Neutralization titrations endpoint detection

Other detection methods are based on optical transmittance [228-231], Alcohol sulfates have been determined by spectrophotometric titration with barium chloride in aqueous acetone at pH 3 and an indicator [232] or by titration with Septonex (carbethoxypentadecyltrimethylammonium bromide) and neutral red as indicator at pH 8.2-8.4 and 540 nm [233]. In a modified two-phase back-titration method, the anionic surfactant solution is treated with hyamine solution, methylene blue, and chloroform and then titrated with standard sodium dodecyl sulfate. The chloroform passing through a porous PTFE membrane is circulated through a spectrometer and the surfactant is analyzed by determining the absorbance at 655 nm [234]. The use of a stirred titration vessel combined with spectrophotometric measurement has also been suggested [235]. Alternative endpoint detections are based on physical methods, such as stalag-mometry [236] and nonfaradaic potentiometry [237]. 

Endpoint detection in a neutralization titration is ordinarily based on the abrupt change in pH that occurs near the equivalence point. A non-instrumental method of pH measurement much used in simple titrations... 

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. 

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,... 

You keep adding base until the solution turns the faintest shade of pink detectable. 1 call this the endpoint of the titration, the point in which the acid has been exactly neutralized by the base. Figure 12-2 shows the titration setup. 

Neutralization indicators, or acid-base indicators or pH indicators, are auxiliary reagents added to the titrand solution in order to detect the equivalence point in acid-base titrations. They can also be used for an accurate quantitative measure of the pH. Tournesol, a natural pigment extracted from some blue-green lichens, was the first pH indicator to be used (1850). Phenolphthalein and methyl orange were introduced somewhat later (1877 and 1878, respectively). Undeniably, the chief interests in the use of acid-base indicators are their low cost and ease of handling. However, they give rise to less precise and less accurate endpoints than some instrumental methods. 

Add small amounts of a standeirdized sodium hydroxide (NaOH) solution of known molarity (for example, 0.100 M) with a buret. (A buret is a graduated glass tube with a small opening and a stopcock valve, which helps you measure precise volumes of solution.) Keep adding base until the solution turns the faintest shade of pink detectable. 1 call this the endpoint of the titration, the point at which the acid has been exactly neutralized by the base. 

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