Acid/base indicators selection

Figure 19.5 shows the color changes and their pH ranges for some common acid-base indicators. Selecting an indicator requires that you know the approximate pH of the titration end point, which in turn requires that you know which ionic species are present. Because the indicator molecule is a weak acid, the ratio of the two forms is governed by the [H30" ] of the test solution ... 

Given the acid-base indicators in Question 37, select a suitable indicator for the following titrations ... 

For some purposes it is desirable to have a sharp colour change over a narrow and selected range of pH this is not easily seen with an ordinary acid-base indicator, since the colour change extends over two units of pH. The required result may, however, be achieved by the use of a suitable mixture of indicators these are generally selected so that their pK ln values are close together and the overlapping colours are complementary at an intermediate pH value. A few examples will be given in some detail. 

Because the pH changes rapidly near the stoichiometric point, an indicator is suitable for a titration if the pH at the stoichiometric point is within one unit of p K pHstoichiometric point P In 1 Table 18-2 lists a selection of acid-base indicators, and Example shows how to select an appropriate indicator. 

In media selective for enterobacteria a surface-active agent is the main selector, whereas in staphylococcal medium sodium and lithium chlorides are the selectors staphylococci are tolerant of salt concentrations to around 7.5%. Mannitol salt, Baird-Parker (BP) and Vogel-Johnson (VJ) media are three examples of selective staphyloccocal media. Beside salt concentration the other principles are the use of a selective carbon source, mannitol or sodium pyruvate together with a buffer plus acid-base indicator for visualizing metabolic activity and, by inference, growth. BP medium also contains egg yolk the lecithin (phospholipid) in this is hydrolysed by staphylococcal (esterase) activity so that organisms are surrounded by a cleared zone in the otherwise opaque medium. The United States Pharmacopeia (1990) includes a test for staphylococci in pharmaceutical products, whereas the British Pharmacopoeia (1993) does not. 

Recipes for Selected Acid-Base Indicator Solutions... 

Reflectance measurements provided an excellent means for building an ammonium ion sensor involving immobilization of a colorimetric acid-base indicator in the flow-cell depicted schematically in Fig. 3.38.C. The cell was furnished with a microporous PTFE membrane supported on the inner surface of the light window. The detection limit achieved was found to depend on the constant of the immobilized acid-base indicator used it was lO M for /7-Xylenol Blue (pAT, = 2.0). The response time was related to the ammonium ion concentration and ranged from 1 to 60 min. The sensor remained stable for over 6 months and was used to determine the analyte in real samples consisting of purified waste water, which was taken from a tank where the water was collected for release into the mimicipal waste water treatment plant. Since no significant interference fi-om acid compounds such as carbon dioxide or acetic acid was encountered, the sensor proved to be applicable to real samples after pH adjustment. The ammonium concentrations provided by the sensor were consistent with those obtained by ion chromatography, a spectrophotometric assay and an ammonia-selective electrode [269]. 

A selective and quantitative detector is the acid-base automatic titration detector used in the first work on the GC of volatile fatty acids and bases by James and Martin [17]. The column effluent enters a cell containing a solution of an acid—base indicator. The change in the pH and thence the colour of the solution is titrated automatically by means of a photocell relay. The amount of titrant added to the cell is plotted against time, thus producing a selective integral chromatogram [47]. 

Use an acid—base indicator, which marks the endpoint of a titration by changing color. Although the equivalence point of a titration, defined by the stoichiometry, is not necessarily the same as the endpoint (where the indicator changes color), careful selection of the indicator will ensure only negligible error. 

Table 3-3 TRANSITION INTERVALS OF SELECTED ACID-BASE INDICATORS...

The most frequently used detector in FI systems with gas-diffusion separation is the spectrophotometer. Quite often the gas-diffusion process offers sufficient selectivity to allow relatively non-specific chemical reactions in the acceptor stream to detect the analyte. Thus, carbon dioxide, sulfur dioxide, hydrogen sulfide, ammonia may all be determined using suitable acid-base indicators in appropriate buffer solutions used as the acceptor streams. The concentration of the buffer solutions may be adjusted to suit a certain concentration range for the analyte. In order to further enhance the selectivity and/or sensitivity more specific reagents may be introduced in the acceptor streams. In the previously mentioned example on the determination of cyanide [20] a modified pyrazolone-isonicotinic acid reaction was used for such purposes. Interferences due to Schlieren effects seem not to have been reported in gas diffusion spectrophotometric systems. This is understandable, since the matrix composition of acceptor streams is usually quite uniform, and the refractive index is little affected after absorbing the gaseous analytes. 

Table 16.4 shows the pH range of selected acid-base Indicators. Use this table to help you decide the colour of phenolphthalein and bromocresol green at a pH of... 

Errors (2) and (3) are negligibly small in comparison to error (1) and consequently in the selection of a suitable indicator only the magnitude of the chemical error is of great importance. Thus the appropriate acid-base indicator must have its transition pH range within the equivalence region. 

Tucker SA, Bates HC, and Acree WE (1995) Acid-base indicators — transition colors and pH ranges determined in select aqueous-organic mixed-solvents. Analyst 120 2277-2279. 

See also Buffer Solutions. Extraction Solvent Extraction Principles. Indicators Acid-Base. Ion-Selective Electrodes Glass. Sensors Ovenriew. Quality Assurance Internal Standards. Titrimetry Potentiometric. 

Use a web search engine to find lists of acid-base indicators, (a) Select an indicator that is not mentioned in this chapter and give its color and pH at each end of its transition range, (b) Determine what is meant by the statement Vanilla extract has been described as an olfactory (acid-base) indicator. ... 

Table 18.6 summarizes the use of phenolphthalein and methyl orange as acid-base indicators. The principles described here can he used to select other suitable indicators for acid-base titrations. 

Acid-base indicators show differing colours with varying hydrogen ion concentration in a solution. The change in colour occurs in general over a colour change interval of some two pH units. It is necessary to select indicators for particular titrations which show clear colours at pH values close to those known to hold at the equivalence point. 

Given three acid-base indicators—methyl orange (end point at pH 4), bromthymol blue (end point at pH 7), and phenolphthalein (end point at pH 9)—which would you select for the following acid-base titrations ... 

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