Acid-base indicators colours

Used as acid-base indicator (colour change rose yellow). Cryst. 

Used as 0.1% MeOH soln. as an acid-base indicator (colour change pink -> yellow). Cryst. Sol. MeOH si. sol. H2O. p i 6.15 (H2O). 

Used as acid-base indicator (colour change red - yellow pH range 3.8-5.8). Orange cryst. (H2O). Sol. EtOH, Mc2CO mod. sol. H2O. 

Acid-base indicator (colour change red - blue) redox indicator. Used as 0.02% aq. soln. of Na salt. Dark green cryst. powder. Sol. H2O, EtOH, MejCO. pK 5.7. E° -b 0.668 V (30°). 

Isol. from logwood under alkaline oxidn. conds. Used as 0.03% soln. in aq. MeOH for photometric detn. of B (2, 505 nm, e 23000), Hf (2, 520 nm, e 58000), Ga, Sn. Acid-base indicator (colour change yellow - red purple) biological stain. Brownish-red needles (EtOH). Mp 250° dec. 

Redox indicator acid-base indicator (colour change brownish-red blue). Used as a 0.02% aq. soln. Red cryst. Sol. H2O. pA j 8.7. E° +0.544 V. 

Used as an acid-base indicator (colour change yellow blue pH range 6.0-7.6). Dark red cryst. powder. Sol. EtOH, Me2CO, EtOAc, alkalis spar. sol. H2O. 

A large number of substances, called neutralisation or acid-base indicators, change colour according to the hydrogen-ion concentration of the solution. The... 

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. 

Calmagite. This indicator, l-(l-hydroxyl-4-methyl-2-phenylazo)-2-naphthol-4-sulphonic acid, has the same colour change as solochrome black, but the colour change is somewhat clearer and sharper. An important advantage is that aqueous solutions of the indicator are stable almost indefinitely. It may be substituted for solochrome black without change in the experimental procedures for the titration of calcium plus magnesium (see Sections 10.54 and 10.62). Calmagite functions as an acid-base indicator ... 

Bromopyrogallol red. This metal ion indicator is dibromopyrogallol sulphon-phthalein and is resistant to oxidation it also possesses acid-base indicator properties. The indicator is coloured orange-yellow in strongly acidic solution, claret red in nearly neutral solution, and violet to blue in basic solution. The dyestuff forms coloured complexes with many cations. It is valuable for the determination, for example, of bismuth (pH = 2-3. nitric acid solution endpoint blue to claret red). 

Scheme 10.1 Colour change in the protonation of methyl orange 6241k an acid-base indicator...

The interest in colour indicators has recently increased as they are used for the direct determination of pH (acid-base indicators) and free calcium ions (fluorescent derivatives based on the calcium chelator EGTA as metallochromic indicators) in biological systems at cellular level. 

According to the early view of Ostwald, acid-base indicators are weak acids or bases, the undissociated form of which differs in colour from the ionic form. For example, the molecule of an indicator HI dissociates in water according to the equation... 

Table 5.1 summarizes the details of some useful acid-base indicators. Exact agreement with the pH range expressed by equation (5.5) is by no means always observed. This is because some colour changes are easier to see than others and so the general approximation made in deriving equation (5.5) is not uniformly close. Structurally, the indicators form three groups phthaleins (e g. phenolphthalein) sulphonephthaleins (e.g. phenol red) and azo compounds (e.g. methyl orange). 

Figure 6.6 Anthrocyanidins impart colour to many natural substances, such as strawberries and cherries. The choice of side chains can cause a huge change in the anthrocyanidin s colour. If the side chain is pH sensitive then the anthrocyanidin acts as an acid-base indicator structures of an anthrocyanidin at three pHs (red in high acidity and low pH, blue in low acidity and high pH and mauve in inter-midiate pHs)...

Litmus was probably the most popular choice of acid-base indicator, but it is not a good choice for colour-blind chemists. The use of phenolphthalein as an acid-base indicator comes a close second. Phenolphthalein (XIII) is another weak organic acid. It is not particularly water soluble, so we generally dissolve it in aqueous ethanol. The ethanol explains the pleasant, sweet smell of phenolphthalein solutions. 

In a solution containing more than 99.9% of violet to blue 77sem ( sem lO - 10 cf. 6.1.) a yellow to red colour is immediately produced by excess acid. With base the colour of 71 sem is so inunediately restored, a phenomenon which resembles that of a normal acid-base indicator. UV-spectra, however, discloK an acid-driven disproportionation which transforms 77sem quantitatively into 77qx and and 77 red H or 77 red 2H respectively. If the free azine 77 red is reformed by base it comproportionates with 77ox. buildup up the original concentration of... 

For the titration of a strong base with a weak acid, the equivalence point is reached when the pH is greater than 7. The half equivalence point is when half of the total amount of base needed to neutralize the acid has been added. It is at this point that the pH = pK of the weak acid. In acid-base titrations, a suitable acid-base indicator is used to detect the endpoint from the change of colour of the indicator used. An acid-base indicator is a weak acid or a weak base. The following table contains the names and the pH range of some commonly used acid-base indicators. 

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]. 

For most neutralization reactions, there are no visible signs that a reaction is occurring. How can you determine that a neutralization reaction is taking place One way is to use an acid-base indicator. This is a substance that changes colour in acidic and basic solutions. Most acid-base indicators are weak, monoprotic acids. The undissociated weak acid is one colour. Its conjugate base is a different colour. 

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