View source for Silica gel

{{Short description|Porous form of silicon dioxide}}
{{about||the South Korean band|Silica Gel (band)}}
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{{Redirect|Do not eat|the packet found in foods|Oxygen scavenger}}
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{{Chembox
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| verifiedrevid = 477002212
| ImageFile1 = Silica_gel_pb092529.jpg
| ImageSize1 = 300px
|Section1={{Chembox Identifiers
| ChemSpiderID_Ref = {{chemspidercite|changed|}}
| ChemSpiderID = none
| CASNo = 112926-00-8
| UNII_Ref = {{fdacite|correct|FDA}}
| UNII = Y6O7T4G8P9
| CASNo_Ref = {{cascite|correct|CAS}}
| CASNo_Comment = <ref name="y1">{{cite web|url=http://hazard.com/msds/mf/baker/baker/files/s1610.htm|title=Silica gel|archive-url=https://web.archive.org/web/20170531133940/http://hazard.com/msds/mf/baker/baker/files/s1610.htm |date=31 May 2017|website=jtbaker.com|archive-date=31 May 2017 }}</ref><ref name="y2">{{cite web|url=http://www.chemcas.org/chemical/msds/cas/AA0_4/AA42764-18.asp|title=Silica gel|archive-url=https://web.archive.org/web/20170609204215/http://www.chemcas.org/chemical/msds/cas/AA0_4/AA42764-18.asp |date=9 June 2017|website=chemcas.org|archive-date=9 June 2017 }}</ref><ref name="y3">{{cite web|url=https://echa.europa.eu/de/substance-information/-/substanceinfo/100.128.559|title=Silicon dioxide|archive-url=https://web.archive.org/web/20180315200129/https://echa.europa.eu/de/substance-information/-/substanceinfo/100.128.559 |date=15 March 2018|website=echa.europa.eu|archive-date=15 March 2018 }}</ref>
| CASNo1 = 1343-98-2
| CASNo1_Ref = {{cascite|correct|CAS}}
| CASNo1_Comment = (''Silicic acid'')
}}
|Section2={{Chembox Properties
| Formula = SiO<sub>2</sub>
| MolarMass = 60.08 g/mol
| Appearance = Transparent beads
| Odor = Odorless
}}
}}
[[File:Opaleszens Kolloid SiO2.jpg|thumb|[[Colloid]]al silica gel with light [[opalescence]]]]
'''Silica gel''' is an [[amorphous]] and [[porosity|porous]] form of [[silicon dioxide]] (silica), consisting of an irregular three-dimensional framework of alternating [[silicon]] and [[oxygen]] atoms with [[nanometer]]-scale voids and pores.  The voids may contain water or some other liquids, or may be filled by gas or [[vacuum]].  In the last case, the material is properly called silica [[Gel#Xerogels|xerogel]].

Silica xerogel with an average pore size of 2.4 nanometers has a strong affinity for water molecules and is widely used as a [[desiccant]].  It is hard and [[translucence|translucent]], but considerably softer than massive [[silica glass]] or [[quartz]], and remains hard when saturated with water.

Silica xerogel is usually commercialized as coarse granules or beads, a few millimeters in diameter. Some grains may contain small amounts of indicator substance that changes color when they have absorbed some water.  Small paper envelopes containing silica xerogel pellets, usually with a "do not eat" warning, are often included in dry food packages to absorb any [[humidity]] that might cause [[decomposition|spoilage]] of the food.

"Wet" silica gel, as may be freshly prepared from [[alkali]] [[silicate]] solutions, may vary in consistency from a soft transparent [[gel]], similar to [[gelatin]] or [[agar]], to a hard solid, namely a water-logged xerogel.  It is sometimes used in laboratory processes, for example to suppress [[convection]] in liquids or prevent [[precipitation|settling]] of suspended particles.<ref name=hen>{{cite book|url=https://books.google.com/books?id=821PCzQ-rhcC|first1=H.K.|last1=Henisch|year=1988|title=Crystals in Gels and Liesegang Rings|publisher=Cambridge University Press|location=Cambridge|isbn=0521345030}}</ref>

