Editing Titration (section)

==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]]s and [[redox titration]]s.

===Acid–base titration===
{{Main|Acid–base titration}}
[[File:Acidobazna titracija 004.jpg|thumbnail|Methyl orange]]
{|border="2" cellpadding="5" align="center" style="text-align: center;" class=wikitable
|-
!style="background:#efefef;"|Indicator
!style="background:#efefef;"|Color on acidic side
!style="background:#efefef;"|Range of color change<br>(pH)
!style="background:#efefef;"|Color on basic side
|-
!style="background:#efefef;"|[[Methyl violet]]
| Yellow || 0.0—1.6 || Violet
|-
!style="background:#efefef;"|[[Bromophenol blue]]
| Yellow || 3.0—4.6 || Blue
|-
!style="background:#efefef;"|[[Methyl orange]]
| Red || 3.1—4.4 || Yellow
|-
!style="background:#efefef;"|[[Methyl red]]
| Red || 4.4—6.3 || Yellow
|-
!style="background:#efefef;"|[[Litmus]]
| Red || 5.0—8.0 || Blue
|-
!style="background:#efefef;"|[[Bromothymol blue]]
| Yellow || 6.0—7.6 || Blue
|-
!style="background:#efefef;"|[[Phenolphthalein]]
| Colorless || 8.3—10.0 || Pink
|-
!style="background:#efefef;"|[[Alizarine Yellow R|Alizarin yellow]]
| Yellow || 10.1—12.0 || Red
|}

Acid–base titrations depend on the [[Neutralization (chemistry)|neutralization]] between an acid and a base when mixed in solution.  In addition to the sample, an appropriate [[pH indicator]] 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.<ref>
{{Cite web
  | title = pH measurements with indicators
  | url = http://www.ph-meter.info/pH-measurements-indicators
  | access-date = 29 September 2011
}}</ref> When more precise results are required, or when the reagents are a weak acid and a weak base, a [[pH meter]] or a conductance meter are used.

For very strong bases, such as [[organolithium reagent]], [[metal amides]], and [[hydride]]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]] are used.<ref>{{cite web |website=shenvilab.org/education |url= https://www.shenvilab.org/_files/ugd/24e834_d95df1b2e78146659e0e20809de02a9e.pdf | title=Titrating Soluble RM, R<sub>2</sub>NM and ROM Reagents }}</ref><ref>{{cite web |url= https://www.chem.tamu.edu/rgroup/gladysz/documents/alkylreagents.pdf |title= Methods for Standardizing Alkyllithium Reagents (literature through 2006) |access-date= 2014-06-04 }}</ref>

[[File:Faint pink color of Phenolphthalein.jpg|thumb|Phenolphthalein, a commonly used indicator in acid and base titration.]]

The approximate pH during titration can be approximated by three kinds of calculations. Before beginning of titration, the concentration of <chem>[H+]</chem> is calculated in an 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]], 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>[H+]</chem> is obtained from the [[Henderson–Hasselbalch equation|Henderson-Hasselbalch equation]] and titration mixture is considered as buffer. In Henderson-Hasselbalch equation the {{chem|[acid]}} and {{chem|[base]}} are said to be the molarities that would have been present even with dissociation or hydrolysis. In a buffer, <chem>[H+]</chem> can be calculated exactly but the dissociation of {{chem|HA}}, the hydrolysis of <chem>A-</chem> and self-ionization of water must be taken into account.<ref name=Harris>{{cite book|title = Quantitative Chemical Analysis |edition = Seventh|first= Daniel C.|last = Harris|publisher = Freeman and Company |date=2007|isbn =978-0-7167-7041-1|url = https://www.academia.edu/32945832}}</ref> Four independent equations must be used:<ref>
{{Cite book
  | last1 = Skoog
  | first1 = D.A.
  | last2 = West
  | first2 = D.M.
  | last3 = Holler
  | first3 = F.J.
  | title = Analytical Chemistry: An Introduction, seventh edition
  | publisher = Emily Barrosse
  | year = 2000
  | pages = [https://archive.org/details/isbn_9780030202933/page/265 265-305]
  | isbn = 0-03-020293-0
  | url = https://archive.org/details/isbn_9780030202933/page/265
  }}</ref>
:<math chem>[\ce{H+}][\ce{OH-}] = 10^{-14}</math>
:<math chem>[\ce{H+}] = K_a\ce{\frac{[HA]}{[A^{-}]}}</math>
:<math chem>[\ce{HA}] + [\ce{A-}] = \frac{(n_\ce{A} + n_\ce{B})}{V}</math>
:<math chem>[\ce{H+}]  + \frac{n_\ce{B}}{V} = [\ce{A-}] + [\ce{OH-}]</math>
In the equations, <math chem>n_\ce{A}</math> and <math chem>n_\ce{B}</math> are the moles of acid ({{chem|HA}}) and salt ({{chem|XA}} where X is the cation), respectively, used in the buffer, and the volume of solution is {{mvar|V}}. The [[law of mass action]] is applied to the ionization of water and the dissociation of acid to derived the first and second equations. The mass balance is used in the third equation, where the sum of <math chem>V[\ce{HA}]</math> and <math chem>V[\ce{A-}]</math> must equal to the number of moles of dissolved acid and base, respectively. Charge balance is used in the fourth equation, where the left hand side represents the total charge of the cations and the right hand side represents the total charge of the anions: <math chem>\frac{n_\ce{B}}{V}</math> is the molarity of the cation (e.g. sodium, if sodium salt of the acid or sodium hydroxide is used in making the buffer).<ref>
{{Cite book
  | last = Henry
  | first = N.
  |author2= M.M. Senozon
   | title = The Henderson-Hasselbalch Equation: Its History and Limitations
  | publisher = Journal of Chermical Education
  | year = 2001
  | pages = 1499–1503
}}</ref>

