Acids acid-base indicators

The pH scale is a convenient way to represent the acidity or basicity of dilute acid and base solutions. Pure water has a pH of 7 acidic solutions have pH values < 7 and basic solutions have pH values > 7. Each change of one unit of pH is a tenfold change in acidity. Acid-base indicators, such as fit-mus and phenolphthalein, can be used to measure whether a solution is acidic or basic. A natural tmiversal indicator, red cabbage extract, can be used to determine the pH within 2 pH xmits. A buffer contains equal amounts of either a weak acid and its conjugate base or a weak base and its conjugate acid. 

Used as a 0.04% soln. in aq. EtOH for extraction-photometric detn. of K (2 450 nm), photometric detn. of Mg, detn. of Zn, titrimetric detn. of organic acids, acid-base indicator (pH range 1.3-3.2 colour change, red -> yellow) redox indicator. Orange-yellow cryst. Sol. H2O, EtOH spar. sol. Et20 insol. QH. ... 

Bishop, E., Indicators, Pergamon, Oxford, 1972 detn. Mg, Zn, organic acids, acid-base indicator)... 

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 ... 

H H Acid-base indicator recommended for titration of acids... 

Name pH Range Color Change Acid to Base Indicator Solution... 

Use acid-base indicator solutions. Oxidation causes bleaching of indicator to colorless... 

Harvey, D. T. Statistical Evaluation of Acid/Base Indicators, /. Chem. Educ. 1991, 68, 329-331. 

Standardization—External standards, standard additions, and internal standards are a common feature of many quantitative analyses. Suggested experiments using these standardization methods are found in later chapters. A good project experiment for introducing external standardization, standard additions, and the importance of the sample s matrix is to explore the effect of pH on the quantitative analysis of an acid-base indicator. Using bromothymol blue as an example, external standards can be prepared in a pH 9 buffer and used to analyze samples buffered to different pHs in the range of 6-10. Results can be compared with those obtained using a standard addition. 

Why is the acid-base indicator methyl red added to the solution ... 

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 acidity constant for an acid-base indicator was determined by preparing three solutions, each of which has a total indicator concentration of 5.00 X 10- M. The first solution was made strongly acidic with HCl and has an absorbance of 0.250. The second solution was made strongly basic and has an absorbance of 1.40. The pH of the third solution was measured at 2.91, with an absorbance of 0.662. What is the value of K, for the indicator ... 

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,... 

We will also consider the equilibrium involved when an acid-base indicator is used to estimate pH (Section 14.2). ... 

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. 

An acid-base indicator is derived from a weak acid HIn ... 

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

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