![Acid and Base Titration](https://upload.wikimedia.org/wikipedia/commons/f/fa/Acid_and_Base_Titration.jpg)
Titration (also known as titrimetry and volumetric analysis) is a common laboratory method of
quantitative
Quantitative may refer to:
* Quantitative research, scientific investigation of quantitative properties
* Quantitative analysis (disambiguation)
* Quantitative verse, a metrical system in poetry
* Statistics, also known as quantitative analysis ...
chemical analysis
Analytical chemistry studies and uses instruments and methods to separate, identify, and quantify matter. In practice, separation, identification or quantification may constitute the entire analysis or be combined with another method. Separati ...
to determine the
concentration
In chemistry, concentration is the abundance of a constituent divided by the total volume of a mixture. Several types of mathematical description can be distinguished: '' mass concentration'', ''molar concentration'', ''number concentration'', an ...
of an identified
analyte
An analyte, component (in clinical chemistry), or chemical species is a substance or chemical constituent that is of interest in an analytical procedure. The purest substances are referred to as analytes, such as 24 karat gold, NaCl, water, etc. ...
(a substance to be analyzed). A
reagent
In chemistry, a reagent ( ) or analytical reagent is a substance or compound added to a system to cause a chemical reaction, or test if one occurs. The terms ''reactant'' and ''reagent'' are often used interchangeably, but reactant specifies a ...
, termed the ''titrant'' or ''titrator'', is prepared as a
standard solution In analytical chemistry, a standard solution is a solution containing a precisely known concentration of an element or a substance. A known mass of solute is dissolved to make a specific volume. It is prepared using a standard substance, such as a p ...
of known concentration and volume. The titrant reacts with a solution of ''analyte'' (which may also be termed the ''titrand'') to determine the analyte's concentration. The volume of titrant that reacted with the analyte is termed the ''titration volume''.
History and etymology
The word "titration" descends from the French word ''titrer'' (1543), meaning the proportion of gold or silver in coins or in works of gold or silver; i.e., a measure of fineness or purity. ''Tiltre'' became ''titre'', which thus came to mean the "fineness of alloyed gold", and then the "concentration of a substance in a given sample". In 1828, the French chemist
Joseph Louis Gay-Lussac
Joseph Louis Gay-Lussac (, , ; 6 December 1778 – 9 May 1850) was a French chemist and physicist. He is known mostly for his discovery that water is made of two parts hydrogen and one part oxygen (with Alexander von Humboldt), for two laws ...
first used ''titre'' as a verb (''titrer''), meaning "to determine the concentration of a substance in a given sample".
Volumetric analysis originated in late 18th-century France.
François-Antoine-Henri Descroizilles
Titration (also known as titrimetry and volumetric analysis) is a common laboratory method of quantitative chemical analysis to determine the concentration of an identified analyte (a substance to be analyzed). A reagent, termed the ''titrant'' ...
(
fr) developed the first burette (which was similar to a graduated cylinder) in 1791. Gay-Lussac developed an improved version of the burette that included a side arm, and invented the terms "
pipette
A pipette (sometimes spelled as pipett) is a laboratory tool commonly used in chemistry, biology and medicine to transport a measured volume of liquid, often as a media dispenser. Pipettes come in several designs for various purposes with diffe ...
" and "
burette
A burette is a graduated glass tube with a tap at one end, for delivering known volumes of a liquid, especially in Titration, titrations. It is a long, graduated glass tube, with a stopcock at its lower end and a tapered capillary tube at the stop ...
" in an 1824 paper on the standardization of indigo solutions. The first true burette was invented in 1845 by the French chemist
Étienne Ossian Henry
Étienne-Ossian Henry (27 November 1798 in Paris – 26 August 1873) was a French chemist, son of Nöel Étienne Henry (1769–1832), and trained by his father, who was director of the Central Pharmacy of the Parisian hospitals and professor ...
(1798–1873). A major improvement of the method and popularization of volumetric analysis was due to
Karl Friedrich Mohr
Karl Friedrich Mohr (November 4, 1806 – September 28, 1879) was a German chemist famous for his early statement of the principle of the conservation of energy. Ammonium iron(II) sulfate, (NH4)2Fe(SO4)2.6H2O, is named Mohr's salt after him.
