Acids and Bases Definitions
Introduction to Key Terms & Concepts
There are several methods of defining acids and bases. While these definitions don’t contradict each other, they do vary in how inclusive they are. Antoine Lavoisier, Humphry Davy, and Justus Liebig also made observations regarding acids and bases, but didn’t formalize definitions.
- acids produce H+ ions in aqueous solutions
- bases produce OH- ions in aqueous solutions
- water required, so only allows for aqueous solutions
- only protic acids are allowed; required to produce hydrogen ions
- only hydroxide bases are allowed
Johannes Nicolaus Brønsted – Thomas Martin Lowry
- acids are proton donors
- bases are proton acceptors
- aqueous solutions are permissible
- bases besides hydroxides are permissible
- only protic acids are allowed
Gilbert Newton Lewis
- acids are electron pair acceptors
- bases are electron pair donors
- least restrictive of acid-base definitions
Properties of Acids
- taste sour (don’t taste them!)… the word ‘acid’ comes from the Latin acere, which means ‘sour’
- acids change litmus (a blue vegetable dye) from blue to red
- their aqueous (water) solutions conduct electric current (are electrolytes)
- react with bases to form salts and water
- evolve hydrogen gas (H2) upon reaction with an active metal (such as alkali metals, alkaline earth metals, zinc, aluminum)
Properties of Bases
- taste bitter (don’t taste them!)
- feel slippery or soapy (don’t arbitrarily touch them!)
- bases don’t change the color of litmus; they can turn red (acidified) litmus back to blue
- their aqueous (water) solutions conduct and electric current (are electrolytes)
- react with acids to form salts and water
Examples of Common Acids
- citric acid (from certain fruits and veggies, notably citrus fruits)
- ascorbic acid (vitamin C, as from certain fruits)
- vinegar (5% acetic acid)
- carbonic acid (for carbonation of soft drinks)
- lactic acid (in buttermilk)
Examples of Common Bases
- lye (NaOH)
- household ammonia (aqueous)
Strength of Acids and Bases
Strong electrolytes are completely dissociated into ions in water. The acid or base molecule does not exist in aqueous solution, only ions. Weak electrolytes are incompletely dissociated.
Strong acids completely dissociate in water, forming H+ and an anion. There are six strong acids. The others are considered to be weak acids. You should commit the strong acids to memory:
- HCl – hydrochloric acid
- HNO3 – nitric acid
- H2SO4 – sulfuric acid
- HBr – hydrobromic acid
- HI – hydroiodic acid
- HClO4 – perchloric acid
100% dissociation isn’t true as solutions become more concentrated. If the acid is 100% dissociated in solutions of 1.0 M or less, it is called strong. Sulfuric acid is considered strong only in its first dissociation step.
H2SO4 -> H+ + HSO4-
A weak acid only partially dissociates in water to give H+ and the anion. Examples of weak acids include hydrofluoric acid, HF, and acetic acid, CH3COOH. Weak acids include:
- Molecules that contain an ionizable proton. A molecule wih a formula starting with H usually is an acid.
- Organic acids containing one or more carboxyl group, -COOH. The H is ionizable.
- Anions with an ionizable proton. (e.g., HSO4- –> H+ + SO42-)
- transition metal cations
- heavy metal cations with high charge
- NH4+ dissociates into NH3 + H+
Strong bases dissociate 100% into the cation and OH- (hydroxide ion). The hydroxides of the Group I and Group II metals usually are considered to be strong bases.
- LiOH – lithium hydroxide
- NaOH – sodium hydroxide
- KOH – potassium hydroxide
- RbOH – rubidium hydroxide
- CsOH – cesium hydroxide
- *Ca(OH)2 – calcium hydroxide
- *Sr(OH)2 – strontium hydroxide
- *Ba(OH)2 – barium hydroxide
* These bases completely dissociate in solutions of 0.01 M or less. The other bases make solutions of 1.0 M and are 100% dissociated at that concentration. There are other strong bases than those listed, but they are not often encountered.
Examples of weak bases include ammonia, NH3, and diethylamine, (CH3CH2)2NH.
- Most weak bases are anions of weak acids.
- Weak bases do not furnish OH- ions by dissociation. Instead, they react with water to generate OH- ions.