===Redox titration===
{{Main|Redox titration}}

Redox titrations are based on a [[redox|reduction-oxidation reaction]] between an oxidizing agent and a reducing agent. A [[potentiometer]] or a [[redox indicator]] is usually used to determine the endpoint of the titration, as when one of the constituents is the oxidizing agent [[potassium dichromate]].  The color change of the solution from orange to green is not definite, therefore an indicator such as sodium diphenylamine is used.<ref>
{{Cite book
  | last = Vogel
  | first = A.I.
  |author2=J. Mendham
   | title = Vogel's textbook of quantitative chemical analysis
  | publisher = Prentice Hall
  | edition = 6
  | year = 2000
  | pages = 423
  | isbn = 0-582-22628-7
}}</ref> Analysis of wines for [[sulfur dioxide]] requires iodine as an oxidizing agent. In this case, starch is used as an indicator; a blue starch-iodine complex is formed in the presence of excess iodine, signalling the endpoint.<ref>
{{Cite book
  | last = Amerine
  | first = M.A.
  |author2=M.A. Joslyn
   | title = Table wines: the technology of their production
  | publisher = University of California Press
  | volume = 2
  | edition = 2
  | year = 1970
  | pages = 751–753
  | isbn = 0-520-01657-2
}}</ref>

Some redox titrations do not require an indicator, due to the intense color of the constituents.  For instance, in [[permanganometry]] a slight persisting pink color signals the endpoint of the titration because of the color of the excess oxidizing agent [[potassium permanganate]].<ref>
{{Cite book
  | last = German Chemical Society. Division of Analytical Chemistry
  | title = Fresenius' Journal of Analytical Chemistry
  | publisher = J.F. Bergmann
  | volume = 166-167
  | year = 1959
  | location = University of Michigan
  | pages = 1
  | language = de
}}</ref> In [[iodometry]], at sufficiently large concentrations, the disappearance of the deep red-brown [[triiodide]] ion can itself be used as an endpoint, though at lower concentrations sensitivity is improved by adding [[starch indicator]], which forms an intensely blue complex with triiodide.

[[File:Iodometric titration mixture.jpg|thumb|center|400px|Color of [[Iodometry|iodometric]] titration mixture before (left) and after (right) the end point.]]

