Acid-base titration endpoint

An analytical solution for molecules with alkaline functionality is acid/base titration. In this technique, the polymer is dissolved, but not precipitated prior to analysis. In this way, the additive, even if polymer-bound, is still in solution and titratable. This principle has also been applied for the determination of 0.01 % stearic acid and sodium stearate in SBR solutions. The polymer was diluted with toluene/absolute ethanol mixed solvent and stearic acid was determined by titration with 0.1 M ethanolic NaOH solution to the m-cresol purple endpoint similarly, sodium stearate was titrated with 0.05 M ethanolic HC1 solution [83]. Also long-chain acid lubricants (e.g. stearic acid) in acrylic polyesters were quantitatively determined by titration of the extract. 

Figure 14 Some examples of endpoint determination in titrations using chemiluminescent indicators. (A) Acid-base titration the endpoint is detected by the emission of light (B) complexometric titration the endpoint is detected by disappearance of light. M, metal acting as a catalyst X, excited state from the CL precursor acting as indicator.

In an acid-base titration you may either add acid to base or base to acid. This addition continues until there is some indication that the reaction is complete. Often a chemical known as an indicator will indicate the endpoint of a titration reaction, the experimental end of the titration. If we perform the experiment well, the endpoint should closely match the equivalence point of the titration, the theoretical end of the reaction. All the calculations in this section assume accurate experimental determination of the endpoint, and that this value is the same as the equivalence point. 

An acid-base titration is a laboratory procedure that we use to determine the concentration of an unknown solution. We add a base solution of known concentration to an acid solution of unknown concentration (or vice versa) until an acid-base indicator visually signals that the endpoint of the titration has been reached. The equivalence point is the point at which we have added a stoichiometric amount of the base to the acid. 

The equivalence point of an acid—base titration is the point at which the moles of H+ from the acid equals the moles of OH- from the base. The endpoint is the point at which the indicator changes color, indicating the equivalence point. 

When an indicator is used in a titration, the range of pH values at which its endpoint occurs must include, or be close to, the equivalence point. Some representative acid-base titration curves, shown in Figures 8.11, 8.12, and 8.13, will illustrate this point. 

Elemental composition B 17.50%, H 4.88%, O 77.62%. Boric acid may be analysed by adding calcium chloride (in excess) and sorbitol or mannitol to its solution, followed by acid-base titration using a strong base to phenolph-thalein endpoint. Elemental boron may be analyzed by AA or ICP spectrophotometry. 

For the titration of a strong base with a weak acid, the equivalence point is reached when the pH is greater than 7. The half equivalence point is when half of the total amount of base needed to neutralize the acid has been added. It is at this point that the pH = pK of the weak acid. In acid-base titrations, a suitable acid-base indicator is used to detect the endpoint from the change of colour of the indicator used. An acid-base indicator is a weak acid or a weak base. The following table contains the names and the pH range of some commonly used acid-base indicators. 

Earlier, the work of Zetlmeisl and Laurence [22] was described. With their instrument, the current decayed exponentially during the titration. Control of the current via an algorithm can be done with the computer. An example of this is found in the work of Earle and Fletcher [64]. Their titrator was based on the Intel 8008, an early 8-bit microprocessor. For acid-base titrations, the applied current was reduced linearly with the difference in pH, ApH, between the measured value and the endpoint pH. An algorithm compared ApH with a set of rate functions specified in units of mA/pH. The magnitude of the current was then computed by multiplying ApH by the rate function. This process was repeated every 65.536 ms. The coulombs passed were computed for each of these time intervals and summed until the endpoint was reached. The result was then... 

In an acid-base titration a buret is used to deliver the second reactant to the flask and an indicator or pH meter is used to detect the endpoint of the reaction. 

You may be familiar with acid-base titration that use phenolphthalein as the endpoint indicator. You might not have noticed, however, what happens when a solution that contains phenolphthalein in the presence of excess base is allowed to stand for a few minutes. Although the solution initially has a pink colour, it gradually turns colourless as the phenolphthalein reacts with the OH- ion in a strongly basic solution. 

Even if you cannot influence the sequence of data collection, you can control the size of the interval between points. The general rule is Take points more closely spaced when the observed quantity is varying more rapidly. Thus data points should be taken more frequently at early times for chemical kinetics and near the neutralization endpoint for an acid-base titration, as illustrated in Fig. 1. 

The point at which the indicator changes color is called the endpoint. The goal is to choose an indicator whose endpoint coincides with the stoichiometric point. An indicator very commonly used by acid-base titrations is phenolphtbalein, which is colorless in acid and turns pink at the endpoint when an acid is titrated with a base. 

Acid-base indicator a substance that marks the endpoint of an acid-base titration by changing color. (8.6)... 

As shown in Fig. 7 there is a rapid change in the value of E as the titration is proceeded through the endpoint. In fact, the titration curve has the same general form as that of an acid-base titration. An exact value for the endpoint can be calculated using the Nernst equations for the half-reactions. 

