Acid-base indicators, neutralization

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

Bromopyrogallol red. This metal ion indicator is dibromopyrogallol sulphon-phthalein and is resistant to oxidation it also possesses acid-base indicator properties. The indicator is coloured orange-yellow in strongly acidic solution, claret red in nearly neutral solution, and violet to blue in basic solution. The dyestuff forms coloured complexes with many cations. It is valuable for the determination, for example, of bismuth (pH = 2-3. nitric acid solution endpoint blue to claret red). 

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. 

Phenolphthalein is another acid-base indicator. It is often used by magicians (and chemistry teachers) to perform a trick that turns water into wine. In acidic and neutral conditions, phenolphthalein is colorless and looks like water. A pH of approximately 8.3, however, turns phenolphthalein a deep reddish-violet color. In basic conditions, phenolphthalein looks like red wine. 

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... 

When NaOH(a ) is added to CH3C02H(aacid-base indicator phenol-phthalein, the color of the indicator changes from colorless to pink in the pH range 8.2-9.8 because of neutralization of the acetic acid. 

Analysis of the electron density indicates the presence of a large number of different interactions between the atoms of the inner part of the systems considered, for instance, chalcogenic interactions between sulfur (or selenium) and the opposed N atom anion. Special emphasis was put on acid-base equilibria (neutral/anion/ dianion). The relationship between the electron density and the Laplacian at the bcp indicate that these interactions are similar to those encountered in intermolecular interactions, to the point that they can be analyzed together. 

It was recognized centuries ago that many substances could be divided into the two general categories. Acids have a sour taste (as in lemon juice), dissolve many metals, and turn litmus paper red. Bases have a bitter taste (as in soaps), feel slippery, and turn litmus paper blue. The chemical reaction between an acid and a base is called neutralization. The products of neutralization reactions are neither acids nor bases. Litmus paper is an example of an acid-base indicator, a substance that changes color when added to an acid to a base. 

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). 

For most neutralization reactions, there are no visible signs that a reaction is occurring. How can you determine that a neutralization reaction is taking place One way is to use an acid-base indicator. This is a substance that changes colour in acidic and basic solutions. Most acid-base indicators are weak, monoprotic acids. The undissociated weak acid is one colour. Its conjugate base is a different colour. 

SiLLCA Gel Chromatotraphy. The acid-, base-, and neutral-nitro-gen-free asphaltene (.126 g) was dissolved in n-pentane (10 mL) and placed on a silica gel column (30 g) that had been wet-packed with n-pentane. The column was eluted with n-pentane (500 mL) to remove the saturate hydrocarbons. Aromatic hydrocarbons were eluted from the column using 85% n-pentane-15% benzene (250 mL) and 60% benzene-40% methanol (250 mL). UV analyses of the saturate fraction indicated that trace amounts of aromatic hydrocarbons were present. The amount of saturates in the aromatic fraction, if any, is unknown. 

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. 

When the solution is neutral, you know that you have added exactly enough base to react with the amount of acid present, and you are at what is known as the endpoint of the titration. But how do you know when the endpoint is reached Probably the best way to indicate the endpoint of a titration is to use an acid-base indicator. Different indicators change color at different pH values. Look at the indicators in Figure 15.17. For a titration of NaOH and HCl, the endpoint is reached when the solution reaches a pH of 7. Therefore, you need an indicator that changes color as close to pH = 7 as possible. The best indicator for this titration is bromothy-mol blue, which changes from yellow to blue close to pH = 7. 

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

More magic in Figure 290 infusion of violets is actually an acid-base indicator (the first was discovered by Boyle in 1675) When vinegar, an acid, is added to the neutral blue infusion of violet, the solution turns red When excess aqueous ammonia base is added, the solution goes from red to blue to green The first human blow-hard neutralizes the solution back to blue by blowing in carbon dioxide, which forms carbonic acid in water The second blow-hard returns the color to red by adding more carbonic acid ... 

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... 

What acid-base indicators, shown in Figure 18.24, would be suitable for the neutralization reaction whose titration curve is shown in Figure 18.30 Why ... 

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