Colour indicators

Check for leaks with soap solution, aqueous ammonia or colour indicator tubes. 

On this basis NH4+ salts can be considered as solvo-acids in liquid NH3 and amides as solvobases. Neutralization reactions can be followed conductimetrically, potentiometrically or even with coloured indicators such as phenolphthalein ... 

Despite this, they are good solvents for chloride-ion transfer reactions, and solvo-acid-solvo-base reactions (p. 827) can be followed conductimetri-cally, voltametrically or by use of coloured indicators. As expected from their constitution, the trihalides of As and Sb are only feeble electron-pair donors (p. 198) but they have marked acceptor properties, particularly towards halide ions (p. 564) and amines. 

In alkaline solution biuret, HN(CONH2)2 reacts with copper(II) sulfate to give a characteristic violet colour due to the formation of the complexes [Cu2(/l-OH)2(NHCONHCONH)4] (Fig. 28.6a) and [Cu(NHCONHCONH)2] . This is the basis of the biuret test in which an excess of NaOH solution is added to the unknown material together with a little CUSO4 soln a violet colour indicates the presence of a protein or other compound containing a peptide linkage. 

Key Colour % indicates preparation but no report of colour) mp/°C (na indicates value not reported) coordination 9 ttp = tricapped trigonal prismatic 8 d = dodecahedral 8 sa = square antiprismatic 8 btp = bicapped trigonal prismatic 8,7 = mixed 8- and 7-coordination (SrBr2 structure) 7 cc = capped octahedral 7 pbp = pentagonal bipyramidal 6 o = octahedral 6 och = octahedral chain, 6 ol = octahedral layered. 

Pipelines should be marked clearly as to the contents. This can be achieved by marking them in accordance with BS 1710, System (contents) and BS 4800, Colours (indicating contents). 

The titrations should be carried out slowly so that the indicator change, which is a time reaction, may be readily detected. If the determinations are to be executed rapidly, the volume of the bromate solution to be used must be known approximately, since ordinarily with irreversible dyestuff indicators there is no simple way of ascertaining when the end point is close at hand. With the highly coloured indicators (xylidine ponceau, fuchsine, or naphthalene black 12B), the colour fades as the end point is approached (owing to local excess of bromate) and another drop of indicator can be added. At the end point the indicator is irreversibly destroyed and the solution becomes colourless or almost so. If the fading of the indicator is confused with the equivalence point, another drop of the indicator may be added. If the indicator has faded, the additional drop will colour the solution if the end point has been reached, the additional drop of indicator will be destroyed by the slight excess of bromate present in the solution. 

To determine the purity of a sample of arsenic(III) oxide follow the general procedure outlined in Section 10.127 but when the 25 mL sample of solution is being prepared for titration, add 25 mL water, 15 mL of concentrated hydrochloric acid and then two drops of indicator solution (xylidine ponceau or naphthalene black 12B see Section 10.125). Titrate slowly with the standard 0.02M potassium bromate with constant swirling of the solution. As the end point approaches, add the bromate solution dropwise with intervals of 2-3 seconds between the drops until the solution is colourless or very pale yellow. If the colour of the indicator fades, add another drop of indicator solution. (The immediate discharge of the colour indicates that the equivalence point has been passed and the titration is of little value.)... 

The filtrate should possess a yellow or orange colour, indicating the presence of excess of precipitant. If a turbidity appears, a portion should be heated if the turbidity disappears, it may be assumed to be due to excess of reagent crystallising out, and is harmless. Otherwise, more reagent should be added, and the solution filtered again. 

Do not rely on the sense of smell to detect hydrogen sulphide leaks. Strips of v/et lead acetate paper turn black on exposure to hydrogen sulphide and offer a simple indicator, as do colour indicator tubes. For plant-scale operations, instrumental multi-point detectors and alarms are likely to be more appropriate. 

Flammable atmospheres can be assessed using portable gas chromatographs or, for selected compounds, by colour indicator tubes. More commonly, use is made of explos-imeters fitted with Pellistors (e.g. platinum wire encased in beads of refractory material). The beads are arranged in a Wheatstone bridge circuit. The flammable gas is oxidized on the heated catalytic element, causing the electrical resistance to alter relative to the reference. Instruments are calibrated for specific compounds in terms of 0—100% of their lower flammable limit. Recalibration or application of correction factors is required for different gases. Points to consider are listed in Table 9.10. 

Lewis (1938) was not content with a purely conceptual view of adds and bases, for he also listed certain phenomenological criteria for an add-base reaction. The process of neutralization is a rapid one, an add or base displaces a weaker add or base from its compounds, adds and bases may be titrated against each other using coloured indicators, and both adds and bases have catalytic effects. 

B. Jaillard, L. Ruiz, and J. C. Arvicu, pH mapping in transparent gel using colour indicator video-densitometry. Plant Soil /83 85 (1996). 

It follows that at half-neutralization pH - pKa, while at A = 1/11 the pH = pjFira - land at A = 10/11 the pH = pKa + 1 in fact this means that the whole titration takes place within 2 pH units, which agrees with the maximum pH range of acid-base colour indicators. 

Kolthoff and Bruckenstein59, considering the determination of Ki and KA of acids and bases, applied a spectrophotometric measurement of the interaction of the acid or base of interest with a well chosen colour indicator base I referring for details to the original papers, we must confine ourselves to a concise explanation for the cases of an acid HX and a base B. 

Debus, Allen George. Sir Thomas Browne and the study of colour indicators. Ambix 10. 

Fig. 1.11 Dissociation ranges of colour indicators for determination of the acidity function H0 in H2S04-H20 mixtures (1) p-nitroaniline, (2) o-nitroaniline, (3) p-chloro-o-nitroaniline, (4) p-nitrodiphenylamine, (5) 2,4-dichloro-6-nitroaniline, (6) p-nitroazobenzene, (7) 2,6-dinitro-4-methylaniline, (8) benzalacetophenone, (9) 6-bromo-2,4-dinitroaniline, (10) anthraquinone, (11) 2,4,6-trinitroaniline. (According to L. P. Hammett and A. J. Deyrup)...

Here, A(BH+) denotes the acid dissociation constant of a basic coloured indicator B (for the theory, see Section 1.4.7). 

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. 

The temperatures of all stars were determined from colour indices, including our own observations and data from the literature. The log g values were derived by fundamental relations using distances from [5], and the metallicities were derived from equivalent widths of Fe II lines. 

Fig. 8.39. Chemo-spectrophotometric evolution of the solar neighbourhood (left) and the whole Milky Way (right) as a function of time. Panels aA show the oxygen and iron abundances, bB the mass of stars and gas and the star formation rate, cC the extinction in B, V and K bands along a line of sight normal to the plane, dD the luminosity in solar units (taking extinction into account), eE the colour indices and fF the supernova rates. Note that panels aA are in linear units (see Fig. 8.16), while the others are all logarithmic. After Boissier and Prantzos (1999).

One important group of colour indicators is derived from 1 10 phenantholine ortho-phenanthroline) which forms a 3 1 complex with iron(II). The complex known as ferroin undergoes a reversible redox reaction accompanied by a distinct colour change... 

---