Indicators, acid-base universal

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,... 

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.

The fascinating book Sensational Chemistry, in the Open University s Our Chemical Environment series, OU Press, Milton Keynes, 1995, is an excellent introduction to the topic. The Web has several hundred relevant sites, most of which are simple to follow just go to a reputable search engine, and type acid-base indicator . 

Bassam Z. Shakhashiri, "Color- ful Acid-Base Indicators, " Chemical Demonstrations, A Handbook for Teachers of Chemistry, Vol. 3 (The University of Wisconsin Press, Madison, 1989), pp. 33-40. 

The pH of a solution can be determined using a pH meter (Figure 18-1) or by the indicator method. Acid-base indicators are intensely colored complex organic compounds that have different colors in solutions of different pH (Section 19-4). Many are weak acids or weak bases that are useful over rather narrow ranges of pH values. Universal indicators are mixtures of several indicators they show several color changes over a wide range of pH values. 

Universal indicators are mixtures of several acid-base indicators that display a continuous range of colors over a wide range of pH values. Figure 18-2 shows concentrated solutions of a universal indicator in flat dishes so that the colors are very intense. The juice of red (purple) cabbage is a universal indicator. Figure 19-2 shows the color of red cabbage juice in solutions within the pH range 1 to 13. 

The model is restricted to compounds that display acid-base bonding and therefore it cannot be used to describe metallic or organic bonding. Within these limits, the Valence Sum Rule is found to be widely obeyed and has proved remarkably useful in the analysis of inorganic structures. The Equal Valence Rule is less universal and fails when the environment of an atom is distorted as a result of its internal electronic structure or as the result of the application of the Distortion Theorem in constrained structures. In some highly constrained structures the Valence Sum Rule may also be violated, indicating the presence of internal strain and flagging the possible existence of a complex crystal chemistry. 

There is no book available in the market directly on acid-base indicators even though use of acid-base indicators is widespread, growing rapidly and has exploded in the past two decades. There was a need to pubUsh a book that provided an immediate incentive for compiUng the notes to update the scientific conununity with the wealth of information on acid-base indicators. The literature in dyestuff chemistry, particularly on acid-base indicators, is largely in patents. This book provides a systematic and up-to-date library of information on 200+ add-base indicators as a reference handbook. It was compiled as a resource guide for chemists and non-chemists in industry and university. 

Prior to the development of the glass electrode and pH meter, the color indicator application found a wide use for the direct determination of hydrogen-ion concentration. William Mansfield Clark (De Lamur Professor Emeritus of Physiological Chemistry at Johns Hopkins University) spent most of his lifetime investigating organic substances suitable for acid-base indicators. He never accepted the pH meter. In his classic book Oxidation-Reduction Potential of Organic Systems he stated ( ) ... 

Universal indicators are mixtures of several acid-base indicators that display a continuous range of colors over a wide range of pH values. Figure 18-2 shows concentrated solu-... 

The spotting of acid-base indicator papers with a drop of a solution or suspension of the sample in order to detect acidity or alkalinity is probably the earliest application of spot test analysis. The so-called universal indicator papers, which contain a mixture of different indicators, serve this broad purpose they cover the pH range from 1 to 11 with their characteristic color changes. However, difficulties may arise if such papers are employed in the examination of colored materials. For example, the detection of dilute acid or caustic alkali in strong potassium permanganate solutions fails if these papers are used. The following tests are based on the reaction of OH and H+ ions with production of volatile compounds that can be identified in the gas phase by appropriate tests. 

Who among us does not enjoy the vision of a rainbow The spectral colors spread out across the sky inspire awe and pleasure no matter how many times we see them. For this reason, we have attempted to replicate rainbows artificially. We can go to any toyshop and find rainbow glasses, any rock shop and find rainbow quartz made by plasma ionization techniques. Jewelry and craft shops sell rainbow jewelry made by electroplating titanium or niobium oxide. Teachers use rainbow tubes with a universal indicator to demonstrate pH changes and acid-base reactions. Dye mixtures can be separated by rainbow electrophoresis and rainbow chromatography. Rainbow thermometers and toys are made from liquid crystals. Thermochromic materials can be heated to produce rainbow colors. Every toy shop is awash with rainbow colors, a phenomenon not available prior to the many inventions of the twentieth century [39]. 

The major application of the potentiometric method is for acid-base measurements in both aqueous and nonaqueous solvent systems. Although the glass electrode is universally the most common indicating electrode system for such measurements, many other electrodes have been developed. However, except in extremely specialized... 

The method developed by Epton [212,213] became the universally accepted method for the analysis of active matter of anionic and cationic surfactants. Epton s method, also known as the two-phase titration, is based on the titration of the anionic surfactant with cetylpyridinium bromide, a cationic surfactant, in the presence of methylene blue as indicator. A solution of the anionic surfactant is mixed with the indicator dissolved in dilute sulfuric acid, followed by further addition of chloroform, and then it is titrated with the cationic surfactant. Methylene blue forms a complex with the anionic salt that is soluble in chloroform, giving the layer a blue color. As the titration proceeds there is a slow transference of color to the water layer until the equivalence point. At the equivalence point colors of the chloroform and water layers are visually the same. On successive additions of titrant the chloroform layer lightens in shade and finally becomes colorless. 

A 10 mM ionic strength universal buffer mixture, consisting of Good zwitterio-nic buffers, [174] and other components (but free of phosphate and boric acid), is used in the pION apparatus [116,556], The 5-pKa mixture produces a linear response to the addition of base titrant in the pH 3-10 interval, as indicated in Fig. 7.53. The robotic system uses the universal buffer solution for all applications, automatically adjusting the pH with the addition of a standardized KOH solution. The robotic system uses a built-in titrator to standardize the pH mapping operation. 

Show that ammonia is a base. Neutralise acids by adding 2 mol dm 3 aqueous ammonia to dilute acids in the presence of universal indicator. 

The indicators do not give an estimate of the strength of an acid or a base. To know the strength of an acid or a base an universal indicator can be used. A combination of indicators is used to make the universal indicator. The strength, -w of an acid or an alkali is given by its pH value. 

---