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.
|Indicator||pH Range||Quantity per 10 ml||Acid||Base|
|Thymol Blue||1.2-2.8||1-2 drops 0.1% soln. in aq.||red||yellow|
|Pentamethoxy red||1.2-2.3||1 drop 0.1% soln. in 70% alc.||red-violet||colorless|
|Tropeolin OO||1.3-3.2||1 drop 1% aq. soln.||red||yellow|
|2,4-Dinitrophenol||2.4-4.0||1-2 drops 0.1% soln. in 50% alc.||colorless||yellow|
|Methyl yellow||2.9-4.0||1 drop 0.1% soln. in 90% alc.||red||yellow|
|Methyl orange||3.1-4.4||1 drop 0.1% aq. soln.||red||orange|
|Bromphenol blue||3.0-4.6||1 drop 0.1% aq. soln.||yellow||blue-violet|
|Tetrabromphenol blue||3.0-4.6||1 drop 0.1% aq. soln.||yellow||blue|
|Alizarin sodium sulfonate||3.7-5.2||1 drop 0.1% aq. soln.||yellow||violet|
|a-Naphthyl red||3.7-5.0||1 drop 0.1% soln. in 70% alc.||red||yellow|
|p-Ethoxychrysoidine||3.5-5.5||1 drop 0.1% aq. soln.||red||yellow|
|Bromcresol green||4.0-5.6||1 drop 0.1% aq. soln.||yellow||blue|
|Methyl red||4.4-6.2||1 drop 0.1% aq. soln.||red||yellow|
|Bromcresol purple||5.2-6.8||1 drop 0.1% aq. soln.||yellow||purple|
|Chlorphenol red||5.4-6.8||1 drop 0.1% aq. soln.||yellow||red|
|Bromphenol blue||6.2-7.6||1 drop 0.1% aq. soln.||yellow||blue|
|p-Nitrophenol||5.0-7.0||1-5 drops 0.1% aq. soln.||colorless||yellow|
|Azolitmin||5.0-8.0||5 drops 0.5% aq. soln.||red||blue|
|Phenol red||6.4-8.0||1 drop 0.1% aq. soln.||yellow||red|
|Neutral red||6.8-8.0||1 drop 0.1% soln. in 70% alc.||red||yellow|
|Rosolic acid||6.8-8.0||1 drop 0.1% soln. in 90% alc.||yellow||red|
|Cresol red||7.2-8.8||1 drop 0.1% aq. soln.||yellow||red|
|a-Naphtholphthalein||7.3-8.7||1-5 drops 0.1% soln. in 70% alc.||rose||green|
|Tropeolin OOO||7.6-8.9||1 drop 0.1% aq. soln.||yellow||rose-red|
|Thymol blue||8.0-9.6||1-5 drops 0.1% aq. soln.||yellow||blue|
|Phenolphthalein||8.0-10.0||1-5 drops 0.1% soln. in 70% alc.||colorless||red|
|a-Naphtholbenzein||9.0-11.0||1-5 drops 0.1% soln. in 90% alc.||yellow||blue|
|Thymolphthalein||9.4-10.6||1 drop 0.1% soln. in 90% alc.||colorless||blue|
|Nile blue||10.1-11.1||1 drop 0.1% aq. soln.||blue||red|
|Alizarin yellow||10.0-12.0||1 drop 0.1% aq. soln.||yellow||lilac|
|Salicyl yellow||10.0-12.0||1-5 drops 0.1% soln. in 90% alc.||yellow||orange-brown|
|Diazo violet||10.1-12.0||1 drop 0.1% aq. soln.||yellow||violet|
|Tropeolin O||11.0-13.0||1 drop 0.1% aq. soln.||yellow||orange-brown|
|Nitramine||11.0-13.0||1-2 drops 0.1% soln in 70% alc.||colorless||orange-brown|
|Poirrier’s blue||11.0-13.0||1 drop 0.1% aq. soln.||blue||violet-pink|
|Trinitrobenzoic acid||12.0-13.4||1 drop 0.1% aq. soln.||colorless||orange-red|
Lange’s Handbook of Chemistry, 8th Edition, Handbook Publishers Inc., 1952.
Volumetric Analysis, Kolthoff & Stenge, Interscience Publishers, Inc., New York, 1942 and 1947.
Formulas of Common Acids & Bases
Binary Acids, Ternary Acids, and Bases
Here are the names and formulas of some of the common acids and bases.
A binary compound consists of two elements. Binary acids have the prefix hydro in front of the full name of the nonmetallic element. They have the ending -ic. Examples include hydrochloric and hydrofluoric acid.
