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Description

Monograph

Non-aqueous titration of acids and bases with potentiometric endpoint indication Peter Bruttel

Non-aqueous titration of acids and bases with potentiometric endpoint indication Peter Bruttel

All rights reserved, including translation rights. Printed in Switzerland by Metrohm Ltd., CH-9101 Herisau 08.1999 – 8.024.5003

Contents

Contents 1.

Preface ................................................................................................... 3

2.

Important terms ...................................................................................... 4

3.

Remarks concerning solvents for non-aqueous titrations ....................... 5

4.

Apparatus and accessories used ........................................................... 9

5.

Titration of acids ..................................................................................... 9

6.

Titration of bases .................................................................................. 12

7.

pKA values of some selected acids ....................................................... 14

8.

pKB values of some selected bases ...................................................... 15

9.

Dielectric constants (DC) of some selected solvents ............................ 16

10.

Literature references ............................................................................ 17

Compilation of results: bases as titrants ......................................................... 18 Standard deviations of the electrode signals ........................................ 24 Titration curves obtained with the Solvotrode LiCl sat. in ethanol ........................................................................... 25 0.4 mol/L TEA-Br in ethylene glycol ................................................ 31 Compilation of results: acids as titrants .......................................................... 37 Standard deviations of the electrode signals ........................................ 44 Titration curves obtained with the Solvotrode LiCl sat. in ethanol ........................................................................... 45 0.4 mol/L TEA-Br in ethylene glycol ................................................ 49

2

Peter Bruttel

Preface

1 Preface Non-aqueous titrations continue to play an important role in analysis, no matter whether in the determination of acid or base numbers in oils and fats, for quantifying products with different acidic or basic strengths separately or for titrating substances that are insoluble in water. This monograph is not intended to be a textbook; it should rather help the practical chemist to decide which solvents, electrodes, titrants and instruments are the most suitable ones for the particular application. The monograph makes no claim to be complete; it also assumes a certain basic knowledge of potentiometric analysis. Non-aqueous titrations are not without their problems. Effects are experienced that do not occur in this form in purely aqueous systems. The main problems are: a) interferences caused by static electricity and b) the response behaviour of the electrodes used. As far as a) is concerned a wide range of different measures have already been introduced or at least tried. These include everything from titration in earthed solutions through shielding the complete titration system (Faraday cage) and up to the use of differential amplifiers and the three-electrode technique. This latter technique has its advantages in many cases, above all wherever work is carried out in nonpolar solvents with nonpolar samples (e.g. petrochemical products). The type of electrode(s) used also plays a major role, which brings us to point b). Many years of experience involving non-aqueous titration have resulted in Metrohm developing a special electrode that is particularly suitable for non-aqueous titration  the 6.0229.100 Solvotrode. It meets all the demands that must be placed on such an electrode: large membrane surface, as small a membrane resistance as possible, rapid response, optimally designed ground-joint diaphragm, excellent shielding and appropriate electrolyte solutions.

Non-aqueous titration of acids and bases with potentiometric endpoint indication

3

Important terms

2 Important terms Amphiprotic Appreciable self-dissociation (e.g. 2 CH3OH < as polar.

>

CH3OH2 + + CH3O ), also known

Aprotic No self-dissociation, also known as nonpolar. Dielectric constant (DC) Proportionality factor between the electric displacement D and the electric field strength E in a vacuum. The DC depends strongly on temperature. Solvents with a higher DC promote the electrolytic dissociation of electrolytes. The DC is a substance-specific constant that depends to a large extent on the molecular structure. It is measured with so-called decameters. In nonpolar solvents (having a small DC) even strong electrolytes (e.g. NaCl, HClO4, etc.) are only weakly dissociated. Dissociation Formation of ions, e.g. CH3COOK

<

> CH3COO + K+

Levelling Equalising of base or acid strengths. In this case separation is not possible. Example: sulphuric acid: During the titration, e.g. in acetone, two well-differentiated steps are obtained. In an aqueous titration levelling occurs; only one potential jump is obtained, which corresponds to the sum of the two acids of different strengths. pK value Acidity or basicity constants KA = [H3O+] [A] / [HA] KB = [BH+] [OH] / [B] The negative logarithms of these values are known as pKA and pKB, respectively; they are analogous to the pH value. The smaller the value, the «stronger» the acid or base. In order to achieve a separation in an aqueous titration the difference between the pK values should be approx. 5. In suitable non-aqueous solvents a difference of 2 to 3 is sufficient.

