Indicator color changes

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). 

In practice it is usually possible to detect an indicator color change over a range of about two pH units. Consider, for example, what happens with bromthymol blue as the pH... 

The glass reaction vessel is equipped with a light source, photocell, and 550-nm filter to detect the indicator color change (red to pale yellow). The signal from the photocell is continuously monitored using a strip recorder. Quantification is obtained by comparison of the time required for ozonolysis of the sample compared to that required for a pure compound with known saturation. 

In a typical acid—base titration, the analyte is a solution of a base and the titrant is a solution of an acid or vice versa. An indicator a water-soluble dye (Section J), helps us detect the stoichiometric point, the stage at which the volume of titrant added is exactly that required by the stoichiometric relation between titrant and analyte. For example, if we titrate hydrochloric acid containing a few drops of the indicator phenolphthalein, the solution is initially colorless. After the stoichiometric point, when excess base is present, the solution in the flask is basic and the indicator is pink. The indicator color change is sudden, so it is easy to detect the stoichiometric point (Fig. L.3). Toolbox L.2 shows how to interpret a titration the procedure is summarized in diagram (3), where A is the solute in the titrant and B is the solute in the analyte. 

Senese, Fred. The molecular basis of indicator color changes, Frostburg State University s Department of Chemistry Web site. Available online. URL http //antoine.frostburg.edu/chem/senese/ 101/features/water2wine.shtml. Accessed on March 13, 2008. 

The molecular basis of indicator color changes. F. A. Senese, Department of Chemistry, Frostburg State University, http //antoine.fsu.umd.edu/chem/senese/101/features/ water2wine.shtml anthocyanin... 

The reaction is coupled to an indicator color change which is much faster than the metal substitution step. L refers to the multidentate ligands which were employed to avoid multiple relaxations r is the relaxation time. 

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. 

Explain the origin of the rule of thumb that indicator color changes occur at pACHln 1. 

M. D. Alexander, "Reactions of the Alkali Metals with Water A Novel Demonstration/ J. Chem. Educ., Vol. 69,1992,418. The reaction of sodium metal with water to produce an aqueous solution of sodium hydroxide and hydrogen gas is performed at the interface between paint thinner and the more dense water. Periodically, bubbles of hydrogen gas carry the sodium metal into the organic layer, temporarily stopping the reaction. The presence of the aqueous layer is shown by a phenol-phthalein indicator color change. 

Many reactions are catalyzed by acid sites on the surface of the catalyst. Isomerization, polymerization, aromatiza-tion, and cracking are catalyzed by Lewis and/or Bronsted acid sites. The precise nature of these sites is open to debate however, intuitively one can use an alkaline material to titrate acid sites and hence determine the number of such sites present. Beses, such as n-butylamine, with a series of Hammett indicators have been used for titrating acid sites. However, the system must be free from water contamination and the catalyst must be colorless to enable one to note indicator color changes. Diffusion of the indicators into the porous network can be very slow and require long equilibration times. 

The titration of an acid with a base, (a) The titrant (the base) is in the buret, and the beaker contains the acid solution along with a small amount of indicator, (b) As base is added drop by drop to the acidic solution in the beaker during the titration, the indicator changes color, but the color disappears on mixing, (c) The stoichiometric (equivalence) point is marked by a permanent indicator color change. The volume of the base added is the difference between the final and initial buret readings. 

Bromthymol blue, an indicator with a Ka value of 1.0 X 10-7, is yellow in its HIn form and blue in its In- form. Suppose we put a few drops of this indicator in a strongly acidic solution. If the solution is then titrated with NaOH, at what pH will the indicator color change first be visible ... 

Indicator color change will be sharp, occurring with the addition of a single drop of titrant. 

The steepn s of the titration curve in the immediate vicinity of the end point is the decisive quantity as to whether determining an end point is feasible. The relative precision of location of an end point is the fraction of the stoichiometric quantity of reagent required to traverse the region represented by 0.1 pH unit on either side of the end point. The change of 0.1 pH unit represents about the limit of visual observation of indicator color change using a color-comparison technique. A relative precision of 10 indicates an expected end-point precision of about 1 ppt. Modern... 

According to (11-23) and (11-24) the indicator color change is affected by the hydrogen ion concentration. From a practical viewpoint a conditional indicator constant which depends on the pH of the buffered solution, may be conveniently defined by... 

Indicator Color change oxidized-reduced Transition potential, V... 

Titrations are widely used in analytical chemistry to determine acids, bases, oxidants, reductants, metal ions, proteins, and many other species. Titrations are based on a reaction between the analyte and a standard reagent known as the titrant. The reaction is of known and reproducible stoichiometry. The volume, or the mass, of the titrant needed to react essentially completely with the analyte is determined and used to obtain the quantity of analyte. A volume-based titration is shown in this figure, in which the standard solution is added from a buret, and the reaction occurs in the Erlenmeyer flask. In some titrations, known as coulometric titrations, the quantity of charge needed to completely consume the analyte is obtained. In any titration, the point of chemical equivalence, experimentally called the end point, is signaled by an indicator color change or a change in an instrumental response. 

Observation At first, the green indicator solution shows no electrical conductivity. As soon as hydrogen chloride is released, the conductivity of the solution increases and the indicator color changes from green to red. 

Procedure (a) In the preliminary test, add a few drops of phenolphthalein solution to 10 ml of sodium hydroxide solution until it becomes wine-red, then add hydrochloric acid until the indicator color changes. Heat part of the... 

The acidity titration is essentially the reverse of the alkalinity titration. The solution is titrated with a strong base, such as NaOH, to an end-point which is commonly a fixed pH of 8.3, as determined by an indicator color change or pH electrode. 

What is the minimum pH change required for a sharp indicator color change at the end point Why ... 

There are numerous automatic titrators that employ potentiometric end-point detection. They usually can automatically record the first or second derivative of the titration curve and read out the end-point volume. The sample is placed in the titration vessel, and the titrant, drawn from a reservoir, is placed in a syringe-driven buret. The volume is digitally read from the displacement of the syringe plunger by the electronic driver. Titrators may also employ photometric detection of indicator color changes. An automatic titrator is shown in Figure 14.7. Automatic titrators make volumetric analyses rapid, reproducible, and convenient. While instrumental methods provide many advantages, classical volumetric analyses are still widely used and are very useful, especially for major constituents, for example, in the pharmaceutical industry. 

Determination of replaceable hydrogen in an unknown acid. If the unknown is a weak acid (e.g., KHP, acetic acid, or vinegar) you will be instructed to use phenolphthalein indicator (color change same as above). If it is a strong acid, you may use another indicator, such as chlorophenol red (color change from yellow to violet). 

The unknown soda ash is titrated with standard HCl using a potentiometric (pH) end point measured with a pH meter using a pH glass electrode-saturated calomel reference electrode combination. The end-point breaks are compared with indicator color changes. 

Determination of lead on leaves. For each plastic bag containing a leaf sample, heat 20 mL of 0.1 M HNO3 to about 70°C. Add 20 mL to each bag, close, and shake for about 2 min. Pour into clean 100-ml- beakers. Add one drop thymol blue indicator solution to each, followed by dropwise addition of 2 M NH3 until the indicator color change is complete (to blue) and add a couple of extra drops. The solution should smell of ammonia. Then, add only if instructed to) 60 mL of the ammonia-cyanide-sulfite solution—and then add with a pipet 25 mL of the CH2Cl2-dithizone solution and proceed with the extraction measurement as with the standards. 

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