==History==
Silica gel was in existence as early as the 1640s as a scientific curiosity.<ref name="y4">{{cite journal |first1=Maryann|last1= Feldman |first2=Pierre|last2= Desrochers |title=Research Universities and Local Economic Development: Lessons from the History of the Johns Hopkins University |journal=Industry and Innovation |date=March 2003 |volume=10 |issue=1 |pages=5–24 |s2cid=154423229 |doi=10.1080/1366271032000068078 |url=http://www.rotman.utoronto.ca/feldman/papers/Research%20Universities%20and%20Local%20Economic%20Development.pdf |archive-url=https://web.archive.org/web/20051112233703/http://www.rotman.utoronto.ca/feldman/papers/Research%20Universities%20and%20Local%20Economic%20Development.pdf |archive-date=12 November 2005}}</ref> It was used in [[World War I]] for the [[adsorption]] of vapors and gases in [[respirator cartridge|gas mask canisters]]. The synthetic route for producing silica gel was patented in 1918 by Walter A. Patrick, a chemistry professor at [[Johns Hopkins University]].

==Types==
{{bulleted list

|Type A – clear pellets, approximate pore diameter: 2.5&nbsp;nm, drying and moistureproof properties, can be used as catalyst carriers, adsorbents, separators and variable-pressure adsorbent.

|Type B – translucent white pellets, pore diameter: 4.5⁠–⁠7.0&nbsp;nm, liquid adsorbents, drier and perfume carriers, also may be used as catalyst carriers, cat litter.

|Type C – translucent, micro-pored structure, raw material for preparation of silica gel cat litter. Additionally dried and screened, it forms macro-pored silica gel which is used as drier, adsorbent and catalyst carrier.
}}

Silica alumina gel - light yellow, chemically stable, flame-resistant, insoluble except in alkali or hydrofluoric acid. Superficial polarity, thermal stability, performance greater than fine-pored silica gel.

Stabilizing silica gel - non-crystalline micro-porous solid powder, nontoxic, flame-resisting, used in brewery of grains for beer to improve taste, clearness, color, and foam and for removal of non-micro-organism impurities.

==Properties==
Silica gel's high [[specific surface area]]{{emdash}}around {{convert|750|–|800|m2/g|abbr=on}}<ref name="Greenwood">{{Greenwood&Earnshaw}}</ref>{{emdash}}allows it to adsorb water readily, making it useful as a desiccant (drying agent). Silica gel is often described as "absorbing" moisture, which may be appropriate when the gel's microscopic structure is ignored, as in silica gel packs or other products. However, material silica gel removes moisture by adsorption onto the surface of its numerous pores rather than by [[absorption (chemistry)|absorption]] into the bulk of the gel.

Silica gel is able to adsorb up to 37% of its own weight in moisture in high-humidity environments.<ref name=Chand>{{cite web |last1=Chandradhas |first1=Susheel |title=How To Recharge Silica Gel Crystals |website=Beyond Photo Tips |date=27 August 2016 |url=https://www.beyondphototips.com/recharge-silica-gel-crystals/ |access-date=2 November 2022 |archive-date=2 November 2022 |archive-url=https://web.archive.org/web/20221102123954/https://www.beyondphototips.com/recharge-silica-gel-crystals/ |url-status=live }}</ref> This moisture can be released upon heating at about {{convert|120|°C|°F}}<ref name="hooman2021"/><ref name=Chand /> for 1–2 hours.<ref name=Chand /> This makes it reusable multiple times with very little, if any, loss of efficiency.<ref name=Chand />

==Preparation==
An aqueous solution of [[sodium silicate]] is acidified to produce a gelatinous precipitate that is washed, then dehydrated to produce colorless silica gel.<ref name="Greenwood"/> When a visible indication of the moisture content of the silica gel is required, ammonium tetrachlorocobaltate(II) (NH<sub>4</sub>)<sub>2</sub>[CoCl<sub>4</sub>] or [[cobalt(II) chloride]] CoCl<sub>2</sub> is added.<ref name="Greenwood"/> This will cause the gel to be blue when dry and pink when hydrated.<ref name="Greenwood"/> Due to a link between the cobalt chloride and cancer, it has been forbidden in Europe for use in silica gel.<ref name="y5">{{cite web |title=Blue Silicagel & Conclusions: Safety information on blue silicagel |url=http://www.descase-europe.com/products/european-regulations-reach/desiccants-available/blue-silicagel-concluisons.html/2/1/12/80/84 |url-status=dead |archive-url=https://web.archive.org/web/20160105075837/http://www.descase-europe.com/products/european-regulations-reach/desiccants-available/blue-silicagel-concluisons.html/2/1/12/80/84 |archive-date=5 January 2016}}</ref> An alternative indicator is [[methyl violet]] which is orange when dry and green when hydrated.