Hydrofluoric Acid – HF
Hydrochloric Acid – HCl
Hydrobromic Acid – HBr
Hydroiodic Acid – HI
Hydrosulfuric Acid – H2S
Ternary acids commonly contain hydrogen, a nonmetal, and oxygen. The name of the most common form of the acid consists of the nonmetal root name with the -ic ending, The acid containing one less oxygen atom than the most common form is designated by the -ous ending. An acid containing one less oxygen atom than the -ous acid has the prefix hypo- and the -ous ending. The acid containing one more oxygen than the most common acid has the per- prefix and the -ic ending.
Nitric Acid – HNO3
Nitrous Acid – HNO2
Hypochlorous Acid – HClO
Chlorous Acid – HClO2
Chloric Acid – HClO3
Perchloric Acid – HClO4
Sulfuric Acid – H2SO4
Sulfurous Acid – H2SO3
Phosphoric Acid – H3PO4
Phosphorous Acid – H3PO3
Carbonic Acid – H2CO3
Acetic Acid – HC2H3O2
Oxalic Acid – H2C2O4
Boric Acid – H3BO3
Silicic Acid – H2SiO3
Sodium Hydroxide – NaOH
Potassium Hydroxide – KOH
Ammonium Hydroxide – NH4OH
Calcium Hydroxide – Ca(OH)2
Magnesium Hydroxide – Mg(OH)2
Barium Hydroxide – Ba(OH)2
Aluminum Hydroxide – Al(OH)3
Ferrous Hydroxide or Iron (II) Hydroxide – Fe(OH)2
Ferric Hydroxide or Iron (III) Hydroxide – Fe(OH)3
Zinc Hydroxide – Zn(OH)2
Lithium Hydroxide – LiOH
Chemistry of Neutralization and Hydrolysis
HA + BOH –> BA + H2O
Depending on the solubility of the salt, it may remain in ionized form in the solution or it may precipitate out of solution. Neutralization reactions usually proceed to completion.
The reverse of the neutralization reaction is called hydrolysis. In a hydrolysis reaction a salt reacts with water to yield the acid or base:
BA + H2O –> HA + BOH
More specifically, there are four combinations of strong and weak acids and bases:
- strong acid + strong base, e.g., HCl + NaOH –> NaCl + H2OWhen strong acids and strong bases react, the products are salt and water. The acid and base neutralize each other, so the solution will be neutral (pH=7) and the ions that are formed will not reaction with the water.
- strong acid + weak base, e.g., HCl + NH3 —> NH4ClThe reaction between a strong acid and a weak base also produces a salt, but water is not usually formed because weak bases tend not to be hydroxides. In this case, the water solvent will react with the cation of the salt to reform the weak base. For example:HCl (aq) + NH3 (aq) <–> NH4+ (aq) + Cl- while
NH4- (aq) + H2O <–> NH3 (aq) + H3O+ (aq)
- weak acid + strong base, e.g., HClO + NaOH –> NaClO + H2OWhen a weak acid reacts with a strong base the resulting solution will be basic. The salt will be hydrolyzed to form the acid, together with the formation of the hydroxide ion from the hydrolyzed water molecules.
- weak acid + weak base, e.g., HClO + NH3 <–> NH4ClOThe pH of the solution formed from the reaction of a weak acid with a weak base depends on the relative strengths of the reactants. For example, if the acid HClO has a Ka of 3.4 x 10-8 and the base NH3 has a Kb = 1.6 x 10-5, then the aqueous solution of HClO and NH3 will be basic because the Ka of HClO is less than the Ka of NH3.
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Determine the Molarity of an Acid or Base
What Is Titration
Titration is a procedure used in chemistry in order to determine the molarity of an acid or a base. A chemical reaction is set up between a known volume of a solution of unknown concentration and a known volume of a solution with a known concentration. The relative acidity (basicity) of an aqueous solution can be determined using the relative acid (base) equivalents. An acid equivalent is equal to one mole of H+ or H3O+ ions. Similarly, a base equivalent is equal to one mole of OH- ions. Keep in mind, some acids and bases are polyprotic, meaning each mole of the acid or base is capable of releasing more than one acid or base equivalent. When the solution of known concentration and the solution of unknown concentration are reacted to the point where the number of acid equivalents equals the number of base equivalents (or vice versa), the equivalence point is reached. The equivalence point of a strong acid or a strong base will occur at pH 7. For weak acids and bases, the equivalence point need not occur at pH 7. There will be several equivalence points for polyprotic acids and bases.
