See also:

• INDICATOR (from Lat. indicare, to point out)

INDICATOR (from See also:

• LAT

Lat. indicare, to point out)  , that which points out or records . In See also:

• ENGINEERING

engineering, the word is specifically given to a See also:

• MECHANICAL

mechanical See also:

• DEVICE

device for registering the pressure of the working fluid in an See also:

• ENGINE
• ENGINE (Lat. ingenium)

engine See also:

• CYLINDER (Gr. KvAwSpos, from KvXivaety, to roll)

cylinder during a stroke of the See also:

• PISTON (through Fr. from Ital. pistone or pestone, a great pestle, from Late Lat. pistare, to pound, a frequentative form of classical Lat. pinsere)

piston, the See also:

• RECORD (Lat. recordari, to recall to mind, from cor, heart or mind)

record so provided being termed the " See also:

• INDICATOR (from Lat. indicare, to point out)

indicator See also:

• DIAGRAM

diagram " (see See also:

• STEAM
• STEAM (0. Eng. steam, vapour, smoke, cf. Du. steam; the origin is unknown)

STEAM-ENGINE) . In See also:

• CHEMISTRY
• CHEMISTRY (formerly "chymistry"; Gr. xvµela; for derivation see ALCHEMY)

chemistry, the word is generically applied to re-agents or chemicals which detect usually small quantities or traces of other substances; it is, however, more customarily restricted to re-agents which show whether a methods, and it is therefore a See also:

• SIMPLE

simple See also:

• MATTER

matter to prepare solutions of definite ionic concentrations and to test these with the See also:

• OBJECT

object of obtaining a See also:

• LIST (O.E. lisle, a Teutonic word, cf. Dut. lust, Ger. Leiste, adapted in Ital. lista and Fr. lisle)
• LIST, FRIEDRICH (1789-1846)

list of indicators according to their sensitiveness . It is found that See also:

• LITMUS (apparently a corruption of lacmus, Dutch lacmoes, lac, lac, and moes, pulp, due to association with " lit," an obsolete word for dye, colour; the Ger. equivalent is Lac/emus, Fr tournesol)

litmus responds to concentrations of 10-6H. and to-60H', a result which shows this dye to be the best indicator of true See also:

• NEUTRALITY

neutrality . Methyl See also:

• ORANGE
• ORANGE (Citrus Aurantium)
• ORANGE, HOUSE OF

orange responds to between io-4H• and io 6H•; See also:

• PARA
• PARA (officially BELEM; sometimes BELEM DO PARA)

para-nitrophenol to between io-6H• and io-6H•; and See also:

• PHENOLPHTHALEIN

phenolphthalein to between to-'OH' and to-60H' Salm (Zeit . Elektrochent., 1904, 10, P . 341) gives a Iist of twenty-seven indicators classified on this principle . Other papers bearing on this subject are Friedenthal, ibid., p . 113; Salessky, ibid., p . 204; Fels, ibid., p . 208; Scholtz, ibid., p . 549; M .

Handa, Ber., 1909, 42, p . 3179 . The actual mechanism by which the indicator changes See also:

• COLOUR (Lat. color, connected with celare, to hide, the root meaning, therefore, being that of a covering)

colour with varying concentrations of hydrion or hydroxidion is now to be considered . Ostwald formulated his ionization theory which assumes the See also:

• CHANGE (derived through the Fr. from the Late Lat. cambium, cambiare, to barter; the ultimate derivation is probably from the root which appears in the Gr. «a urmu', to bend)

change to be due to the transition of the non-dissociated indicator to the ionized See also:

• CONDITION (Lat. condicio, from condicere, to agree upon, arrange; not connected with conditio, from condere, conditum, to put together)

condition, which are necessarily of different See also:

• COLOURS, MILITARY

colours . On this theory, an indicator must be weakly basic or See also:

• ACID (from the Lat. root ac-, sharp; acere, to be sour)

acid, for if it were a strong acid or See also:

• BASE

base high See also:

• DISSOCIATION

dissociation would occur when it was in the See also:

• FREE

free See also:

• STATE
• STATE, GREAT OFFICERS OF

state, and there would be no change of colour when the See also:

• SOLUTION (from Lat. solvere, to loosen, dissolve)

solution was neutralized . Take the See also:

• CASE
• CASE, JOHN (d. 1600)

case of a weakly acid indicator such as phenolphthalein . The presence of an acid depresses the very slight dissociation of the indicator, and the colour of the solution is that of the non-dissociated See also:

• MOLECULE (from mod. Lat. molecula, the diminutive of moles, a mass)

molecule . The addition of an See also:

• ALKALI

alkali, if it be strong, brings about the formation of a See also:

• SALT (a common Teutonic word, cf. Dutch zout, Ger. Salz, Scand. salt; cognate with Gr. &Xs, Lat. sal)
• SALT, SIR TITUS, BART

salt of phenolphthalein, which is readily ionized, and so reveals the intense red coloration of the anion; a weak base, however, fails to give free ions . An acid indicator of See also:

• MEDIUM

medium strength is methyl orange . When free this substance is ionized and the solution shows an orange colour, due to a mixing of the red of the non-dissociated molecule and the yellow of the ionized molecule . Addition of hydrions lessens the dissociation and the solution assumes the red colour, while a base increases the dissociation and so brings about the yellow colour . If the alkaline solution be titrated with a strong acid, the hydrions See also:

• PRESENT

present in a very small amount of the acid suffices to See also:

• REVERSE

reverse the colour; a weak acid, however, must he added in considerable excess of the quantity properly required to neutralize the solution, owing to its weak dissociation .

This indicator is therefore only useful when strong acids are being dealt with, while its strongly acid nature renders it serviceable for both strong and weak bases . It seems, however, that in addition to a change in the ionic condition of an indicator, there are cases where the coloration is associated with tautomeric change . For example, J . T . See also:

• HEWITT, ABRAM STEVENS (1822-1903)

Hewitt (See also:

• ANALYST

Analyst, 1908, 33, p . 85) regards phenolphthalein and similar indicators as obeying the following See also:

• EQUILIBRIUM (from the Lat. aequus, equal, and libra, a balance)

equilibrium in solution, 0: X„•HX,•O•H-<=>_X„•O'+H•, X„ and X„ being isomeric . This indicates the presence of two tautomeric forms, one being of a quinonoid structure, and an ionized molecule . A similar view is advanced by A . Hantzsch and F . -Hilscher (Ber., 1908, 41, p . 1187) who find that helianthin is quinonoid when solid, whilst in solution there is an equilibrium between an aminoazo- and sulphonic acid-See also:

• FORM (Lat. forma)

form; on the other See also:

• HAND
• HAND (a word common to Teutonic languages; cf. Ger. Hand, Goth. handus)
• HAND, FERDINAND GOTTHELF (1786-185r)

hand, the See also:

• SODIUM [symbol Na, from Lat. natrium; atomic weight 23.00 (0=16)]

sodium salt, methyl orange, is a sulphonate under both conditions .