==Uses==

===Desiccant===
[[File:Silica gel bag open with beads.jpg|thumb|Silica gel, as beads packed in a permeable bag, is a commonly used desiccant.]]
{{See also|Desiccant}}
====Initial use====
Moisture can cause mold and spoilage in many items.<ref name="y6">{{Cite web |date=30 January 2013 |title=Prevent bacteria from feasting on your fresh produce, causing spoilage |url=https://www.canr.msu.edu/news/prevent_bacteria_from_feasting_on_your_fresh_produce_causing_spoilage |access-date=8 March 2024 |website=MSU Extension |language=en-us}}</ref><ref name="y7">{{Cite book |last=Command |first=United States Naval Education and Training |url=https://books.google.com/books?id=UckIU2-LPwoC&dq=In+many+items,+moisture+encourages+the+growth+of+mold+and+spoilage&pg=PA91 |title=Mess Management Specialist 3 & 2 |date=1978 |publisher=Department of Defense, Navy Department, Office of the Chief of Naval Operations, Naval Education and Training Command |pages=91 |language=en |archive-date=22 September 2024 |access-date=1 April 2024 |archive-url=https://web.archive.org/web/20240922174037/https://books.google.com/books?id=UckIU2-LPwoC&dq=In%20many%20items%2C%20moisture%20encourages%20the%20growth%20of%20mold%20and%20spoilage&pg=PA91 |url-status=live }}</ref> It can also damage electronics by causing condensation and shorten the lifespan of chemicals, like those in vitamins.{{citation needed|date=March 2024}} Silica gel packets help by absorbing moisture and extending the life of these items.<ref name="y8">{{Cite book |url=https://books.google.com/books?id=Y1InAQAAMAAJ&q=Through+the+inclusion+of+silica+gel+packets,+these+items+can+be+preserved+longe+r. |title=Machinery and Production Engineering |date=1951 |publisher=Machinery Publishing Company |language=en}}</ref><ref name="y9">{{Cite book |last=Anderson |first=Gary A. |url=https://books.google.com/books?id=dOnqE3iJOoAC&q=Through+the+inclusion+of+silica+gel+packets,+these+items+can+be+preserved+longe+r. |title=Floral Design and Marketing |date=1988 |publisher=Ohio Agricultural Education Curriculum Materials Service, the Ohio State University |language=en |archive-date=22 September 2024 |access-date=1 April 2024 |archive-url=https://web.archive.org/web/20240922174038/https://books.google.com/books?id=dOnqE3iJOoAC&q=Through+the+inclusion+of+silica+gel+packets,+these+items+can+be+preserved+longe+r. |url-status=live }}</ref><ref name="Varlamoff-1998">{{Cite book |last1=Varlamoff |first1=Marie-Thérèse |url=https://books.google.com/books?id=3X_0Lg6Nk8gC&q=Through+the+inclusion+of+silica+gel+packets,+these+items+can+be+preserved+longe+r. |title=IFLA Principles for the Care and Handling of Library Material |last2=Kremp |first2=Virginie |last3=Conservation |first3=IFLA Programme on Preservation and |last4=Resources |first4=Council on Library and Information |date=1998 |publisher=International Federation of Library Associations and Institutions, Core Programme on Preservation and Conservation |isbn=978-2-912743-00-8 |language=en}}</ref>