4

Peter Bruttel

Remarks concerning solvents for non-aqueous titrations

3 Remarks concerning solvents for non-aqueous titrations 3.1 Amphiprotic solvents  acidic 3.1.1 Glacial acetic acid (HAc) Solvent for bases in particular. The water content should be less than 1%. In some cases acetic anhydride is added. However, this can lead to acetylation and therefore to incorrect results (primary, secondary and tertiary amines). For the titration of weak bases it is better to add an aprotic solvent, e.g. dioxane or toluene. 3.1.2 Formic acid Is only used for the titration of the weakest bases, e.g. caffeine (pKB = 13.4) or urea (pKB = 13.8). The water content should not exceed 1%. Should not be used with short-chain alcohols  esterification! 3.1.3 Trifluoroacetic acid Suitable as a solvent for the weakest bases, where acetic anhydride cannot be used because of acetylation. An expensive solvent! 3.1.4 Others Propionic acid, cresols and phenol are not very important. Cresols are used in mixtures, e.g. with chloroform, as solvents for determinations of terminal amino groups in polyamides. As phenol has excellent solubilising properties for the salts of organic bases, its use with a mixture of chloroform and acetonitrile has been suggested for these applications. 3.2 Amphiprotic solvents  basic 3.2.1 Ethylenediamine Used as a solvent for weak acids. Has good solubilising properties. Has a levelling effect for strong acids  carboxylic acids and phenols can nevertheless be separated. It fumes when exposed to the air, is toxic and corrosive and also has an unpleasant odour. Takes up water and carbon dioxide from the air (blank values). 3.2.2 Butylamine Similar properties to ethylene diamine, but has slightly smaller solubilising properties. Offensive odour. 3.2.3 Benzylamine If it has to be a solvent from this class (3.2), then this one. Similar to butylamine, but better solubilising properties and less offensive odour. Non-aqueous titration of acids and bases with potentiometric endpoint indication

5

Remarks concerning solvents for non-aqueous titrations

3.3 Amphiprotic solvents  neutral 3.3.1 Methanol and ethanol Compared with water they have lower dielectric constants but similar acidity and basicity. Not suitable for the determination of weak acids such as phenol. However, the carboxyl groups of dicarboxylic acids and the two acid groups of sulphuric acid can be separated. They also have certain solubilising properties for organic compounds that water does not possess. 3.3.2 Isopropanol Has excellent solubilising properties, e.g. for hydrocarbons, and is often used as a solubility promoter between these and water. As a secondary alcohol it is a weaker «acid» than the primary alcohols and can be used instead of them without any problems. The determination of weak acids is also not possible in this solvent. 3.3.3 tert. Butanol Unfortunately has a relatively high melting point of 25.8 °C. Approx. 5% isopropanol (IPA) should therefore be added. Tertiary alcohols can almost be classified as being aprotic solvents. Has no levelling properties and is therefore particularly suitable for the step-by-step titration of mixtures of acids  up to the phenols. 3.3.4 Diols (ethylene glycol/propylene glycol) Particularly suitable as solvents for the salts of weak acids because of their high polarity. However, mostly used in 1:1 mixtures with chloroform, toluene or IPA. 3.3.5 Ethylene glycol monomethyl ether (methylglycol, methylcellosolve) Can be used for the titration of weak bases and has good solubilising properties. Is used together with acetic anhydride for the determination of small amounts of quaternary amines in other amines (the other amines are acetylated and are therefore not determined).

3.4 Aprotic solvents  acidic 3.4.1 Nitromethane and nitroethane As a result of their high polarity, which favours the dissociation of salts, both would be very useful solvents for the titration of weak bases. Unfortunately both present explosion and fire hazards and are therefore seldom used. A special application would be the titration of mixtures of primary, secondary and tertiary aliphatic or aromatic amines in a solvent mixture made up of glacial acetic acid/ dioxane/nitromethane at a ratio of 5:75:20.

