End Uses of Sodium Hydroxide

The end uses of caustic soda, shown in Table 1.2, are more varied than the uses of chlorine. The origin of its extensive use lies in its ability to neutralize acids, as it is a strong base, and its ability to react with some metals and oxides, such as aluminum or AI2O3 to form anionic aluminates. 

Caustic soda is used either directly or indirectly in the production of organic and inorganic chemicals, described below. 

TABLE 1.2. End Uses of Sodium Hydroxide (with permission from SRI Consulting) 

Sodium orthosilieato Sodium pteraimti Sodium slinnafo — 

Salfordyes Synthslie rubber Petroleum addittves Elselroplating 

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The term chlor-alkali refers to those products obtained from the commercial electrolysis of aqueous sodium chloride. These are chlorine, sodium hydroxide, and sodium carbonate. The first two are produced simultaneously during the electrolysis while the latter is included because it is also produced in small quantities and shares many of the end uses of sodium hydroxide. Perfluorinated ionomer membranes are permeable to sodium ions but not the chloride ions, and hence they are useful for these electrolytic cells. The arrangement of a typical membrane cell is shown in Figure 10.2. 

P-Hydroxy-a-naphthaldehyde, Equip a 1 litre three-necked flask with a separatory funnel, a mercury-sealed mechanical stirrer, and a long (double surface) reflux condenser. Place 50 g. of p-naphthol and 150 ml. of rectified spirit in the flask, start the stirrer, and rapidly add a solution of 100 g. of sodium hydroxide in 210 ml. of water. Heat the resulting solution to 70-80° on a water bath, and place 62 g. (42 ml.) of pure chloroform in the separatory funnel. Introduce the chloroform dropwise until reaction commences (indicated by the formation of a deep blue colour), remove the water bath, and continue the addition of the chloroform at such a rate that the mixture refluxes gently (about 1 5 hours). The sodium salt of the phenolic aldehyde separates near the end of the addition. Continue the stirring for a further 1 hour. Distil off the excess of chloroform and alcohol on a water bath use the apparatus shown in Fig. II, 41, 1, but retain the stirrer in the central aperture. Treat the residue, with stirring, dropwise with concentrated hydrochloric acid until... 

The hydrogen ions thus set free can be titrated with a standard solution of sodium hydroxide using an acid-base indicator or a potentiometric end point alternatively, an iodate-iodide mixture is added as well as the EDTA solution and the liberated iodine is titrated with a standard thiosulphate solution. 

Assemble the apparatus shown in Fig. 7/7,56, 1, using a 1-litre distilling flask replace the filter flask receiver by a small funnel attached to the end of the condenser by a short length of rubber tubing and dipping about 0 5 cm. below 100 ml. of dilute hydrochloric acid (1 1) contained in a beaker. Place a solution of 60 g. of sodium hydroxide in 150 ml. of water, together wdth a few fragments of porous porcelain, in the flask and the bromoacetamide solution in the separatory funnel. Warm the solution ill the flask until the thermometer in the liquid reads 60-70°. Allow the bromoacetamide solution to run slowly into the flask at such a rate that the temperature does not rise above about 70° heat is evolved... 

Quantitative Determination and Determination of the Sodium Carbonate Content.—Titrate u solution of I gm. of sodium hydroxide in 100 ee. of water with normal hydrochloric arid ill the eold, using phruolpldhiilriu in the iudi cahir. At least 1.5 ee. of normal neitl. -honid be required I.o discharge the color. Now add I drop of me li l orange, and titrate further tin til llir color again eltaugra to red. In this second titration at most O.fi rr. of Hie acid should be necessary (5, per end of i n..( ( ),). ... 

Commercial processors of potatoes remove the skin by using a 10-20% by mass solution of sodium hydroxide. The potatoes are soaked in the solution for a few minutes at 60-70°C, after which the peel can be sprayed off using fresh water. You work in the laboratory at a large food processor and must analyse a batch of sodium hydroxide solution. You pipette 25.00 mL of NaOH(aq), and find it has a mass of 25.75 g. Then you titrate the basic solution against 1.986 mol/L HCI, and find it requires 30.21 mL of acid to reach an end point. 

The first observation of the instability of carbohydrate orthoesters toward alkali came from Haworth, Hirst and Miller in connection with their experiments on the simultaneous deacetylation and methylation of L-rhamnose methyl 1,2-orthoacetate. These authors noticed that methylation by methyl iodide and silver oxide in the presence of solid sodium hydroxide resulted in the formation of crystalline methyl tri-methyl-/3-L-rhamnopyranoside. A similar result was obtained by Bott, Haworth and Hirst on the simultaneous deacetylation and methylation of triacetyl-D-mannose methyl 1,2-orthoacetate by the use of excessive quantities of dimethyl sulfate and alkali. The reaction produced a mixture of a. and /3 forms of methyl tetramethyl-D-mannopyranoside but the yield was only 40%. When the acetylated orthoester was submitted to methylation with silver oxide and methyl iodide in the presence of sodium hydroxide, the product was mainly trimethyl-rhamnose methyl 1,2-orthoacetate. This result indicates that for the alkaline hydrolysis of orthoesters, hydroxyl ions are necessary. Such ions are present in the dimethyl sulfate-alkali process, but are absent in the methyl iodide treatment except when the reaction mixture contains a little water either by accident or from the decomposition of the sugar molecule. Haworth, Hirst and Samuels examined the behavior of dimethyl-L-rhamnose methyl 1,2-orthoacetate in alkaline solution. When the substance was heated under various conditions with 0.1 A alkali at 70 there was no appreciable hydrolysis at the end of ninety minutes, whereas at 80 for... 

The sodium amalgam cathode product of the electrolyzer of the mercury cell is the chief difference between this and the diaphragm cell. The production of sodium hydroxide from this stream uses a separate set of electrochemical reactions conducted in a decomposer or denuder. This unit is usually located below the electrolyzer so as to allow gravity feed of the sodium amalgam from the electrolyzer to the top end of the decomposer (Fig. 8.4). Deionized water is fed into the bottom of the decomposer to provide countercurrent flows of sodium amalgam and water. Mercury, stripped (or denuded) of sodium, is continuously drawn off the bottom of the decomposer, and a hot solution of 50-70% sodium hydroxide in water, plus hydrogen gas, from the top. 

We can titrate an acid solution of unknown concentration hy adding a standardized solution of sodium hydroxide dropwise from a buret (see Figure 11-1). A common huret is graduated in large intervals of 1 mL and in smaller intervals of 0.1 mL so that it is possible to estimate the volume of a solution dispensed to within at least 0.02 mL. (Experienced individuals can often read a buret to 0.01 mL.) The analyst tries to choose an indicator that changes color clearly at the point at which stoichiometrically equivalent amounts of acid and base have reacted, the equivalence point. The point at which the indicator changes color and the titration is stopped is called the end point. Ideally, the end point should coincide with the equivalence point. Phenolphthalein is colorless in acidic solution and reddish violet in basic solution. In a titration in which a base is added to an acid, phenolphthalein is often used as an indicator. The end point is signaled by the first appearance of a faint pink coloration that persists for at least 15 seconds as the solution is swirled. 

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