preface to fifteenth edition

PRACTICAL LABORATORY INFORMATION 11.83
alkaline arsenite solution as follows: measure, accurately, 30 to 40 mL of arsenite solution into
an Erlenmeyer flask and dilute to 150 mL. Add slowly, to prevent excessive frothing, 20 mL of
4N sulfuric acid, 2 drops of 0.01M osmium tetraoxide solution, and 4 drops of 1,10-phenanthroline
iron(II) complex indicator. Titrate with the ceric sulfate solution to a faint blue endpoint.
Compute the normality of the ceric solution from the normality of the arsenite solution.
Iron(II) ammonium sulfate hexahydrate, 0.1N Fe(II) to Fe(III). Dissolve 39.2139 g of
FeSO 4· 2(NH 4) 2SO 4 · 6H2O in 500 mL of 1N sulfuric acid and dilute to 1L. If desired, check
against standard dichromate or permanganate solution.
Iodine, 0.1N (0 to 1). Dissolve 12.690 g of resublimed iodine in 25 mL of a solution containing
15gof KI which is free from iodate. After all the solid has dissolved, dilute to 1 L. If desired,
check against a standard arsenite or standard thiosulfate solution.
Potassium bromate, 0.1N (5 to 1). Weigh out 2.7833 g of KBrO 3 , dissolve in water, and
dilute to 1L.
Potassium dichromate, 0.1N Cr(VI) to Cr(III). Weigh out 4.9030 g of K 2 Cr 2 O 7 that has been
dried at 120C, dissolve in water, and dilute to 1L.
Potassium iodate, 0.1N (5 to 1). Weigh out exactly 3.5667 g of KIO 3 (free from iodide),
dried at 120C, and dissolve in water containing about 15gof KI, and dilute to 1 L.
Potassium permanganate, 0.1N (7 to 2). Dissolve about 3.3 g in a liter of distilled water.
Allow this to stand for 2 or 3 days, then siphon it carefully through clean glass tubes or filter it
through a Gooch crucible into the glass container in which it is to be kept, discarding the first
25 mL and allowing the last inch of liquid to remain in the bottle. In this way any dust or reducing
substance in the water is oxidized, and the MnO 2 formed is removed. Permanganate solutions
should never be allowed to come into contact with rubber, filter paper, or any other organic
matter, and should be stored away from light. To standardize the KMnO 4 , weigh accurately
samples of about 0.3 g of primary standard grade Na 2 C 2 O 4 into Erlenmeyer flasks, add 150 mL
of distilled water and 4mLof concentrated H 2 SO 4 , and heat to 70C and maintain at this temperature
throughout the titration with the permanganate solution. The end point is a faint, permanent
pink color throughout the solution. Equivalent weight of Na 2 C 2 O 4 /2 is 67.000 g.
Sodium thiosulfate, 0.1N. Weigh 24.818 g of fresh crystals of Na2S2O 3· 5H2O,
dissolve in distilled
water. Add 0.5 g of Na 2 CO 3 and 0.5 mL of chloroform as preservative. Dilute to 1L.
Equations for the principal methods for the redox determinations of the elements are given in
Table 11.29. Volumetric factors in redox titrations for the common titrants are given in Table 11.28.
11.6.4 Indicators for Redox Titrations
A selected list of redox indicators will be found in Table 8.26. A redox indicator should be selected
so that its E 0 is approximately equal to the electrode potential at the equivalent point, or so that the
color change will occur at an appropriate part of the titration curve. If n is the number of electrons
involved in the transition from the reduced to the oxidized form of the indicator, the range in which
0
the color change occurs is approximately given by E 0.06/n volt (V) for a two-color indicator
whose forms are equally intensely colored. Since hydrogen ions are involved in the redox equilibria
of many indicators, it must be recognized that the color change interval of such an indicator will
vary with pH.
In Table 8.26, E 0 represents the redox potential at which the color change of the indicator would
normally be perceived in a solution containing approximately 1M H. For a one-color indicator this
is the potential at which the concentration of the colored form is just large enough to impart a visible
color to the solution and depends on the total concentration of indicator added to the solution. If it
is the reduced form of the indicator that is colorless, the potential at which the first visible color