Life ...
, who redesigned the burette into a simple and convenient form, and who wrote the first textbook on the topic, ''Lehrbuch der chemisch-analytischen Titrirmethode'' (''Textbook of analytical chemistry titration methods''), published in 1855.
Procedure
![Titration for Soil](https://upload.wikimedia.org/wikipedia/commons/e/e3/Titration_for_Soil.JPG)
A typical titration begins with a
beaker or
Erlenmeyer flask
An Erlenmeyer flask, also known as a conical flask (British English) or a titration flask, is a type of laboratory flask which features a flat bottom, a conical body, and a cylindrical neck. It is named after the German chemist Emil Erlenmeyer ...
containing a very precise amount of the analyte and a small amount of
indicator
Indicator may refer to:
Biology
* Environmental indicator of environmental health (pressures, conditions and responses)
* Ecological indicator of ecosystem health (ecological processes)
* Health indicator, which is used to describe the health o ...
(such as
phenolphthalein
Phenolphthalein ( ) is a chemical compound with the chemical formula, formula carbon, C20hydrogen, H14oxygen, O4 and is often written as "HIn", "HPh", "phph" or simply "Ph" in shorthand notation. Phenolphthalein is often used as an indicator in ...
) placed underneath a calibrated
burette
A burette is a graduated glass tube with a tap at one end, for delivering known volumes of a liquid, especially in Titration, titrations. It is a long, graduated glass tube, with a stopcock at its lower end and a tapered capillary tube at the stop ...
or
chemistry pipetting syringe containing the titrant. Small volumes of the titrant are then added to the analyte and indicator until the indicator changes color in reaction to the titrant saturation threshold, representing arrival at the
endpoint
An endpoint, end-point or end point may refer to:
* Endpoint (band), a hardcore punk band from Louisville, Kentucky
* Endpoint (chemistry), the conclusion of a chemical reaction, particularly for titration
* Outcome measure, a measure used as an e ...
of the titration, meaning the amount of titrant balances the amount of analyte present, according to the reaction between the two. Depending on the endpoint desired, single drops or less than a single drop of the titrant can make the difference between a permanent and temporary change in the indicator.
Preparation techniques
Typical titrations require titrant and analyte to be in a liquid (solution) form. Though solids are usually dissolved into an aqueous solution, other solvents such as
glacial acetic acid
Acetic acid , systematically named ethanoic acid , is an acidic, colourless liquid and organic compound with the chemical formula (also written as , , or ). Vinegar is at least 4% acetic acid by volume, making acetic acid the main component ...
or
ethanol
Ethanol (abbr. EtOH; also called ethyl alcohol, grain alcohol, drinking alcohol, or simply alcohol) is an organic compound. It is an Alcohol (chemistry), alcohol with the chemical formula . Its formula can be also written as or (an ethyl ...
are used for special purposes (as in
petrochemistry
Petrochemicals (sometimes abbreviated as petchems) are the chemical products obtained from petroleum by refining. Some chemical compounds made from petroleum are also obtained from other fossil fuels, such as coal or natural gas, or renewable sou ...
, which specializes in petroleum.) Concentrated analytes are often diluted to improve accuracy.
Many non-acid–base titrations require a constant
pH during the reaction. Therefore, a
buffer solution
A buffer solution (more precisely, pH buffer or hydrogen ion buffer) is an aqueous solution consisting of a mixture of a weak acid and its conjugate base, or vice versa. Its pH changes very little when a small amount of strong acid or base is ...
may be added to the titration chamber to maintain the pH.
In instances where two reactants in a sample may react with the titrant and only one is the desired analyte, a separate
masking solution may be added to the reaction chamber which eliminates the effect of the unwanted ion.
Some reduction-oxidation (
redox
Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate (chemistry), substrate change. Oxidation is the loss of Electron, electrons or an increase in the oxidation state, while reduction ...
) reactions may require heating the sample solution and titrating while the solution is still hot to increase the
reaction rate
The reaction rate or rate of reaction is the speed at which a chemical reaction takes place, defined as proportional to the increase in the concentration of a product per unit time and to the decrease in the concentration of a reactant per unit ...
. For instance, the oxidation of some oxalate solutions requires heating to to maintain a reasonable rate of reaction.
