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Saturday, March 21, 2009 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.

pH of salts formed from reactions of acids & bases	Strong Base	Weak Base
Strong Acid	pH = 7	pH <>
Weak Acid	pH > 7	pH = 7

http://www.ausetute.com.au/indicata.html

Examples of Indicators:

pH	0	1	2	3	4	5	6	7	8	9	10	11	12	13	14	pH range
	Acid							neutral	Base
[H⁺] (mol/L)	10⁰	10^-1	10^-2	10^-3	10^-4	10^-5	10^-6	10^-7	10^-8	10^-9	10^-10	10^-11	10^-12	10^-13	10^-14
Universal indicator	red	red	orange- red	orange	pale orange	orange- yellow	pale yellow	green- yellow	green	dark- green	blue	blue	blue	blue	blue
cyanidin (red cabbage water)	red	red	red	cerise	purple	blue	blue	blue	aqua- marine	emerald- green	lime	lime	yellow	yellow	yellow
blue litmus indicator	red	red	red	red	red	red	red	blue	blue	blue	blue	blue	blue	blue	blue	5.0 - 8.0
red litmus indicator	red	red	red	red	red	red	red	red	blue	blue	blue	blue	blue	blue	blue	5.0 - 8.0
phenol- phthalein indicator	colour -less	colour -less	colour -less	colour -less	colour -less	colour -less	colour -less	colour -less	pink	pink	pink	pink	pink	pink	pink	8.3 - 10.0
thymol blue indicator	yellow	yellow	yellow	yellow	yellow	yellow	yellow	yellow	yellow	blue	blue	blue	blue	blue	blue	8.0 - 9.6
phenol red indicator	yellow	yellow	yellow	yellow	yellow	yellow	yellow	yellow	red	red	red	red	red	red	red	6.8 - 8.4
bromo- thymol blue indicator	yellow	yellow	yellow	yellow	yellow	yellow	yellow	blue	blue	blue	blue	blue	blue	blue	blue	6.2 - 7.6
methyl red indicator	pink	pink	pink	pink	pink	pink	yellow	yellow	yellow	yellow	yellow	yellow	yellow	yellow	yellow	4.4 - 6.0
bromo- cresol green indicator	yellow	yellow	yellow	yellow	yellow	pale blue- green	blue- green	blue- green	blue- green	blue- green	blue- green	blue- green	blue- green	blue- green	blue- green	3.8 - 5.4
methyl orange indicator	red	red	red	red	yellow	yellow	yellow	yellow	yellow	yellow	yellow	yellow	yellow	yellow	yellow	3.1 - 4.4
bromo- phenol blue	yellow	yellow	yellow	yellow	blue	blue	blue	blue	blue	blue	blue	blue	blue	blue	blue	3.0 - 4.6
cresol red	red	red	red	yellow	yellow	yellow	yellow	yellow	yellow	yellow	yellow	yellow	yellow	yellow	yellow	0.2 - 1.8
pH	0	1	2	3	4	5	6	7	8	9	10	11	12	13	14	pH range

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