===Gas phase titration===
Gas phase titrations are titrations done in the [[gas phase]], specifically as methods for determining reactive species by reaction with an excess of some other [[gas]], acting as the titrant.  In one common gas phase titration, gaseous [[ozone]] is titrated with nitrogen oxide according to the reaction

:O<sub>3</sub> + NO → O<sub>2</sub> + NO<sub>2</sub>.<ref>
{{Cite book
  | last = Hänsch
  | first = T.W.
  | title = Metrology and Fundamental Constants
  | publisher = IOS Press
  | year = 2007
  | pages = 568
  | isbn = 978-1-58603-784-0
}}</ref><ref>
{{Cite web
  | title = Gas phase titration
  | publisher = Bureau International des Poids et Mesures
  | url = http://www.bipm.fr/en/scientific/chem/gas_titration.html
  | access-date = 29 September 2001
}}</ref>

After the reaction is complete, the remaining titrant and product are quantified (e.g., by [[Fourier transform spectroscopy]]) (FT-IR); this is used to determine the amount of analyte in the original sample.

Gas phase titration has several advantages over simple [[spectrophotometry]].  First, the measurement does not depend on path length, because the same path length is used for the measurement of both the excess titrant and the product.  Second, the measurement does not depend on a linear change in absorbance as a function of analyte concentration as defined by the [[Beer–Lambert law]].  Third, it is useful for samples containing species which interfere at wavelengths typically used for the analyte.<ref>
{{Cite journal
  | last = DeMore
  | first = W.B.
  |author2=M. Patapoff
   | title = Comparison of Ozone Determinations by Ultraviolet Photometry and Gas-Phase Titration
  | journal = Environmental Science & Technology
  | volume = 10
  | issue = 9
  | pages = 897–899
  | date = September 1976
  | doi = 10.1021/es60120a012
| bibcode = 1976EnST...10..897D
  }}</ref>

===Complexometric titration===
{{Main|Complexometric titration}}

Complexometric titrations rely on the formation of a [[complex (chemistry)|complex]] between the analyte and the titrant. In general, they require specialized [[complexometric indicator]]s that form weak complexes with the analyte.  The most common example is the use of [[starch indicator]] to increase the sensitivity of iodometric titration, the dark blue complex of starch with iodine and iodide being more visible than iodine alone.  Other complexometric indicators are [[Eriochrome Black T]] for the titration of [[calcium]] and [[magnesium]] ions, and the [[chelating agent]] [[EDTA]] used to titrate metal ions in solution.<ref>
{{Cite book
  | last = Khopkar
  | first = S.M.
  | title = Basic Concepts of Analytical Chemistry
  | publisher = New Age International
  | edition = 2
  | year = 1998
  | pages = 63–76
  | isbn = 81-224-1159-2
}}</ref>

===Zeta potential titration===
{{Main|Zeta potential titration}}

Zeta potential titrations are titrations in which the completion is monitored by the [[zeta potential]], rather than by an [[pH indicator|indicator]], in order to characterize [[heterogeneous]] systems, such as [[colloid]]s.<ref>
{{Cite journal
  | last = Somasundaran
  | first = P.
  | title = Calculation of Zeta-Potentials from Electrokinetic Data
  | journal = Encyclopedia of Surface and Colloid Science
  | volume = 2
  | edition = 2
  | pages = 1097
  | publisher = CRC Press
  | year = 2006
  | isbn = 0-8493-9607-7
}}</ref>  One of the uses is to determine the [[iso-electric point]] when [[surface charge]] becomes zero, achieved by changing the [[pH]] or adding [[surfactant]]. Another use is to determine the optimum dose for [[flocculation]] or [[stabilizer (chemistry)|stabilization]].<ref name="dukhin2002">Dukhin, A. S. and Goetz, P. J. ''Characterization of liquids, nano- and micro- particulates and porous bodies using Ultrasound'', Elsevier, 2017 
{{ISBN|978-0-444-63908-0}}</ref>

===Assay===
{{Main|Assay|Virus quantification}}

An assay is a type of biological titration used to determine the concentration of a [[virus]] or [[bacterium]].  Serial dilutions are performed on a sample in a fixed ratio (such as 1:1, 1:2, 1:4, 1:8, etc.) until the last dilution does not give a positive test for the presence of the virus. The positive or negative value may be determined by inspecting the infected cells visually under a [[microscope]] or by an immunoenzymetric method such as [[ELISA|enzyme-linked immunosorbent assay]] (ELISA). This value is known as the [[titer]].<ref>
{{Cite book
  | last = Decker
  | first = J.M.
  | title = Introduction to immunology
  | publisher = Wiley-Blackwell
  | series = Eleventh Hour
  | edition = 3
  | year = 2000
  | pages = 18–20
  | isbn = 0-632-04415-2
}}</ref>