INDICATORS SMK Negeri 13 Bandung

Teknologi dan Rekayasa I. General Consideration  Definition An indicator may be defined as a substance which presence during a titration renders the end-point visible

Teknologi dan Rekayasa ● The indicator effect may consist of : 1.The colour change of a solution at or near equivalence-point 2.The formation of a precipitate in a clear solution or the clearing up of a turbidity 3.The formation of a disappearence of a coloured precipitate 4.The colour change of a precipitate formed during titration

Teknologi dan Rekayasa ☼ Classification Indicators may be classified according to the type of reaction in which they are to be used ♦ The grouping of indicators : a. Indicators for acid-base reactions b. Indicators for precipitate reactions c. Indicators for oxidation-reduction reactions

Teknologi dan Rekayasa II. Indicators for Acid-Base Reaction  Indicators for neutralization titration reac specifically to hydogen / hydroxyl ions  Organic compounds, behave as weak acids / bases having different colour in the undissociated condition from that which they have in the form of their ion  An indicator, regarded as an acid, exist in a state of equilibrium with a certain concentration of its ions in solution, according to the reaction:

Teknologi dan Rekayasa HIn ↔ H + + In - ◘ In this equation, HIn represents the acid form which has the “acid colour” While In - detones the alkaline form, which has the “alkaline colour” ◘ If the indicator is a base, the same considerations apply as for an indicator acid InOH ↔ In + + OH -

Teknologi dan Rekayasa * In the table 1 are listed the transition intervals and colours of important indicators

Teknologi dan Rekayasa ♪ Selection of Indicators ♠ The selection of an indicator to be used in a titration is based upon the neutralization curve of the substance being determined ♣ In principle it is advisable to choose an indicator which transition interval will include the pH at the equivalence-point

Teknologi dan Rekayasa ♪ Selection of Indicators  If the pH change at this point is very large, several indicators will have transition intervals within the proper range  For example: the titration of a strong acid with a strong base, the equivalence-point pH is 7, so that any indicators changing near this value may be used

Teknologi dan Rekayasa T ABLE T RANSITION I NTERVALS AND C OLOUR C HANGES OF A CID- B ASE I NDICATORS Indicator Interval in pHColour AcidAlkaline Thymol blue1.2 – 2.8redyellow Pentamethoxy red1.2 – 3.2Red-violetcolourless 2,4-Dinitrophenol2.4 – 4.0colourlessyellow Methyl yellow2.9 – 4.0redyellow Methyl orange3.1 – 4.4redorange Bromophenol blue3.0 – 4.6yellowblue-violet α - naphtyl red3.7 – 5.0redyellow Bromcresol green4.0 – 5.6yellowblue Methyl red4.4 – 6.2redyellow Bromcresol purple5.2 – 6.8yellowpurple Bromthymol blue6.2 – 7.6yellowblue p- nitrophenol5.0 – 7.0colourlessyellow Phenol red6.4 – 8.0yellowred Cresol red7.2 – 8.8yellowred α- naphtholphthalein7.3 – 8.7rosegreen Thymol blue8.0 – 9.6yellowblue Phenolphthalein8.0 – 10.0colourlessred α naptholbenzein9.0 – 11.0yellowblue Alizarin yellow10.0 – 12.0yellowlilac Salicyl yellow10.0 – 12.0yellowOrange-brown Diazo violet10.1 – 12.0yellowviolet Nitramine11.0 – 13.0yellowOrange-brown