How to Estimate the Equivalence Point
There are two common methods of estimating the equivalence point:
- Use a pH Meter
For this method, a graph is made plotting the pH of the solution as a function of the volume of added titrant.
- Use an Indicator
This method relies on observing a color change in the solution. Indicators are weak organic acids or bases that are different colors in their dissociated and undissociated states. Because they are used in low concentrations, indicators do not appreciably alter the equivalence point. The point at which the indicator changes color is called the end point. For a properly performed titration, the volume difference between the end point and the equivalence point is small. Sometimes the volume difference (error) is ignored; in other cases a correction factor may be applied. The volume added to achieve the end point may be calculated using this formula:VANA = VBNB
where V is volume, N is normality, A is acid, and B is base.
Home and Garden pH Indicators
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Common Household Items
There are many common household products and garden plants that can be used as pH indicators. Most plants contain pH sensitive anthocyanins, so experiment with other plants, too.
A very basic solution will change the color of beets or beet juice from red to purple.
- ‘Black’ Berries
Blackberries, black currants, and black raspberries change from red in an acidic environment to blue or violet in a basic environment.
Blueberries are blue around pH 2.8-3.2, but turn red as the solution becomes even more acidic.
Cherries and their juice are red in an acidic solution, but turn blue to purple in a basic solution.
- Curry Powder
Curry contains the pigment curcumin, which changes from yellow at pH 7.4 to red at pH 8.6.
- Delphinium Petals
The anthocyanin delphinin changes from bluish-red in an acidic solution to violet blue in a basic solution.
- Geranium Petals
Geraniums contain the anthocyanin pelargonin, which changes from orange-red in an acidic solution to blue in a basic solution.
Red and purple grapes contain multiple anthocyanins. Blue grapes contain a monoglucoside of malvinidin which changes from deep red in an acidic solution to violet in a basic solution.
- Horse Chestnut Leaves
Soak horse chestnut leaves in alcohol to extract the fluorescent dye esculin. Esculin is colorless at pH 1.5 but becomes fluorescent blue at pH 2. Get the best effect by shining a black light on the indicator.
- Morning Glories
Morning glories contain a pigment named ‘heavenly blue anthocyanin’ which changes from purplish-red at pH 6.6 to blue at pH 7.7.
Onions are olfactory indicators. You don’t smell onions in strongly basic solutions. Red onion also changes from pale red in an acidic solution to green in a basic solution.
- Pansy Petals
- Petunia Petals
The anthocyanin petunin changes from reddish-purple in an acidic solution to violet in a basic solution.
- Poison Primrose
Primula sinensis has orange or blue flowers. The orange flowers contain a mixture of pelargonins (see Geranium). The blue flowers contain malvin, which turns from red to purple as a solution goes from acidic to basic.
- Poppy Petals
- Purple Peonies
Peonin changes from reddish-purple or magenta in an acidic solution to deep purple in basic solution.
- Red (Purple) Cabbage
Red cabbage contains a mixture of pigments used to indicate a wide pH range.
- Red Radish
- Rose Petals
The oxonium salt of cyanin turns from red to blue in basic solution.
- Thyme Extract in Alcohol
This spice contains a yellow pigment, curcumin, which changes from yellow at pH 7.4 to red at pH 8.6.
- Tulip Petals
- Violet Petals
- Baking Soda (NaHCO3)
Baking soda will fizz when added to an acidic solution (such as vinegar), but will not fizz in an alkaline solution. The reaction doesn’t readily reverse itself, so baking soda can be used to test a solution, but can’t be ‘reused’. The reaction is:HCO3-(aq) + H+(aq) = H2O(l) + CO2(g)
- Colorchange Lipstick
You’ll need to test your colorchange lipstick to determine its pH range, but most cosmetics that change color respond to changes in pH (different from cosmetics that change color according to angle of light).
- ExLax Tablets
The tablets contain contain phenolphthalein, which is a pH indicator that is colorless in solutions more acidic than pH 8.3 and pink to deep red at solutions more basic than pH 9.
- Vanilla Extract
Vanilla extract is an olfactory indicator. You can’t smell the characteristic scent at high pH (alkaline solution) because the molecule is in its ionic form.
- Washing Soda
As with baking soda, washing soda fizzes in an acidic solution but not in a basic solution.