Silica gel may also be used to keep the [[relative humidity]] inside a high frequency radio or satellite transmission system [[waveguide (electromagnetism)|waveguide]] as low as possible (see also ''[[humidity buffering]]'').<ref name="y11">{{Cite book |last=Sabry |first=Fouad |url=https://books.google.com/books?id=gdiTEAAAQBAJ&dq=Silica+gel+may+also+be+used+to+keep+the+relative+humidity+inside+a+high+frequency+radio+or+satellite+transmission+system+waveguide&pg=PT89 |title=Aerogel: Want to Colonize Mars? Aerogel could help us farm and survive on Mars "in our lifetimes" |date=16 January 2022 |publisher=One Billion Knowledgeable |language=en |archive-date=22 September 2024 |access-date=1 April 2024 |archive-url=https://web.archive.org/web/20240922174038/https://books.google.com/books?id=gdiTEAAAQBAJ&dq=Silica+gel+may+also+be+used+to+keep+the+relative+humidity+inside+a+high+frequency+radio+or+satellite+transmission+system+waveguide&pg=PT89 |url-status=live }}</ref> Excessive moisture buildup within a waveguide can cause arcing inside the waveguide itself, damaging the power amplifier feeding it. Also, the beads of water that form and condense inside the waveguide change the characteristic impedance and frequency, degrading the signal. It is common for a small compressed air system (similar to a small home aquarium pump) to be employed to circulate the air inside the waveguide over a jar of silica gel.

Silica gel is also used to dry the air in industrial compressed air systems. Air from the compressor discharge flows through a bed of silica gel beads. The silica gel adsorbs moisture from the air, preventing damage at the point of use of the compressed air due to condensation or moisture. The same system is used to dry the compressed air on railway locomotives, where condensation and ice in the brake air pipes can lead to brake failure.

Prior to widespread use of air-conditioning, salt shakers with caps containing silica gel beads to keep the salt dry enough to prevent clumping were marketed in the US, replacing the practice of including a few grains of rice in salt shakers to effect the same drying.

Silica gel is sometimes used as a [[preservation (library and archival science)|preservation]] tool to control relative humidity in museum and library exhibitions and storage.

Other applications include diagnostic test strips, inhalation devices, [[syringe]]s, [[drug test|drug test kits]], and hospital sanitation kits.

Silica gel is commonly described as a way to remove moisture from phones and electronics accidentally exposed to water, but there are no known controlled studies that compare its actual efficacy compared to simple air or fan exposure. The average person is also unlikely to have a sufficient quantity of ready-to-use activated or [[#Regeneration|regenerated]] silica gel.

====Regeneration====
Once saturated with water, the gel may be regenerated by heating it to about {{convert|120|°C|°F}}<ref name=Chand /><ref name="hooman2021"/> for 1–2 hours.<ref name=Chand /> Some types of silica gel will "pop" when exposed to enough water. This is caused by breakage of the silica spheres when contacting the water.<ref name="y12">{{cite web|author=Spence Konde|url=http://m.wpi.edu/Pubs/E-project/Available/E-project-042607-140508/unrestricted/MQP-RWT-606.pdf|title=Preparation of High-Silica Zeolite Beads From Silica Gel|archive-url=https://web.archive.org/web/20160304082813/http://m.wpi.edu/Pubs/E-project/Available/E-project-042607-140508/unrestricted/MQP-RWT-606.pdf |date=4 March 2016 |archive-date=4 March 2016 }}</ref>