6

Peter Bruttel

Remarks concerning solvents for non-aqueous titrations

3.5 Aprotic solvents  basic 3.5.1 Pyridine Were it not for its unpleasant odour and its harmful side-effects this solvent would be particularly suitable for the separation of mixtures of acids. Levelling occurs only with strong mineral acids (HCl, HClO4, 1st proton of H 2SO4). Otherwise acids up to phenol can be separated. A particular use of pyridine is for the determination of polyacids. A special advantage is that pyridine does not undergo any side reactions with strong acids, which allows exact determinations to be carried out. 3.5.2 Dimethylformamide (DMF) DMF is an excellent solvent for polar compounds, is not toxic and also has no odour. Can be used for the determination of weak acids, but only when water is almost completely absent (saponification). Unfortunately has a relatively high reactivity, which prevents the determination of mixtures that contain strong acids (errors of up to 20%). Mixtures containing alcohols are also unfavourable as the titration curves become extremely flat (also with alcoholic titrants). 3.5.3 Dimethylsulphoxide (DMSO) Similar properties to DMF, without its tendency to saponify. Warning: DMSO reacts violently with HClO4!!!! 3.6 Aprotic solvents  neutral 3.6.1 Acetone and MIBK The ideal solvents for mixtures of acids, from HClO4 up to phenol. Practically no levelling occurs. Absorption of CO2 from the air takes place so slowly that, if need be, the titration can be carried out in an open beaker. 3.6.2 Acetonitrile Is used for the determination of basic amines having different basicities. Acids can also be separated. Acetates of Cu, Ni, etc. can be titrated in this solvent; this may be due to the formation of complexes. 3.6.3 Nitrobenzene Would give an excellent differentiation of amines having different basicities, but should not be used because of its toxicity.

Non-aqueous titration of acids and bases with potentiometric endpoint indication

7

Remarks concerning solvents for non-aqueous titrations

3.6.4 Ethers (diethyl ether, dioxane, tetrahydrofuran, ethylene glycol dimethyl ether) Mostly only used for dilution purposes. HClO4 in dioxane is used as a titrant. Ethers often contain peroxides, which can react like acids during the titration. In pure dioxane and diethyl ether no potentiometric indication is possible  the resistance is too high. Even a differential amplifier does not help. If these solvents have to be used, then for weak bases. 3.6.5 Hydrocarbons and chlorinated hydrocarbons Examples: hexane, benzene, toluene, chlorobenzene, trichloroethylene, chloroform, tetrachloroethane, etc. Mostly only used for diluting or dissolving. Solvents such as benzene and chlorobenzene are very questionable. Chlorinated hydrocarbons are disappearing more and more from the specifications. In pure solvents of this class the endpoint cannot be indicated potentiometrically or conductometrically. Only colour indicators can be used. 3.6.6 Acetic anhydride Is used for the removal of residual water in the titration of extremely weak bases (e.g. thiourea, acetylpiperidine). The acetyl ion is an even stronger acid than the acetonium ion (CH3COOH2 +). CH3COOCOCH3 + HClO4 → CH3CO+ + ClO4  + CH3COOH The acetyl cation is extremely reactive. Primary, secondary and tertiary amines are acetylated and therefore removed from the titration. It is possible to determine quaternary amines in the presence of all the other amines. The presence of more than 10% acetic acid reduces the titrability of weak bases by buffering. Gel layers on glass electrodes are rapidly dehydrated; this results in a sluggish response.

8

Peter Bruttel

Apparatus and accessories used  Titration of acids

4 Apparatus and accessories used - 702, 716, 736, 751 or 785 Titrino or 726 Titroprocessor, 728 Magnetic stirrer, Metrodata TiNet software, printer and PC - 6.3014.223 Exchange Units - 6.0229.100 Solvotrode*; electrolyte a) 6.2312.000 LiCl sat. in ethanol, electrolyte b) 6.2320.000 TEA-Br 0.4 mol/L in ethylene glycol

5 Titration of acids The titrations were carried out in the «pH range»** of the titrator in the MET mode (volume increments 0.10 mL, fixed delay period 5 s). Electrodes 1) 6.0229.100 Solvotrode, electrolyte LiCl sat. in ethanol 2) 6.0229.100 Solvotrode, electrolyte TEA-Br c = 0.4 mol/L in ethylene glycol (TEA-Br = tetraethyl ammonium bromide) Acids tested (0.1 mol/L in ethanol) - Benzoic acid; pKA = 4.20 - Phenol; pKA = 9.95 Titrants a) c(TBAOH) = 0.1 mol/L in IPA (TBAOH = tetrabutyl ammonium hydroxide, IPA = isopropanol) b) c(KOH) = 0.1 mol/L in IPA Solvents Ethanol Isopropanol (IPA) tert. Butanol/IPA 95:5 Acetone

Methyl isobutyl ketone (MIBK) Dimethyl formamide (DMF) Pyridine Acetonitrile

2.50 mL of the tested acid was treated with 50 mL of the corresponding solvent and titrated with TBAOH or KOH.

