trimethylamine and methanol. Adequately stable are methanolic solutions, which are limited in use due to their levelling effect. Mostly propanol-methanol mixtures are used, which present an acceptable compromise between storage and titration behaviour. Small water content of 1 % increases stability without influencing titration properties. A propanolic solution is preferred as isopropanol has a smaller levelling effect than methanol, which is important especially when titrating very weak acids.
The FIXANAL ampoule Fluka 38335 contains 0.100 mol tetramethylammonium hydroxide dissolved in approx. 20 mL water. Because of the aqueous solution, this FIXANAL ampoule has long storage stability. It can be filled up with the desired solvent to 1L 0.1 mol/L standard solution (or to any other volume resp. molarity).
||Perchloric acid standard solution, ready-to-use, 0.1 mol/L in acetic acid
||Perchloric acid standard solution, ready-to-use, 0.1 N in acetic acid
||500 mL, 2 L
||Perchloric acid standard solution, FIXANAL® concentrate, pkg of 0.1 mol (10.046 g HClO4)
||Trifluoromethanesulfonic acid standard solution, ready-to-use, 0.1 mol/L in acetic acid
||Acetic acid, puriss. p.a., for use as solvent (for perchloric acid titration), 99.5 %
||1 L, 2.5 L, 5 L
||Pyridine, puriss. p.a., for titration in non-aqueous medium, 99.5 %
||1 L, 2.5 L
||Tetrabutylammonium hydroxide standard solution, ready-to-use, 0.1 mol/L in toluene/methanol
||Tetrabutylammonium hydroxide standard solution, ready-to-use, 0.1 mol/L in isopropanol/methanol
||Tetramethylammonium hydroxide standard solution, ready-to-use, 0.1 mol/L in isopropanol/methanol
||Tetramethylammonium hydroxide solution, FIXANAL® concentrate, pkg of 0.1 mol (9.115 g C4H13NO)
|Table 1 Sigma-Aldrich reagents for non-aqueous titration (see complete product listing on sigma-aldrich.com/titration)
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