Editing Universal indicator

{{Short description|Indicator that works over a wide range of pH}}
{{About|the chemical pH indicator|the musical artists|Universal Indicator (collective)}}
[[File:Universal indicator paper.jpg|thumb|A roll of universal indicator paper]]
[[File:Skala boja 2.JPG|thumb|Colors of universal indicator]]

A '''universal indicator''' is a [[pH indicator]] made of a [[Solution (chemistry)|solution]] of several compounds that exhibit various smooth [[Color|colour]] changes over a wide range [[pH]] values to indicate the [[Acid|acidity]] or [[Alkali|alkalinity]] of solutions. A universal indicator can be in paper form or present in a form of a solution.<ref>{{cite book|last1=Walker|first1=Denise|title=Acids and alkalis|date=2007|publisher=Evans|location=London|isbn=978-0-237-53002-0|page=13|edition=1|url=https://books.google.com/books?id=nPnRO_mQymUC&pg=PA13 |access-date=4 June 2015}}</ref>

== History ==

Although there are several commercially available universal pH indicators, most are a variation of a formula patented by Yamada in 1933.<ref>''Jap. Pat.'' 99,664, Feb 21, 1933</ref><ref>''Chem Abstr'', '''28''', 2258 (1934)</ref><ref>{{Cite journal | doi = 10.1021/ed014p274| title = Demonstration experiments using universal indicators| journal = Journal of Chemical Education| volume = 14| issue = 6| pages = 274| year = 1937| last1 = Foster | first1 = L. S. | last2 = Gruntfest | first2 = I. J. | bibcode = 1937JChEd..14..274F}}</ref>

== Composition ==

A universal indicator is usually composed of [[water]], [[1-propanol]], [[phenolphthalein]], [[sodium hydroxide]], [[methyl red]], [[bromothymol blue]], [[sodium bisulfite]], and [[thymol blue]].<ref>{{ cite web | url = http://www.iscid.org/encyclopedia/Universal_Indicator | title = Universal Indicator | archiveurl = https://web.archive.org/web/20060925035727/http://www.iscid.org/encyclopedia/Universal_Indicator | archivedate = September 25, 2006 | publisher = ISCID Encyclopedia of Science and Philosophy }}</ref> The colours that indicate the pH of a solution, after adding a universal indicator, are:

{| class=wikitable
! pH range
! Description
! Colour
|-
|≤ 3
|Strong acid
| style="background:red; color:white" |Red
|-
|4–6
|Weak acid
| style="background:darkorange" |Orange or Yellow
|-
|7
|Neutral
| style="background:green; color:white;" |Green
|-
|8–10
|Weak alkali 
| style="background:blue; color:white;" |Blue
|-
|≥ 11
|Strong alkali 
| style="background:darkviolet; color:white" |Indigo or Violet
|}

The colors from yellow to red indicate an acidic solution, colours blue to violet indicate an alkaline solution and a green colour indicates that a solution is neutral.

{| class="wikitable"
|+ Universal indicator components<ref>{{cite book |last1=Jenkins |first1=Glenn Llewellyn |title=Jenkins' Quantitative Pharmaceutical Chemistry |date=1979 |publisher=Mei Ya |page=67 |url=https://www.google.com.ph/books/edition/Jenkins_Quantitative_Pharmaceutical_Chem/QPaFoAEACAAJ?hl=en |access-date=4 May 2025 |language=en}}</ref>
! Indicator
! Low pH colour
! Transition pH range
! High pH colour
|-
| [[Thymol blue]] (first transition)
| style="background:red; color:white"| Red
| style="text-align:center;"| 1.2&nbsp;–&nbsp;2.8
| style="background:yellow;"| Yellow
|-
| [[Methyl orange]]
| style="background:red; color:white"|Red
| style="text-align:center;"| 3.2&nbsp;–&nbsp;4.4
| style="background:yellow;"| Yellow
|-
| [[Methyl red]]
| style="background:red; color:white"| Red
| style="text-align:center;"| 4.8&nbsp;–&nbsp;6.0
| style="background:yellow; color:black"|Yellow
|-
| [[Bromothymol blue]]
| style="background:yellow;"| Yellow
| style="text-align:center;"| 6.0&nbsp;–&nbsp;7.6
| style="background:blue; color:white"| Blue
|-
| [[Thymol blue]] (second transition)
| style="background:yellow;"| Yellow
| style="text-align:center;"| 8.0&nbsp;–&nbsp;9.6
| style="background:blue; color:white"| Blue
|-
| [[Phenolphthalein]]
| Colourless
| style="text-align:center;"| 8.3&nbsp;–&nbsp;10.0
| style="background:#ff33cc;; color:white"| Fuchsia
|}

Wide-range pH test papers with distinct colours for each pH from 1 to 14 are also available. Colour matching charts are supplied with the specific test strips purchased.

== Types ==

* Paper form: It is a strip of coloured paper which changes colour to red if the solution is acidic and to blue, if the solution is basic. The strip can be placed directly onto a surface of a wet substance or a few drops of the solution can be dropped onto the universal indicator using dropping equipment. If the test solution is of a dark colour, it is preferable to use a paper universal indicator, such as [[Hydrion paper]].
* Solution: The main components of a universal indicator, in the form of a solution, are [[thymol blue]], [[methyl red]], [[bromothymol blue]], and [[phenolphthalein]]. This mixture is important because each component loses or gains [[proton]]s depending upon the [[acidity or alkalinity]] of the solution being tested. It is beneficial to use this type of universal indicator in a colorless solution. This will increase the accuracy level of indication.

== Influence on conductivity ==
[[File:Universal indicator solution in desalinated water.svg|thumb|Influence of universal indicator solution on the conductivity of desalinated water]]

The impact of an ethanol-based universal indicator may seem negligible at first glance. However, in the case of dilute solutions prepared with bidistilled water, this influence becomes readily discernible and measurable.<ref>https://github.com/ddiesing/universal_indicator_conductivity/blob/main/indicator_conc_conduct.csv</ref>

== See also ==

* [[Litmus]]
* [[pH indicator]]

== References ==

{{refs}}

[[Category: PH indicators]]