Potentiometric acid-base titrations are particularly useful for the analysis of mixtures of acids or poly-protic acids (or bases) because often, discrimination between the endpoints can be made. An approximate numerical value for the dissociation constant of a weak acid or base can be estimated from potentiometric titration curves. In theory, this quantity can be obtained from any point along the curve, but it is most easily derived from the pH at the point of halfneutralization. 

Further, like acid-base titrations, a rapid change in potential indicates the endpoint of the titration. A plot of potential E vs volume titrant added enables the equivalence point to be found, Figure 2.8(a). 

When the analyte is a base or an acid, the required titrant is a strong acid or strong base, respectively. This procedure is called an acid-base titration. An indicator very commonly used for acid-base titrations is phenolphthalein, which is colorless in an acidic solution and pink in a basic solution. Thus, when an acid is titrated with a base, the phenolphthalein remains colorless until after the acid is consumed and the first drop of excess base is added. In this case, the endpoint (the solution changes from colorless to pink) occurs approximately one drop of base beyond the stoichiometric point. This type of titration is illustrated in the three photos in Fig. 4.18. 

In this chapter we will also study acid-base titrations to explore how the pH changes when a base is added to an acid and vice versa. This process is important because titrations are often used to determine the amount of acid or base present in an unknown sample. In addition, we will see how indicators can be used to mark the endpoint of an acid-base titration. 

Figure 5.15 shows a comparison of a low-frequency acid-base titration at two different ionic strengths with high-frequency titrations conducted at 3 and 10 MHz. In each case, 50 milliequivalents of HCl is titrated with 0.01 N NaOH. Obviously 10 MHz is the best frequency to use, but because of the curvature several additional titration points need to be taken to increase the precision of the endpoint determination. The M-shaped curve resulting at 3 MHz could lead to misinterpretation and an incorrect endpoint. 

Many titration procedures in volumetric analysis use an indicator that changes color to signal the endpoint of the titration. For example, acid-base titrations are often performed... 

Spectrophotometric titrations have been used for redox titrations, acid-base titrations, and complexation titrations. The spectrophotometer can be used in a light scattering mode to measure the endpoint for a precipitation titration by turbidimetry. Spectrophotometric titrations can be easily automated. 

Use of potentiometry for pH titration allows analyses to be carried out in colored or turbid solutions. Also, it solves the problem of selecting the correct indicator for a particular acid-base titration. The endpoint can be determined more accurately by using a first or second differential curve as described earlier. It also permits pH titrations in nonaqueous solvents for the determination of organic acids and bases as described subsequently. In addition, it can be readily automated for unattended operation. 

It only remains to determine moles of H by means of an acid-base titration in which the volume of base of a known molarity required to neutralize the is measured. An appropriate indicator, in this case, phenolphthalein, is used to detect the endpoint. Since the acid and base react in a 1 1 ratio,... 

T raditionally, titration curve calculations are described in terms of equations that are valid only for parts of the titration. Equations will be developed here that reliably describe the entire curve. This will be done first for acid-base titration curves. In following chapters, titration curves for other reaction systems (metal complexation, redox, precipitation) will be developed and characterized in a similar fashion. For all, graphical and algebraic means of locating the endpoints will be described, colorimetric indicators and how they function will be explained, and the application of these considerations to (1) calculation of titration errors, (2) buffo design and evaluation, (3) sharpness of titrations, and finally, (4) in Chapter 18, the use of titration curve data to the determination of equilibrium constants will be presented. 

As we found with acid-base titrations in the last chapter, however, the most precise method for locating the endpoint is the Gran method. This method results in a linear plot which intercepts the equivalence point at the X axis. Not only can we easily find the best line through linear regression but, as mentioned earlier, the necessary points can be taken at a distance from the equivalence point making this method rapid and convenient. 

The means of detecting the endpoint will be dictated by the type of reaction employed. Acid-base titrations are most easily followed using a glass pH electrode while redox reactions lend themselves to amperometric detection (only a small fraction of the species detected is consumed at the indicator electrode). Other options are ion-selective electrodes and conductometric detection. 

An acid-base titration is a quick and convenient method for the quantitative analysis of substances with acidic or basic properties. Many inorganic and organic acids and bases can be titrated in aqueous media, but others, mainly organic, are insoluble in water. Fortunately, most of them are soluble in organic solvents hence they are conveniently determined by nonaqueous acid-base titrimetry. Although acid-base titrations can usually be followed potentio-metrically, visual endpoint detection is quicker and can be very precise and accurate if the appropriate indicator is chosen. 

Many different types of indicator electrodes can be used as endpoint indicators in potentiometric titrations. For example, an acid-base titration can be performed with a glass electrode as endpoint detector instead of using colored indicators, or chloride ions can be titrated with silver(I) using a chloride-ion- or silver-ion-selective electrode. 

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