Neutralization or Acid-Base Indicators

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. 

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Acid-base indicator Acid (or base). Neutralization is complete as determined by color change of indicator. 

Litmus is not the only plant material that turns a different color in response to acidic or basic conditions. For example, when red cabbage or beets are boiled, the solids can be separated from the liquid. The liquid is then cooled for use as an acid-base indicator. Red cabbage juice is red or purple in acidic conditions, while bases cause it to turn blue or yellow. When a solution is neutral, the juice is a bluish-purple. 

Although litmus paper, cabbage juice, and phenolphthalein can indicate whether a substance is acidic or basic, they have limitations in that they cannot determine an exact pH. To do this, an acid-base indicator called universal indicator can be used. Universal indicator is actually a mixture of several different acid-base indicators (usually phenolphthalein, methyl red, bromthymol blue, and thymol blue). This mixture produces a wide range of colors to indicate different pHs. Under very acidic conditions, universal indicator is red. It turns orange and then yellow between the pHs of 3 to 6. It is green at neutral pH and turns greenish-blue as a solution becomes more alkaline. In very basic conditions, universal indicator turns a dark purple color. 

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. 

INDICATOR (Chemical). A substance which shows by a color change, or other visible manifestation, some change in. or particular condition of. the chemical nature of a system. Thus acid-base indicators may be used to indicate the end point of a particular neutralization reaction, or they may... 

In a typical acid-base titration, an acid-base indicator (such as phenolphthalein) or a pH meter is used to monitor the course of a neutralization reaction. The usual goal of titration is to determine an unknown concentration of an acid (or base) by neutralizing it with a known concentration of base (or acid). 

A cuprimetric method for the determination of ascorbic acid is based on a direct potentiometric titration with CUSO4 using a copper-based mercury film electrode as the indicator electrode. The electrochemical reaction taking place is ascorbic acid reducing Cu(n) ions quantitatively to Cu(I) ions in neutral or acidic aqueous solutions. The linearity... 

To monitor the progress of the chemical reaction, the acid-base indicator phenolphthalein will be used. Phenolphthalein is colorless when acidic and pink in color when neutral or basic. In this activity, we will know that all of the acid has been consumed by the base when the test-tube solution starts to turn pink. We can monitor the progress of the reaction so that a single drop of the base results in a sudden change from colorless to pink. At that point, we will know that all of the acid has reacted with the base. 

Acid-base indicators are weak organic acids or bases that change color at the equivalence point in an acid-base neutralization reaction. 

The photometric end point has been applied lo many types of reactions. For example, most standard oxidizing agents have characteristic absorption spectra and thus produce photometrically detectable end points. Although standard acids or bases not absorb, the introduction of acid-base indicators permits photoineiric neutralization titrations. The photometric end point ha.s also been used to gicat advantage in titrations with F.DTA (cthylenediaiiiinetciraacetic... 

As shown in eqn [3], in nonaqueous solvents activity coefficients must be taken into account to relate the color of an indicator and the pH. Moreover, acid-base indicators may be not only neutral (model I), but also positively or negatively charged. The ionic strength of the solution modifies activity coefficients in a different way for each kind of dissociation model, according to the ionic charge of chemical species. 

Figure 16.11 Depending on the ions involved, salt solutions can be neutral, acidic, or basic. These three solutions contain the acid-base indicator bromthymol blue, (a) The NaCI solution is neutral (pH = 7.0) (b) the NH4CI solution is acidic (pH = 3.5) (c) the NaCIO solution is basic (pH = 9.5). 

The equivalence point is the point at which the acid has been neutralized completely by the added base. The equivalence point in a titration can be determined by monitoring the pH over the course of the titration, or it can be determined using an acid-base indicator. An acid-base indicate is usually a weak raganic acid or base for which the ionized and un-ionized forms are different colors. 

AgN03(c9) + HiO(/). 4.113 (a) Check with litmus paper, test reactivity with carbonate, or mix with NaOH(c ) and demonstrate neutralization (use an acid-base indicator), (b) Titrate a known quantity of acid with standard NaOH( ) solution, (c) Visually compare the conductivity of an acid solution with that of a sodium chloride solution of the same molarity. 4.115 (a) The complete reaction is Pb (c9) + 2NO3 (09) -1- Na2S04( )... 

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