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preface to fifteenth edition

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11.84 SECTION 11<br />

appears becomes less positive as the <strong>to</strong>tal concentration of indica<strong>to</strong>r increases. For a two-color<br />

indica<strong>to</strong>r, the potential at which the middle tint appears is independent of the <strong>to</strong>tal indica<strong>to</strong>r concentration,<br />

but may differ from the potentiometrically determined formal potential of the indica<strong>to</strong>r<br />

in either direction, depending on which of the two forms is more intensely colored. If the reduced<br />

form is the more intense color, the middle tint will appear at a potential more positive than the<br />

potentiometrically measured formal potential, which is the potential at which the two forms are<br />

present at equal concentrations.<br />

In addition <strong>to</strong> those indica<strong>to</strong>rs listed in Table 8.26, there are indica<strong>to</strong>rs for bromometric and<br />

iodometric titrations:<br />

Specific reagents for titrations with bromine or bromate<br />

Methyl orange or methyl red<br />

Use acid-base indica<strong>to</strong>r solutions. Oxidation<br />

causes bleaching of indica<strong>to</strong>r <strong>to</strong> colorless<br />

Bordeaux acid red 17 Dissolve 2gdye in 1Lwater. The red solution<br />

is oxidized <strong>to</strong> pale yellowish green or colorless.<br />

Naphthol blue black Dissolve 2gdye in 1Lwater. The blue solution<br />

is oxidized <strong>to</strong> pale red.<br />

Specific reagents for iodometric titrations<br />

Organic solvents such as CCl 4 , CHCl 3 Up <strong>to</strong> 5mLsolvent is usually added per titration.<br />

Near the end point the mixture is shaken vigorously<br />

after each addition of titrant, and the<br />

appearance or disappearance of the I 2 color in<br />

the organic layer is observed.<br />

Starch Suspend 5 g of soluble starch in 50 mL of saturated<br />

NaCl solution, and stir slowly in<strong>to</strong><br />

500 mL of boiling saturated NaCl solution.<br />

Cool and bottle. Free iodine produces a blueblack<br />

color.<br />

TABLE 11.29<br />

Al<br />

As 0<br />

As(III)<br />

As(V)<br />

Equations for the Redox Determinations of the Elements with Equivalent Weights<br />

Al(C 9 H 6 NO) 3 3 HCl AlCl 3 3C 9 H 7 NO (8-hydroxyquinoline)<br />

3C 9 H 7 NO 6Br 2 3C 9 H 5 Br 2 NO 6 HBr<br />

Al/12 2.2485; Al 2 O 3 /24 4.2483<br />

As 5 Ce(IV) 4H 2 O H 3 AsO 4 5 Ce(III) 5H <br />

As/5 14.9843<br />

5 H 3 AsO 3 2 KMnO 4 6 HCl 5H 3 AsO 4 2 MnCl 2 3H 2 O<br />

H 3 AsO 3 2 Ce(SO 4 ) 2 H 2 O H 3 AsO 4 Ce 2 (SO 4 ) 3 H 2 SO 4<br />

As/2 37.4608; As 2 O 3 /4 49.460<br />

3H 3 AsO 3 KBrO 3 ( HCl) 3H 3 AsO 4 KBr<br />

H 3 AsO 3 I 2 2H 2 O H 3 AsO 4 2I 2H <br />

As/2 37.4608; As 2 O 3 /4 49.460<br />

H 3 AsO 4 2 KI (excess) 2 HCl H 3 AsO 3 I 2 2 KCl H 2 O<br />

I 2 2Na 2 S 2 O 3 2NaI Na 2 S 4 O 6<br />

As/2 37.4608; As 2 O 3 /4 49.460

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