Regenerating silica gel is important for both economic and environmental reasons.<ref name="y13">{{cite web|url=https://www.humipak.com.my/desiccants/ways-to-reuse-silica-gel/|title=Ways to Reuse Silica Gel|date=6 March 2024|access-date=25 December 2024|archive-date=4 August 2024|archive-url=https://web.archive.org/web/20240804204542/https://www.humipak.com.my/desiccants/ways-to-reuse-silica-gel/|url-status=live}}</ref> It can be regenerated thermally or chemically. Chemical methods involve the use of desiccating agents or solvents to desorb water from silica gel, though these are generally less practical for routine regeneration. Thermal regeneration can be applied also to packed silica gel beds without opening them, and can be done by conventional heating in an oven at a temperature around {{Val|120|u=Celsius}}.<ref name="hooman2021">{{cite journal|quote=Preparation of the dried silica gel sample was achieved by heating it in an oven at 120 C (to ensure full dryness) and controlling the mass until no change in mass was observed.|doi=10.1080/07373937.2021.1929296 |title=Application of ultrasound in regeneration of silica gel for industrial gas drying processes |date=2022 |last1=Daghooghi-Mobarakeh |first1=Hooman |last2=Miner |first2=Mark |last3=Wang |first3=Liping |last4=Wang |first4=Robert |last5=Phelan |first5=Patrick E. |journal=Drying Technology |volume=40 |issue=11 |pages=2251–2259 }}</ref> Another option for thermal regeneration is by using microwaves to apply heat more evenly and reduce regeneration time compared to conventional ovens, but this process should be tightly controlled to avoid overheating.<ref name="mw">{{cite journal|date=12 April 2024|doi=10.1007/s10973-024-13010-0 |title=Regeneration behavior of solid desiccants with microwave drying |last1=Zhang |first1=Xiaoyuan |last2=Chen |first2=Meiqian |last3=Guan |first3=Junli |journal=Journal of Thermal Analysis and Calorimetry |volume=149 |issue=19 |pages=10927–10940 }}</ref> Yet another option to apply heat is using a solar heater with a compound parabolic concentrator.<ref name="y14">{{cite journal| doi=10.1016/j.renene.2006.02.009 | title=The regeneration of silica gel desiccant by air from a solar heater with a compound parabolic concentrator | date=2007 | last1=Pramuang | first1=Surajitr | last2=Exell | first2=R.H.B. | journal=Renewable Energy | volume=32 | issue=1 | pages=173–182 | bibcode=2007REne...32..173P }}</ref> There are also studies on using ultrasonic waves to remove water molecules from silica gel at {{Val|45|-|90|u=Celsius}} which are lower temperatures than that of conventional heating.<ref name="y15">{{cite journal|doi=10.1016/j.energy.2014.01.061 |title=Study on the performance of silica gel dehumidification system with ultrasonic-assisted regeneration |date=2014 |last1=Yao |first1=Ye |last2=Yang |first2=Kun |last3=Liu |first3=Shiqing |journal=Energy |volume=66 |pages=799–809 |bibcode=2014Ene....66..799Y }}</ref><ref name="y16">{{cite journal|doi=10.1021/ef801066m |title=Parametric Study of High-intensity Ultrasonics for Silica Gel Regeneration |date=2009 |last1=Yao |first1=Ye |last2=Zhang |first2=Weijiang |last3=Liu |first3=Shiqing |journal=Energy & Fuels |volume=23 |issue=6 |pages=3150–3158 |bibcode=2009EnFue..23.3150Y }}</ref>

===Chemistry===
[[File:P1000495.JPG|thumb|60px|[[Chromatography column]]]]
In chemistry, silica gel is used in [[chromatography]] as a [[stationary phase (chemistry)|stationary phase]]. In [[column chromatography]], the stationary phase is most often composed of silica gel particles of 40–63 μm. Different particle sizes are used for different kinds of column chromatography as the particle size is related to surface area. The differences in particle size dictate if the silica gel should be used for flash or gravity chromatography. In this application, due to silica gel's polarity, non-polar components tend to [[Elution|elute]] before more polar ones, hence the name [[normal phase chromatography]]. However, when [[hydrophobic]] groups (such as C<sub>18</sub> groups) are attached to the silica gel then polar components elute first and the method is referred to as [[reverse phase chromatography]]. Silica gel is also applied to aluminium, glass, or plastic sheets for [[thin layer chromatography]].

The hydroxy (OH) groups on the surface of silica can be functionalized to afford specialty silica gels that exhibit unique stationary phase parameters. These so-called functionalized silica gels are also used in organic synthesis and purification as insoluble reagents and [[scavenger (chemistry)|scavengers]].

Chelating groups have also been covalently bound to silica gel. These materials have the ability to remove metal ions selectively from aqueous solutions. Chelating groups can be covalently bound to polyamines that have been grafted onto a silica gel surface producing a material of greater mechanical integrity. Silica gel is also combined with [[alkali metal]]s to form a [[M-SG reducing agent]]. (See [[SiGNa chemistry]])

Silica gel is not expected to biodegrade in either water or soil.<ref name="hazard">{{cite web |title=Silica Gel |author=Environmental Health and Safety |date=10 September 2007 |url=http://hazard.com/msds/mf/baker/baker/files/s1610.htm |access-date=12 January 2008 |archive-date=31 May 2017 |archive-url=https://web.archive.org/web/20170531133940/http://hazard.com/msds/mf/baker/baker/files/s1610.htm |url-status=live }}</ref>

===Cat litter===
Silica gel is also used as [[cat litter#Types of litter box filler|cat litter]],<ref name="y17">{{cite web |title=Non-Tech High Tech Litters the Landscape |author=Andrew Kantor |publisher=USA Today |date=10 December 2004 |url=https://www.usatoday.com/tech/columnist/andrewkantor/2004-12-10-kantor_x.htm |access-date=2 March 2008 |archive-date=16 March 2008 |archive-url=https://web.archive.org/web/20080316024055/http://www.usatoday.com/tech/columnist/andrewkantor/2004-12-10-kantor_x.htm |url-status=live }}</ref> by itself or in combination with more traditional materials, such as clays including [[bentonite]]. It is non-tracking and virtually odorless.

