Sulphonic layer

Ion-exchange membranes for chlor-alkali electrolysis generally contain a sulphonic layer which faces the anode and a carboxylic layer which faces the cathode, joined by lamination. The Na+ transport number is higher in the carboxylic layer than in the sulphonic layer, and a region of low Na+ concentration therefore tends to form at the interface between the two layers during electrolysis, as shown in Fig. 17.5. 

The thinness of the anion-exchange layer and the Donan exclusion (i.e. the inability of ions present in low concentrations to enter ion-exchange materials containing a higher density of ion-exchange sites of the same charge type as the ions of interest) effected by the intermediate sulphonated layer provide excellent mass transfer properties by ensuring that sample penetration is confined to the outer lates layer. 

Dissolve 1 0 g. of the compound in 5 ml. of dry chloroform in a dry test-tuhe, cool to 0°, and add dropwise 5g. (2-8 ml.) of redistilled chloro-sulphonic acid. When the evolution of hydrogen chloride subsides, allow the reaction mixture to stand at room temperature for 20 minutes. Pour the contents of the test-tube cautiously on to 25 g. of crushed ice contained in a small beaker. Separate the chloroform layer and wash it with a httle cold water. Add the chloroform layer, with stirring, to 10 ml. of concentrated ammonia solution. After 10 minutes, evaporate the chloroform on a water bath, cool the residue and treat it with 5 ml. of 10 per cent, sodium hydroxide solution the sulphonamide dissolves as the sodium derivative, RO.CgH4.SO,NHNa. Filter the solution to remove any insoluble matter (sulphone, etc.), acidify the filtrate with dilute hydrochloric acid, and cool in ice water. Collect the sulphonamide and recrystallise it from dilute alcohol. 

Method 1. Equip a 1 litre three-necked flask (or bolt-head flask) with a separatory funnel, a mechanical stirrer (Fig. II, 7, 10), a thermometer (with bulb within 2 cm. of the bottom) and an exit tube leading to a gas absorption device (Fig. II, 8, 1, c). Place 700 g. (400 ml.) of chloro-sulphonic acid in the flask and add slowly, with stirring, 156 g. (176 ml.) of pure benzene (1) maintain the temperature between 20° and 25° by immersing the flask in cold water, if necessary. After the addition is complete (about 2 5 hours), stir the mixture for 1 hour, and then pour it on to 1500 g. of crushed ice. Add 200 ml. of carbon tetrachloride, stir, and separate the oil as soon as possible (otherwise appreciable hydrolysis occurs) extract the aqueous layer with 100 ml. of carbon tetrachloride. Wash the combined extracts with dilute sodium carbonate solution, distil off most of the solvent under atmospheric pressure (2), and distil the residue under reduced pressure. Collect the benzenesulphonyl chloride at 118-120°/15 mm. it solidifies to a colourless sohd, m.p. 13-14°, when cooled in ice. The yield is 270 g. A small amount (10-20 g.) of diphen3 lsulphone, b.p. 225°/10 mm., m.p. 128°, remains in the flask. 

Using sulphonic acid ion-exchange resins in ether solvent, selective removal of the trimethylsilyl group from oxygen in bistrimethylsilylated terminal alkynols can be achieved. This method is particularly suitable for low-molecular-weight compounds, where water solubility would make efficient extraction from an aqueous layer difficult. 

Of interest here is the question relating to the value for the slope coefficient, k, from equation (1), when surfactant structures incorporating both ionic (say sulphonate) and nonionic moieties are included together. The Ghanges in electric double layer effects imparted from salt addition might dominate the packing constraints and therefore the phase inversion process, or perhaps oxyethylene dehydration effects from the presence of toluene could also play a role. 

Asahi s investigations showed that a Na+ concentration of 1.1 N was necessary in compartment II to maintain a current efficiency of 96% in the carboxylic membrane during operation with 3.5 N brine in compartment I and 32% caustic soda in compartment III. The Na+ concentration in compartment II was generally far lower than that in compartment I, and clearly indicates the tendency for depression of the Na+ concentration at the interface of the sulphonic and carboxylic layers in the normal... 

Fig. 17.7 Influence of Na+ concentration at interface of sulphonic acid and carboxylic layers on current efficiency 3-4 N NaCI 1.0-5.5 kA rrT2 33% NaOH.

In summary, the results indicate that the Na+ concentration at the interface between the sulphonic and carboxylic layers in the normal membrane is substantially depressed from that of the anolyte compartment, and that the concentration at this interface strongly affects the current density. 

The limiting current density may be defined as the current density at which the depression of the Na+ concentration at the interface of the membrane s sulphonic and carboxylic layers results in an abrupt rise in cell voltage and drop in current efficiency. 

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