Let the Titrations Begin

Pour out liquid Blow out liquid Copper stopcock 

Procedures in which we measure the volume of reagent needed to react with analyte are called volumetric analysis. In this chapter, we discuss principles that apply to all volumetric procedures and then focus on precipitation titrations. We also introduce spectrophotometric titrations, which are especially useful in biochemistry. 

The principal requirements for a titration reaction are that it have a large equilibrium constant and proceed rapidly. That is, each increment of titrant should be completely and 

The equivalence point occurs when the quantity of added titrant is the exact amount necessary for stoichiometric reaction with the analyte. For example, 5 mol of oxalic acid react with 2 mol of permanganate in hot acidic solution  

If the unknown contains 5.000 mmol of oxalic acid, the equivalence point is reached when 2.000 mmol of Mn04 have been added. 

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This method provides a reasonable estimate of the piQ, provided that the weak acid is neither too strong nor too weak. These limitations are easily appreciated by considering two limiting cases. For the first case let s assume that the acid is strong enough that it is more than 50% dissociated before the titration begins. As a result the concentration of HA before the equivalence point is always less than the concentration of A , and there is no point along the titration curve where [HA] = [A ]. At the other extreme, if the acid is too weak, the equilibrium constant for the titration reaction... 

Let us introduce into the titrant one Pt indicator electrode vs. an SCE and maintain in the electric circuit a low constant current + /, as indicated by the broken horizontal line in Fig. 3.71. For this line we shall consider the successive points of its intersection with the voltammetric curves during titration and observe the following phenomena as expressed in the corresponding electrode potentials. Immediately from the beginning of the titration E remains high (nearly 1.44 V), but falls sharply just before the equivalence point (E = 1.107 V), and soon approaches a low E value (below 0.77 V) (see Fig. 3.72, cathodic curve +1). 

Let us describe the mechanism of two-phase titration in some detail, using the titration of sodium dodecylsulfate (SDS) with Hyamine 1622 in the presence of mixed dye indicator as an example. At the beginning of the titration, when only SDS is present in aqueous solution, dimidium bromide reacts with it to form a salt that is insoluble in water, but... 

Let s take, for example, the titration of a weak acid with a strong base. Since we have seen that the titration reaction is the most quantitative at the very beginning of the titration, we can consider without any error of reasoning that it is, actually, complete in this range of the titration ... 

Let us examine some batch results. In trials in which 5 mL of a dye solution was added by pipet (with pressure) to 10 mL of water in a 25-mL flask, which was shaken to mix (as determined visually), and the mixed solution was delivered into a 3-mL rectangular cuvette, it was found that = 3-5 s, 2-4 s, and /obs 3-5 s. This is characteristic of conventional batch operation. Simple modifications can reduce this dead time. Reaction vessels designed for photometric titrations - may be useful kinetic tools. For reactions that are followed spectrophotometrically this technique is valuable Make a flat button on the end of a 4-in. length of glass rod. Deliver 3 mL of reaction medium into the rectangular cuvette in the spectrophotometer cell compartment. Transfer 10-100 p.L of a reactant stock solution to the button on the rod. Lower this into the cuvette, mix the solution with a few rapid vertical movements of the rod, and begin recording the dead time will be 3-8 s. A commercial version of the stirrer is available. 

Let s begin with this titration question How many moles of calcium hydroxide are necessary to titrate 0.250 mol of acetic acid We have information about the acetic acid, and we are seeking information about the calcium hydroxide. We will begin the problem with the acetic acid since we know more about it, and we will end the problem with the calcium hydroxide since we do not know anything about this compound. One way to remember what you need to do is to copy the given information and the question to the balanced chemical equation, as shown here ... 

Now let us try an example needing additional information after the mole ratio step. I low many grams of calcium hydroxide are necessary to titrate 0.200 mol of acetic acid As usual, we begin by adding this information to the balanced chemical equation ... 

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