===Food additive===
Silica gel, also referred to as silicon dioxide or synthetic amorphous silica (SAS), is listed by the FDA in the United States as [[generally recognized as safe]] (GRAS), meaning it can be added to food products without needing approval. Silica is allowed to be added to food in the US at up to 2% as permitted under 21 CFR 172.480. <ref name="Notification-2013">{{cite web |title=Notification of the GRAS Determination of Silicon Dioxide When Added Directly or Indirectly to Human Food |url=http://www.accessdata.fda.gov/scripts/fcn/gras_notices/GRN000321.pdf |url-status=dead |archive-url=https://web.archive.org/web/20130418061428/http://www.accessdata.fda.gov/scripts/fcn/gras_notices/GRN000321.pdf |archive-date=18 April 2013}}</ref> In the EU, it can be in up to 5% concentrations. {{Citation needed|date=November 2025|reason=5% in EU was not present in the reference for the 2% allowance in US food products.}}In 2018, a re-evaluation by the EFSA Panel on Food Additives and Nutrient Sources added to Food found no indications of toxicity even at the highest estimates of exposure level.<ref name="pmid32625658">{{cite journal |last1= Younes|first1=M.| last2= Aggett | first2=P. | last3=Aguilar |first3=F.| title=Scientific Opinion on the re-evaluation of silicon dioxide (E 551) as a food additive |journal=EFSA Journal |volume=16 |issue=1 |pages=5088–5158 |year=2018 |pmid=32625658 |pmc=7009582 |s2cid=79503431 |doi=10.2903/j.efsa.2018.5088}}</ref>

Listed uses include: [[anticaking agent]], defoaming agent, stabilizer, adsorbent, carrier, conditioning agent, chill proofing agent, filter aid, emulsifying agent, viscosity control agent, and anti-settling agent.<ref name="y19">{{cite web |title=GRAS Notice (GRN) No. 298 |url=http://www.accessdata.fda.gov/scripts/fcn/gras_notices/grn0298.pdf |url-status=dead |archive-url=https://web.archive.org/web/20110409092941/http://www.accessdata.fda.gov/scripts/fcn/gras_notices/grn0298.pdf |archive-date=9 April 2011}}</ref> Silica can be found commonly in foods including baked goods, spices and herbs, dairy products, cocoa products, and more.<ref name="pmid32625658" />

===Water filtration===
Given the water adsorption properties of silica gel, it is used in domestic water filters.<ref name="y20">{{Cite web |url=http://zerowater.com/ |title=ZeroWater |access-date=3 January 2015 |archive-date=4 January 2015 |archive-url=https://web.archive.org/web/20150104013230/http://zerowater.com/ |url-status=live }}</ref> The surface structure of silica gel allows the adsorption of some minerals that are dissolved in the water,<ref name="y21">{{cite journal |last1=Peri| first1=J. B. |last2=Hensley Jr.|first2=A. L. |title=The surface structure of silica gel |journal=The Journal of Physical Chemistry |volume=72 |issue=8 |pages=2926–2933 |year=1968 |doi=10.1021/j100854a041}}</ref> or "ion exchange" as it is marketed. Due to the lack of regulations for domestic water filtration products, no studies validate the manufacturer claims regarding the effectiveness of the filtration system. {{Citation needed|date=September 2024}}

