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preface to fifteenth edition

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PRACTICAL LABORATORY INFORMATION 11.75<br />

TABLE 11.27<br />

Primary Standards for Aqueous Acid-Base Titrations (Continued)<br />

Standard<br />

Formula<br />

weight<br />

Preparation<br />

Basic substances for standardizingacidic solutions (continued)<br />

Na 2 CO 3 105.989 Heat reagent-grade material for 1 hr at 255–265C. Cool in an efficient<br />

desicca<strong>to</strong>r. Titrate sample with acid <strong>to</strong> pH 4–5 (first green<br />

tint of bromocresol green), boil the solution <strong>to</strong> eliminate the carbon<br />

dioxide, cool, and again titrate <strong>to</strong> pH 4–5. Equivalent weight<br />

is one-half the formula weight.<br />

NaCl 58.45 Accurately weigh about 6 g NaCl and dissolve in distilled water.<br />

Pass the solution through a well-rinsed cation exchange column<br />

(Dowex 50W) in the hydrogen form. The equivalent amount of<br />

HCl is washed from the column (in 10 column volumes) in<strong>to</strong> a<br />

volumetric flask and made up <strong>to</strong> volume. Equivalent weight is the<br />

formula weight.<br />

Acidic substances for standardizingbasic solutions<br />

C 6 H 5 COOH 122.125 Pure benzoic acid is available from NIST (National Institute for Science<br />

and Technology). Dissolve 0.5 g in 20 mL of neutral ethanol<br />

(run a blank), excludingCO 2 , add 20–50 mL, and titrate usingphenolphthalein<br />

as indica<strong>to</strong>r.<br />

o-C 6 H 4 (COOK)(COOH) 204.22 Potassium hydrogen o-phthalate is available commercially as primary<br />

standard, also from NIST. Dry at 135C. Dissolve in water,<br />

excludingCO 2 , and titrate with phenolphthalein as indica<strong>to</strong>r.<br />

For Ba(OH) 2 solution, perform the titration at an elevated temperature<br />

<strong>to</strong> prevent precipitation of Ba phthalate.<br />

KH(IO 3 ) 2 389.915 Potassium hydrogen bis(iodate) is available commercially in a primary<br />

standard grade. Dry at 110C. Dissolve a weighed amount<br />

of the salt in water, excludingCO 2 , and titrate <strong>to</strong> pH 5–8. [I. M.<br />

Kolthoff and L. H. van Berk, J. Am. Chem. Soc., 48:2800<br />

(1926)].<br />

NH 2 SO 3 H 97.09 Hydrogen amidosulfate (sulfamic acid) acts as a strong acid. Primary<br />

standard grade is available commercially. Since it does undergo<br />

slow hydrolysis, an acid end point (pH 4 <strong>to</strong> 6.5) should be<br />

chosen unless fresh reagent is available, then the end point can be<br />

in the range pH 4 <strong>to</strong> 9. [W. F. Wagner, J. A. Wuellner, and C. E.<br />

Feiler, Anal. Chem., 24:1491 (1952). M. J. Butler, G. F. Smith,<br />

and L. F. Audrieth, Ind. Eng. Chem., Anal. Ed., 10:690 (1938)].

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