Titration curves
A titration curve is a curve in graph the ''x''-coordinate of which represents the volume of
titrant
Titration (also known as titrimetry and volumetric analysis) is a common laboratory method of Quantitative research, quantitative Analytical chemistry, chemical analysis to determine the concentration of an identified analyte (a substance to be ...
added since the beginning of the titration, and the ''y''-coordinate of which represents the concentration of the analyte at the corresponding stage of the titration (in an acid–base titration, the ''y''-coordinate usually represents the pH of the solution).
In an
acid
In computer science, ACID ( atomicity, consistency, isolation, durability) is a set of properties of database transactions intended to guarantee data validity despite errors, power failures, and other mishaps. In the context of databases, a sequ ...
–
base titration, the titration curve represents the strength of the corresponding acid and base. For a strong acid and a strong base, the curve will be relatively smooth and very steep near the equivalence point. Because of this, a small change in titrant volume near the equivalence point results in a large pH change and many indicators would be appropriate (for instance
litmus
Litmus is a water-soluble mixture of different dyes extracted from lichens. It is often absorbed onto filter paper to produce one of the oldest forms of pH indicator, used to test materials for acidity. It is a purple dye that is extracted fro ...
,
phenolphthalein
Phenolphthalein ( ) is a chemical compound with the chemical formula, formula carbon, C20hydrogen, H14oxygen, O4 and is often written as "HIn", "HPh", "phph" or simply "Ph" in shorthand notation. Phenolphthalein is often used as an indicator in ...
or
bromothymol blue
Bromothymol blue (also known as bromothymol sulfone phthalein and BTB) is a pH indicator. It is mostly used in applications that require measuring substances that would have a relatively neutral pH (near 7). A common use is for measuring the pre ...
).
If one reagent is a weak acid or base and the other is a strong acid or base, the titration curve is irregular and the pH shifts less with small additions of titrant near the
equivalence point
The equivalence point, or stoichiometric point, of a chemical reaction is the point at which chemically equivalent quantities of reactants have been mixed. For an acid-base reaction the equivalence point is where the moles of acid and the moles of ...
. For example, the titration curve for the titration between
oxalic acid
Oxalic acid is an organic acid with the systematic name ethanedioic acid and formula . It is the simplest dicarboxylic acid. It is a white crystalline solid that forms a colorless solution in water. Its name comes from the fact that early inve ...
(a weak acid) and
sodium hydroxide
Sodium hydroxide, also known as lye and caustic soda, is an inorganic compound with the formula NaOH. It is a white solid ionic compound consisting of sodium cations and hydroxide anions .
Sodium hydroxide is a highly caustic base and alkali ...
(a strong base) is pictured. The equivalence point occurs between pH 8-10, indicating the solution is basic at the equivalence point and an indicator such as
phenolphthalein
Phenolphthalein ( ) is a chemical compound with the chemical formula, formula carbon, C20hydrogen, H14oxygen, O4 and is often written as "HIn", "HPh", "phph" or simply "Ph" in shorthand notation. Phenolphthalein is often used as an indicator in ...
would be appropriate. Titration curves corresponding to weak bases and strong acids are similarly behaved, with the solution being acidic at the equivalence point and indicators such as
methyl orange
Methyl orange is a pH indicator frequently used in titration because of its clear and distinct color variance at different pH values. Methyl orange shows red color in acidic medium and yellow color in basic medium. Because it changes color at the ...
and
bromothymol blue
Bromothymol blue (also known as bromothymol sulfone phthalein and BTB) is a pH indicator. It is mostly used in applications that require measuring substances that would have a relatively neutral pH (near 7). A common use is for measuring the pre ...
being most appropriate.
Titrations between a weak acid and a weak base have titration curves which are very irregular. Because of this, no definite indicator may be appropriate and a
pH meter
A pH meter is a scientific instrument that measures the hydrogen-ion activity in water-based solutions, indicating its acidity or alkalinity expressed as pH. The pH meter measures the difference in electrical potential between a pH electro ...
is often used to monitor the reaction.
The type of function that can be used to describe the curve is termed a
sigmoid function
A sigmoid function is a mathematical function having a characteristic "S"-shaped curve or sigmoid curve.