===Humidity indicator (color-changing silica gel)===
{{Main article|Humidity indicator}}
[[File:Indicating-silica-gel.png|right|thumb|Indicating silica gel]]
Silica gel may be doped with a moisture indicator that gradually changes its color when it transitions from the [[anhydrous]] (dry) state to the hydrated (wet) state. Common indicators are [[cobalt(II) chloride]] and [[methyl violet]]. Cobalt (II) chloride is deep blue when dry and pink when wet, but it is toxic and carcinogenic, and was reclassified by the [[European Union]] in July 2000 as a toxic material.<ref name="y22">{{cite web |title=Classifications - CL Inventory |url=http://echa.europa.eu/information-on-chemicals/cl-inventory-database/-/cl-inventory/view-notification-summary/15404 |access-date=5 August 2015 |archive-date=5 January 2016 |archive-url=https://web.archive.org/web/20160105075836/http://echa.europa.eu/information-on-chemicals/cl-inventory-database/-/cl-inventory/view-notification-summary/15404 |url-status=live }}</ref> [[Methyl violet]] may be formulated to change from orange to green, or orange to colorless. It also is toxic and potentially carcinogenic,<ref name="y23">{{cite web |title=Methyl Violet Safety Data Sheet |ref=15 |website=labchem |url=http://www.labchem.com/tools/msds/msds/LC17190.pdf |access-date=5 February 2018 |archive-date=6 February 2018 |archive-url=https://web.archive.org/web/20180206073556/http://www.labchem.com/tools/msds/msds/LC17190.pdf |url-status=dead }}</ref> but is safe enough to have medicinal uses.
[[Iron(III)|Ferric]] and [[Iron(II)|ferrous]] salts, sometimes combined with small amounts of [[sodium hydroxide]], provide a better alternative. In particular, [[ferric sulfate]] and [[double salt]]s like [[ammonium iron(III) sulfate]] (iron alum), [[ammonium iron(II) sulfate]], and [[potassium iron(III) sulfate]] all result in a color change from amber/yellow when dry to colorless/white when saturated.<ref name="y24">{{cite patent
| country = WO
| number = WO2000065339A1
| inventor = 
| invent1 = Stephen Moreton
| invent2 = Graham James Earl
| status = patent
| title = Humidity indicators
| pubdate = 2 November 2000
| gdate = 
| fdate = 
| pridate = 
| assign1 = 
| assign2 = 
| url = https://patents.google.com/patent/WO2000065339A1/en
}} {{Webarchive|url=https://web.archive.org/web/20230515063130/https://patents.google.com/patent/WO2000065339A1/en |date=15 May 2023 }}</ref><ref name="y25">{{cite web |title=SORBSIL CHAMELEON Safety Data Sheet |ref=18 |website=OkerChemie |url=https://www.oker-chemie.de/downloads/SORBSIL-CHAMELEON/Datenblaetter/EN/PI_SRB-01-Sorbsil_engl.pdf}}</ref>

==Hazards==
Silica gel is non-toxic, and non-reactive and stable with ordinary usage. It will react with [[hydrogen fluoride]], [[fluorine]], [[oxygen difluoride]], [[chlorine trifluoride]], strong acids, strong bases, and oxidizers.<ref name="hazard"/> Silica gel is irritating to the respiratory tract and may cause irritation of the digestive tract. Dust from the beads may cause irritation to the skin and eyes, so precautions should be taken.<ref name="y26">{{cite web |title=Material Safety Data Sheet: Silica Gel Dessiccant |author=Fisher Scientific |date=9 February 1997 |url=https://www2.atmos.umd.edu/~russ/MSDS/silicagel28200.html |access-date=12 January 2008 |archive-date=7 October 2008 |archive-url=https://web.archive.org/web/20081007184630/http://www.atmos.umd.edu/~russ/MSDS/silicagel28200.html |url-status=live }}</ref> Crystalline silica dust can cause [[silicosis]], but synthetic amorphous silica gel is [[indurated]], so it does not cause silicosis. Additional hazards{{example needed|date=March 2024}} may occur when doped with a [[#Humidity indicator (color-changing silica gel)|humidity indicator]].

==References==
{{Reflist|30em}}

==External links==
{{Commons category}}
* {{CPID|id=2845}}
* [https://web.archive.org/web/20051201173120/http://www.envirogel.co.uk/material_safety_datasheet_blue.htm UK Material Safety Datasheet]

[[Category:Conservation and restoration materials]]
[[Category:Desiccants]]
[[Category:Gels]]
[[Category:Glass types]]
[[Category:Silicon dioxide]]
[[Category:American inventions]]