Acid-base indicators colors

Plate II. Some acid-base indicator colors and the hydrogen ion concentration. 

Acid-base indicator color changes are featured in Demonstration 11-1. Box 11-2 shows how optical absorption by indicators allows us to measure pH. 

The colorations produced in this reaction arise from the action of nitrous acid on the phenol, giving />-nitrosophenol (I) which then reacts with excess of phenol to form an indophenol (II) which is an acid-base indicator ... 

The plT at which an acid-base indicator changes color is determined by its acid dissociation constant. For an indicator that is a monoprotic weak acid, ITIn, the following dissociation reaction occurs... 

A list of several common acid-base indicators, along with their piQs, color changes, and pH ranges, is provided in the top portion of Table 9.4. In some cases. 

Ladder diagram showing the range of pH levels over which a typical acid-base indicator changes color. 

The indicator method is especially convenient when the pH of a weU-buffered colorless solution must be measured at room temperature with an accuracy no greater than 0.5 pH unit. Under optimum conditions an accuracy of 0.2 pH unit is obtainable. A Hst of representative acid—base indicators is given in Table 2 with the corresponding transformation ranges. A more complete listing, including the theory of the indicator color change and of the salt effect, is also available (1). 

It turns out that in low-viscosity blending the acdual result does depend upon the measuring technique used to measure blend time. Two common techniques, wliich do not exhaust the possibilities in reported studies, are to use an acid-base indicator and inject an acid or base into the system that will result in a color change. One can also put a dye into the tank and measure the time for color to arrive at uniformity. Another system is to put in a conductivity probe and injecl a salt or other electrolyte into the system. With any given impeller type at constant power, the circulation time will increase with the D/T ratio of the impeller. Figure 18-18 shows that both circulation time and blend time decrease as D/T increases. The same is true for impeller speed. As impeller speed is increased with any impeller, blend time and circulation time are decreased (Fig. 18-19). 

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

You are probably familiar with a variety of aqueous solutions that are either acidic or basic (Figure 4.6). Acidic solutions have a sour taste and affect the color of certain organic dyes known as acid-base indicators. For example, litmus turns from blue to red in acidic solution. Basic solutions have a slippery feeling and change the colors of indicators (e.g., red to blue for litmus). 

The objective of the titration is to determine the point at which reaction is complete, called the equivalence point. This is reached when the number of moles of OH- added is exactly equal to the number of moles of acetic acid, HC O originally present To determine this point, a single drop of an acid-base indicator such as phenolphthalein is used. It should change color (colorless to pink) at the equivalence point. 

A less accurate but more colorful way to measure pH uses a universal indicator, which is a mixture of acid-base indicators that shows changes in color at different pH values (Figure 13.5, p. 359). A similar principle is used with pH paper. Strips of this paper are coated with a mixture of pH-sensitive dyes these strips are widely used to test the pH of biological fluids,... 

As pointed out in Chapter 4, an acid-base indicator is useful in determining the equivalence point of an acid-base titration. This is the point at which reaction is complete equivalent quantities of acid and base have reacted. If the indicator is chosen properly, the point at which it changes color (its end point) coincides with the equivalence point To understand how and why an indicator changes color, we need to understand the equilibrium principle involved. 

Red cabbage juice, a natural acid-base indicator. The picture shows (left to right) its colors at pH 1,4,7,10. and 13. 

Acid-base indicator, 403-404q colors, 392-393 equivalence point and, 84 Acid-base reactions, 96-97q, 402q amino acids, 622-625 Brensted-Lowry model, 353-354 buffer systems, 383-391 equations for, 82-84 Lewis acid in, 410 Lewis base in, 410 types, 81-82... 

A simple, reliable, and fast method of determining the pH of a solution and of monitoring a titration is with a pH meter, which uses a special electrode to measure H 0+ concentration. An automatic titrator monitors the pH of the analyte solution continuously. It detects the stoichiometric point by responding to the characteristic rapid change in pH (Fig. 11.9). Another common technique is to use an indicator to detect the stoichiometric point. An acid-base indicator is a water-soluble organic dye with a color that depends on the pH. The sudden change in pH... 

An acid—base indicator changes color with pH because it is a weak acid that has one color in its acid form (HIn, where In stands for indicator) and another color in its conjugate base form (In-). The color change results because the proton in HIn changes the structure of the molecule in such a way that the light absorption characteristics of HIn are different from those of In. When the concentration of HIn is much greater than that of In, the solution has the color of the acid form of the indicator. When the concentration of In is much greater than that of HIn, the solution has the color of the base form of the indicator. 

Acid-base indicators are weak acids that change color close to pH = pK,n an indicator should be chosen so that its end point is close to the stoichiometric point of the titration. 

Dissolve the compounds in water and use an acid-base indicator to look for a color change. 

Conductometric titration rests on the marked changes that occur near the titration endpoint in the relation between conductivity and the amount of titrant added (an extreme or inflection point). It is used in particular for the titration of acids with base (and vice versa) in colored and turbid solutions or solutions containing reducing and oxidizing agents (i.e., in those cases where the usual color change of acid-base indicators cannot be seen). 

Litmus paper changes color in the presence of an acid or a base. Substances like litmus paper are called acid-base indicators. An acid-base indicator responds to the concentration of hydrogen ions in a solution by changing color. Litmus paper is a very common acid-base indicator. It turns blue if the pH is above 8.2. Therefore, if litmus turns blue, it means the substance is a base. 

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. 

What makes indicators change color in the presence of acids and bases A color change is often the sign that a chemical reaction has occurred, and this is no exception Acid-base indicators are actually acids or bases themselves. They change colors because the acid and its conjugate base (or the base and its conjugate acid) are different colors. For example, suppose an acidic indicator, abbreviated HIn (this is not really a chemical formula, it is just a way to show an indicator that has hydrogen ions to donate), is dissolved in water. It... 

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. 

Figure 3.4 An acid-base indicator called universal indicator can show the exact pH of a substance. Adding a small amount of universal indicator to a solution changes the color of the solution. Each color represents a different pH.

Acid-base indicator A substance that changes color in the presence of an acid or a base. 

Whether for a class demonstration, a practical joke, or perhaps a clandestine activity, disappearing ink is a fascinating substance. What is the secret to its action One formulation of disappearing ink contains a common acid-base indicator, that is, a substance that by its color shows the acid or basic nature of a solution. One acid-base indicator that shifts from a colorless hue under acidic conditions to a deep blue color in alkaline solutions is thymolphthalein. If the indicator starts off in a basic solution, perhaps containing sodium hydroxide, the typical blue color of an ink is perceived. How does the ink color disappear This behavior is dependent upon the contact of the ink with air. Over time, carbon dioxide in the air combines with the sodium hydroxide in the ink solution to form a less basic substance, sodium carbonate. The carbon dioxide also combines with water in the ink to form carbonic acid. The indicator solution responds to the production of acid and returns to its colorless acid form. A white residue (sodium carbonate) remains as the ink dries. 

Aqueous solutions of strong soluble bases (1) have a bitter taste, (2) have a slippery feeling, (3) change the colors of many acid-base indicators, (4) react with protic acids (acids that have an H) to form salts and water, and (5) conduct an electrical current since they contain ions. 

FIGURE 5.5 Some acid-base indicators and their color change ranges. R = red, Y = yellow, B = blue, P = pink, C = colorless, and A = amber. 

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