*

The Solvotrode is a combined pH glass electrode developed specially for non-aqueous titrations.

**

The concept of pH really applies only to purely aqueous solutions. In organic solvents the values are displaced. The neutral point is no longer at «pH = 7» and negative values or values up to 20 can occur (extension of the pH scale). This means that at «high» pH values high alkali errors must be expected with glass electrodes.

Non-aqueous titration of acids and bases with potentiometric endpoint indication

9

Titration of acids

Results A) Benzoic acid (pKA = 4.20) KOH normally produces larger potential jumps than TBAOH, but TBAOH yields more symmetrical and steeper titration curves. In 11 (63%) out of 16 determinations larger jumps were obtained with Electrode 2 (underlined values). Summary of the delta pH values (mean value of 3 determinations, delta «pH init» to «pH end»): Solvent

KOH/El.1

TBAOH/El.1

KOH/El.2

Ethanol

7.6

7.3

8.1

TBAOH/El.2 7.5

IPA

8.5

7.1

8.7

8.5

tert. Butanol/IPA

8.9

9.5

8.2

8.9

Acetone

9.0

6.4

8.1

7.6

MIBK

7.7

7.6

7.1

7.0

DMF

7.6

5.8

8.7

6.5

Pyridine

6.1

6.1

8.0

7.0

Acetonitrile

7.3

6.7

10.2

7.7

The highest pH values at the end of the titration were: with KOH; 18.2 in DMF/El.2, 16.8 in DMF/El.1 and in pyridine/El.2 with TBAOH; 16.0 in pyridine/El.2, 15.9 in tert. butanol/El.2 The lowest pH values at the end of the titration were: with KOH; 11.6 in ethanol/El.1, 12.4 in MIBK/El.1 with TBAOH; 11.1 in ethanol/El.1, 11.7 in MIBK/El.1 Li ions therefore appear to have a certain influence on the pH value (alkali error). The largest jump was obtained with KOH/El.2 in acetonitrile (delta pH = 10.2). However, acetonitrile is a problematic solvent. During the titration sticky precipitates formed with both titrants, which lead to blockage of the diaphragm. This means that with this solvent the electrode should be cleaned after each titration (H2O and ethanol). Very good solvents for medium-strength acids are tert. butanol/IPA, acetone, DMF and isopropanol (IPA). B) Phenol (pKA = 9.95) With TBAOH as titrant the endpoint is normally reached sooner. Electrode 2 usually gives titration curves that are easier to evaluate.

10

Peter Bruttel

Titration of acids

Summary of the delta pH values (mean value of 3 determinations, delta «pH init» to «pH end»): Solvent

KOH/El.1

TBAOH/El.1

Ethanol

3.7 

3.3 

3.8 

3.3 

4.4 -

3.8 +

5.0 +

4.9 +

IPA tert. Butanol/IPA

KOH/El.2

TBAOH/El.2

4.6 -

4.3 +

4.6 +

6.8 ++

4.4 ++

3.3 ++

3.9 +

3.9 ++

MIBK

6.0 -

6.0 

5.7 -

5.6 

DMF

4.5 ++

3.7 +

5.4 ++

4.3 ++

Pyridine

3.2 +

4.4 ++

5.3 +

4.8 +

Acetonitrile

3.4 +

4.0 ++

5.4 ++

4.1 ++

Acetone

 + ++

no jump suggestion of a jump EP found pronounced jump, easy to evaluate

In 12 (75%) out of 16 determinations larger jumps were obtained with Electrode 2. The highest pH values at the end of the titration were: with KOH; 17.4 in DMF/El.2, 16.4 in pyridine/El.2 with TBAOH; 16.0 in DMF/El.2 and in pyridine/El.2 The lowest pH values at the end of the titration were: with KOH; 11.6 in ethanol/El.1, 12.8 in MIBK/El.1 with TBAOH; 11.1 in ethanol/El.1, 11.5 in MIBK/El.1 The largest jump was obtained with TBAOH/El.2 in tert. butanol/IPA (delta pH = 6.8). Acetonitrile produced nice curves. Unfortunately sticky precipitates on the diaphragm were also produced here, which made it necessary to clean the electrode (H2O and ethanol) after each determination. Very good solvents for weak acids are acetone, DMF and tert. butanol/IPA (in ethanol ...