Acid-Base Indicators
Acid-base indicators are substances that change color depending on the pH of a solution. They are commonly used in chemistry to visually determine whether a solution is acidic, basic, or neutral. Indicators work by undergoing a reversible chemical change that is sensitive to the hydrogen ion concentration in the solution.
3 Key excerpts on "Acid-Base Indicators"
eBook - ePubBiermann's Handbook of Pulp and Paper
Volume 2: Paper and Board Making
- Pratima Bajpai(Author)
- 2018(Publication Date)
- Elsevier(Publisher)
...The color of these species should be intense so only small amounts of indicator are used; after all, the indicators are also reacting with the titrant. Usually these compounds involve conjugated aromatic systems. A commonly used indicator is phenolphthalein. Fig. 21.2 shows the unionized, colorless form and the ionized, deep red-purple form. The p K a of phenolphthalein is about 9. Using Eq. (21.2), this means that at pH 9 about half the indicator will be in the ionized form, whereas at pH 8 only 1/10 the indicator will be ionized. This is the range in which one observes the most color change. Most pH indicators change color over a pH range of about 2, going from 90% of one color to 90% of the other color. If one of the forms is colorless, the color change is over one pH unit. Phenolphthalein is a suitable indicator for strong acid-strong base reactions. It reacts with CO 2, so the CO 2 must be boiled off near the endpoint to give a sharp endpoint for titrations involving carbonate. This will be discussed in greater detail in Chapter 13, Volume 1 on pulping liquor analysis. Figure 21.2 Phenolphthalein indicator forms. Phenolphthalein is suitable for titration of borax if glycerol is present to avoid color fading. The color change goes from colorless up to pH 8.5 to deep red-purple above pH 9. Phenolphthalein is usually used as a few drops of a 1% ethanol–water solution, as it is only slightly soluble in water in its unionized form. Methyl orange is another common pH indicator and goes from red below pH 3.1 to yellow above pH 4.4; it is used as a 0.1% aqueous solution. There are about 60 additional pH indicators available. 21.4. Titration of a Weak Acid With a Strong Base or Weak Base With a Strong Acid The tools are now available to describe the titration curve of a weak acid by a strong base or, more likely in kraft pulping liquor analysis, the titration of a weak base by a strong acid. Consider the titration of 100 mL of 0.1 N acetic acid by 0.1 N NaOH (Fig. 21.3)...
eBook - ePubConn's Biological Stains
A Handbook of Dyes, Stains and Fluorochromes for Use in Biology and Medicine
- Richard Horobin, John Kiernan, Richard Horobin, John Kiernan(Authors)
- 2020(Publication Date)
- Taylor & Francis(Publisher)
...Potentiometric probes report fluctuations in the transmembrane potentials of living cells. Ion-specific indicators are employed to measure local concentrations of cell-signallers such as Ca 2+ and Mg 2+. Generally, indicators used as biological probes have fluorescent properties, to allow their detection at very low concentrations. Whereas acidimetric and metallochromic titrations are reversible by the simple addition or removal of protons or metal ions, redox indicators undergo more drastic changes in structure on being reduced or oxidized. However, the redox indicator behavior remains reversible, and there must be a well-defined difference in color between the two forms. M AJOR APPLICATIONS OF INDICATORS Indicators may be used in either single or multiphase systems, with the complexity of the system governing the technical difficulty of data collection. For instance, use of Phenolphthalein in acid–base titrations is straightforward, requiring only simple mixing of components and a visual check on the color change. However, monitoring the change in potential across a cell membrane requires several refinements, both in sample preparation and in measuring changes in fluorescence. Results are also influenced by distribution of indicators within and outside cells. To predict the distribution of an indicator a thorough knowledge of its properties is required. Thus the lipophilicity of a protonated indicator is usually lower than that of the corresponding neutral species. With fluorescent indicators the wavelength and intensity of emission of each form will be different. Such pH-distribution behavior can be used to demonstrate particular intracellular compartments, such as mitochondria and lysosomes. The basis of the differential uptake is in part related to the degree of polarity of the molecule in its different forms...
eBook - ePubHandbook of Microbiology
Condensed Edition
- Allen I Laskin(Author)
- 2019(Publication Date)
- CRC Press(Publisher)
...0.1 M sodium hydroxide I: 50 ml of 0.05 M disodium hydrogen phosphate + x ml of 0.1 M sodium hydroxide J: 25 ml of 0.2 M potassium chloride + +x ml of 0.2 M sodium hydroxide Properties of Standard Aqueous Buffer Solutions at 25°C pH Values of Standard Solutions at Temperatures from 0 to 95°C Approximate pH Values of Some Acids, Bases, Biologic Materials and Foods All values in the following tableare based on measurements made at 25°C and are rounded otf to the nearest tenth. pH Indicators Fluorescent Indicators DR. JACK DEMENT Fluorescent indicators are substances that show definite changes in fluorescence with change in pH. Some fluorescent materials are not suitable for indicators, since their change in fluorescence is too gradual. Fluorescent indicators find greatest utility in the titration of opaque, highly turbid or deeply colored solutions. A long-wavelength ultraviolet (“black light”) lamp in a dimly lighted room provides the best environment for titrations involving fluorescent indicators, although bright daylight is sometimes sufficient to evoke a response in the bright-green, yellow or orange fluorescent indicators. Titrations are carried out in nonfluorescent glassware. All glassware should be checked before use, to be certain that it does not fluoresce in the wavelength of the light involved in the titration. The meniscus of the liquid in the burette can be followed when a few particles of an insoluble fluorescent solid are dropped onto its surface. The indicators in the following table are arranged by approximate pH range covered. In some of the dyestuffs the end point may vary slightly with the source or manufacturer....