A common example of a sigmoid function is the logistic function shown in the first figure and defined by the formula:
:S(x) = \frac = \f ...
.
Types of titrations
There are many types of titrations with different procedures and goals. The most common types of qualitative titration are
acid–base titration
An acid–base titration is a method of quantitative analysis for determining the concentration of an acid or base by exactly neutralizing it with a standard solution of base or acid having known concentration. A pH indicator is used to monitor ...
s and
redox titration
A redox titration is a type of titration based on a redox reaction between the analyte and titrant. It may involve the use of a redox indicator and/or a potentiometer. A common example of a redox titration is treating a solution of iodine with a ...
s.
Acid–base titration
![Acidobazna titracija 004](https://upload.wikimedia.org/wikipedia/commons/1/16/Acidobazna_titracija_004.jpg)
Acid–base titrations depend on the
neutralization between an acid and a base when mixed in solution. In addition to the sample, an appropriate
pH indicator
A pH indicator is a halochromic chemical compound added in small amounts to a solution so the pH (acidity or basicity) of the solution can be determined visually or spectroscopically by changes in absorption and/or emission properties. Hence, ...
is added to the titration chamber, representing the pH range of the equivalence point. The acid–base indicator indicates the endpoint of the titration by changing color. The endpoint and the equivalence point are not exactly the same because the equivalence point is determined by the stoichiometry of the reaction while the endpoint is just the color change from the indicator. Thus, a careful selection of the indicator will reduce the indicator error. For example, if the equivalence point is at a pH of 8.4, then the phenolphthalein indicator would be used instead of Alizarin Yellow because phenolphthalein would reduce the indicator error. Common indicators, their colors, and the pH range in which they change color are given in the table above. When more precise results are required, or when the reagents are a weak acid and a weak base, a
pH meter
A pH meter is a scientific instrument that measures the hydrogen-ion activity in water-based solutions, indicating its acidity or alkalinity expressed as pH. The pH meter measures the difference in electrical potential between a pH electro ...
or a conductance meter are used.
For very strong bases, such as
organolithium reagent
In organometallic chemistry, organolithium reagents are chemical compounds that contain carbon–lithium (C–Li) bonds. These reagents are important in organic synthesis, and are frequently used to transfer the organic group or the lithium atom ...
,
metal amides Metal amides (systematic name metal azanides) are a class of coordination compounds composed of a metal center with amide ligands of the form NR2−. Amide ligands have two electron pairs available for bonding. In principle, they can be terminal or ...
, and
hydride
In chemistry, a hydride is formally the anion of hydrogen( H−). The term is applied loosely. At one extreme, all compounds containing covalently bound H atoms are called hydrides: water (H2O) is a hydride of oxygen, ammonia is a hydride of ...
s, water is generally not a suitable solvent and indicators whose
pKa are in the range of aqueous pH changes are of little use. Instead, the titrant and indicator used are much weaker acids, and anhydrous solvents such as
THF
Tetrahydrofuran (THF), or oxolane, is an organic compound with the formula (CH2)4O. The compound is classified as heterocyclic compound, specifically a cyclic ether. It is a colorless, water- miscible organic liquid with low viscosity. It is ...
are used.
![Faint pink color of Phenolphthalein](https://upload.wikimedia.org/wikipedia/commons/1/18/Faint_pink_color_of_Phenolphthalein.jpg)
The approximate pH during titration can be approximated by three kinds of calculations. Before beginning of titration, the concentration of
+/chem> is calculated in aqueous solution of weak acid before adding any base. When the number of moles of bases added equals the number of moles of initial acid or so called equivalence point
The equivalence point, or stoichiometric point, of a chemical reaction is the point at which chemically equivalent quantities of reactants have been mixed. For an acid-base reaction the equivalence point is where the moles of acid and the moles of ...
, one of hydrolysis and the pH is calculated in the same way that the conjugate bases of the acid titrated was calculated. Between starting and end points, +/chem> is obtained from the Henderson-Hasselbalch equation and titration mixture is considered as buffer. In Henderson-Hasselbalch equation the and are said to be the molarities that would have been present even with dissociation or hydrolysis. In a buffer, +/chem> can be calculated exactly but the dissociation of , the hydrolysis of A- and self-ionization of water must be taken into account. Four independent equations must be used:
:
:
:
: