Acid-Base Indicators

<STYLE>P{margin-top:2px;margin-bottom:2px;}</STYLE> Useful Weak Acids and Bases   What is an acid-base indicator? An acid-base indicator is a weak acid or a weak base. The undissociated form of the indicator is a different color than the iogenic form of the indicator. An Indicator does not change color from pure acid to pure alkaline at specific hydrogen ion concentration, but rather, color change occurs over a range of hydrogen ion concentrations. This range is termed the color change interval. It is expressed as a pH range. How is an indicator used? Weak acids are titrated in the presence of indicators which change under slightly alkaline conditions. Weak bases should be titrated in the presence of indicators which change under slightly acidic conditions. What are some common acid-base indicators? Several acid-base indicators are listed below, some more than once if they can be used over multiple pH ranges. Quantity of indicator in aqueous (aq.) or alcohol (alc.) solution is specified. Tried-and-true indicators include: thymol blue, tropeolin OO, methyl yellow, methyl orange, bromphenol blue, bromcresol green, methyl red, bromthymol blue, phenol red, neutral red, phenolphthalein, thymolphthalein, alizarin yellow, tropeolin O, nitramine, and trinitrobenzoic acid. Data in this table are for sodium salts of thymol blue, bromphenol blue, tetrabromphenol blue, bromcresol green, methyl red, bromthymol blue, phenol red, and cresol red. <CENTER> <TABLE border=1> <TBODY> <TR> <TH>Indicator</TH> <TH>pH Range</TH> <TH>Quantity per 10 ml</TH> <TH>Acid</TH> <TH>Base</TH></TR> <TR> <TD>Thymol Blue</TD> <TD>1.2-2.8</TD> <TD>1-2 drops 0.1% soln. in aq.</TD> <TD>red</TD> <TD>yellow</TD></TR> <TR> <TD>Pentamethoxy red</TD> <TD>1.2-2.3</TD> <TD>1 drop 0.1% soln. in 70% alc.</TD> <TD>red-violet</TD> <TD>colorless</TD></TR> <TR> <TD>Tropeolin OO</TD> <TD>1.3-3.2</TD> <TD>1 drop 1% aq. soln.</TD> <TD>red</TD> <TD>yellow</TD></TR> <TR> <TD>2,4-Dinitrophenol</TD> <TD>2.4-4.0</TD> <TD>1-2 drops 0.1% soln. in 50% alc.</TD> <TD>colorless</TD> <TD>yellow</TD></TR> <TR> <TD>Methyl yellow</TD> <TD>2.9-4.0</TD> <TD>1 drop 0.1% soln. in 90% alc.</TD> <TD>red</TD> <TD>yellow</TD></TR> <TR> <TD>Methyl orange</TD> <TD>3.1-4.4</TD> <TD>1 drop 0.1% aq. soln.</TD> <TD>red</TD> <TD>orange</TD></TR> <TR> <TD>Bromphenol blue</TD> <TD>3.0-4.6</TD> <TD>1 drop 0.1% aq. soln.</TD> <TD>yellow</TD> <TD>blue-violet</TD></TR> <TR> <TD>Tetrabromphenol blue</TD> <TD>3.0-4.6</TD> <TD>1 drop 0.1% aq. soln.</TD> <TD>yellow</TD> <TD>blue</TD></TR> <TR> <TD>Alizarin sodium sulfonate</TD> <TD>3.7-5.2</TD> <TD>1 drop 0.1% aq. soln.</TD> <TD>yellow</TD> <TD>violet</TD></TR> <TR> <TD>a-Naphthyl red</TD> <TD>3.7-5.0</TD> <TD>1 drop 0.1% soln. in 70% alc.</TD> <TD>red</TD> <TD>yellow</TD></TR> <TR> <TD>p-Ethoxychrysoidine</TD> <TD>3.5-5.5</TD> <TD>1 drop 0.1% aq. soln.</TD> <TD>red</TD> <TD>yellow</TD></TR> <TR> <TD>Bromcresol green</TD> <TD>4.0-5.6</TD> <TD>1 drop 0.1% aq. soln.</TD> <TD>yellow</TD> <TD>blue</TD></TR> <TR> <TD>Methyl red</TD> <TD>4.4-6.2</TD> <TD>1 drop 0.1% aq. soln.</TD> <TD>red</TD> <TD>yellow</TD></TR> <TR> <TD>Bromcresol purple</TD> <TD>5.2-6.8</TD> <TD>1 drop 0.1% aq. soln.</TD> <TD>yellow</TD> <TD>purple</TD></TR> <TR> <TD>Chlorphenol red</TD> <TD>5.4-6.8</TD> <TD>1 drop 0.1% aq. soln.</TD> <TD>yellow</TD> <TD>red</TD></TR> <TR> <TD>Bromphenol blue</TD> <TD>6.2-7.6</TD> <TD>1 drop 0.1% aq. soln.</TD> <TD>yellow</TD> <TD>blue</TD></TR> <TR> <TD>p-Nitrophenol</TD> <TD>5.0-7.0</TD> <TD>1-5 drops 0.1% aq. soln.</TD> <TD>colorless</TD> <TD>yellow</TD></TR> <TR> <TD>Azolitmin</TD> <TD>5.0-8.0</TD> <TD>5 drops 0.5% aq. soln.</TD> <TD>red</TD> <TD>blue</TD></TR> <TR> <TD>Phenol red</TD> <TD>6.4-8.0</TD> <TD>1 drop 0.1% aq. soln.</TD> <TD>yellow</TD> <TD>red</TD></TR> <TR> <TD>Neutral red</TD> <TD>6.8-8.0</TD> <TD>1 drop 0.1% soln. in 70% alc.</TD> <TD>red</TD> <TD>yellow</TD></TR> <TR> <TD>Rosolic acid</TD> <TD>6.8-8.0</TD> <TD>1 drop 0.1% soln. in 90% alc.</TD> <TD>yellow</TD> <TD>red</TD></TR> <TR> <TD>Cresol red</TD> <TD>7.2-8.8</TD> <TD>1 drop 0.1% aq. soln.</TD> <TD>yellow</TD> <TD>red</TD></TR> <TR> <TD>a-Naphtholphthalein</TD> <TD>7.3-8.7</TD> <TD>1-5 drops 0.1% soln. in 70% alc.</TD> <TD>rose</TD> <TD>green</TD></TR> <TR> <TD>Tropeolin OOO</TD> <TD>7.6-8.9</TD> <TD>1 drop 0.1% aq. soln.</TD> <TD>yellow</TD> <TD>rose-red</TD></TR> <TR> <TD>Thymol blue</TD> <TD>8.0-9.6</TD> <TD>1-5 drops 0.1% aq. soln.</TD> <TD>yellow</TD> <TD>blue</TD></TR> <TR> <TD>Phenolphthalein</TD> <TD>8.0-10.0</TD> <TD>1-5 drops 0.1% soln. in 70% alc.</TD> <TD>colorless</TD> <TD>red</TD></TR> <TR> <TD>a-Naphtholbenzein</TD> <TD>9.0-11.0</TD> <TD>1-5 drops 0.1% soln. in 90% alc.</TD> <TD>yellow</TD> <TD>blue</TD></TR> <TR> <TD>Thymolphthalein</TD> <TD>9.4-10.6</TD> <TD>1 drop 0.1% soln. in 90% alc.</TD> <TD>colorless</TD> <TD>blue</TD></TR> <TR> <TD>Nile blue</TD> <TD>10.1-11.1</TD> <TD>1 drop 0.1% aq. soln.</TD> <TD>blue</TD> <TD>red</TD></TR> <TR> <TD>Alizarin yellow</TD> <TD>10.0-12.0</TD> <TD>1 drop 0.1% aq. soln.</TD> <TD>yellow</TD> <TD>lilac</TD></TR> <TR> <TD>Salicyl yellow</TD> <TD>10.0-12.0</TD> <TD>1-5 drops 0.1% soln. in 90% alc.</TD> <TD>yellow</TD> <TD>orange-brown</TD></TR> <TR> <TD>Diazo violet</TD> <TD>10.1-12.0</TD> <TD>1 drop 0.1% aq. soln.</TD> <TD>yellow</TD> <TD>violet</TD></TR> <TR> <TD>Tropeolin O</TD> <TD>11.0-13.0</TD> <TD>1 drop 0.1% aq. soln.</TD> <TD>yellow</TD> <TD>orange-brown</TD></TR> <TR> <TD>Nitramine</TD> <TD>11.0-13.0</TD> <TD>1-2 drops 0.1% soln in 70% alc.</TD> <TD>colorless</TD> <TD>orange-brown</TD></TR> <TR> <TD>Poirrier's blue</TD> <TD>11.0-13.0</TD> <TD>1 drop 0.1% aq. soln.</TD> <TD>blue</TD> <TD>violet-pink</TD></TR> <TR> <TD>Trinitrobenzoic acid</TD> <TD>12.0-13.4</TD> <TD>1 drop 0.1% aq. soln.</TD> <TD>colorless</TD> <TD>orange-red</TD></TR></TBODY></TABLE></CENTER> Primary References: Lange's Handbook of Chemistry, 8th Edition, Handbook Publishers Inc., 1952. Volumetric Analysis, Kolthoff & Stenge, Interscience Publishers, Inc., New York, 1942 and 1947.