What Is Indicating Silica Gel Biology Essay

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A desiccant is a hygroscopic substance that establishes a dry state in its local vicinity through adsorption or absorption of water vapour in the atmosphere. Evaporation is the natural process most objects experience when they are moist, but this process slows down as the moisture content of the object to that of the surrounding environment is reduced. As the object's moisture content is reduced to the same level to that of the atmosphere, it cannot lose any more water and this is where desiccants come in useful. Enclosing the object in a container with a desiccant will allow it to lose more water as the desiccant sucks fluid out [1] (supports our experiment) There are liquid based desiccants as well as solid based desiccants, but we will be focusing on solid desiccants as it applies better in our context.

Indicating Silica Gel

Silica gel is a solid, granular, vitreous, highly porous form of silica made synthetically from sodium silicate. Indicating silica gel is a silica gel bead or granule that has been washed with concentrated heavy metal salt known as cobalt chloride. The dry deep bluish cobalt chloride will turn from blue to purple and then to pink as it gradually becomes saturated with moisture, the change in colour is an indication to replace the saturated silica gel, thus the name indicating silica gel. Because of the addition of cobalt chloride, indicating silica gel should not be used in contact with products for consumption such as food or pharmaceuticals. [2] Silica gel's high surface area ( approximately 800 m²/g ) allows it to absorb water readily. Once it has absorbed enough water and becomes saturated, the gel can be regenerated to its original form by heating it to 120°C for two hours. [3]

Silica, SiO2 is a huge atomic structure where each silicon atom is bonded to four oxygen atoms, and each oxygen atom to two silicon atoms such that each silicon atom is at the centre of a regular tetrahedron of oxygen atoms. Silica has high melting point of 1700oC, density of between 2 and 3 g/cm3. The configuration of the silicon atom suggest that the formation of bonds is based on terahedrally oriented hybrid sp3 orbitals. Naturally occurring silica includes quartz, tridymite, cristobalite, coesite, keatile, stishovite, and opal. The absorption of water vapour by slicia is depends entirely on the formation of hydrogen bonding to single and paired hydroxyl groups(OH). In the early stages of absorption, water draws oxygen resulting in one or two hydrogen bonds with isolated or adjacent silanols. The interaction through one silanol group is with that of OH groups. In the later stages, the hydrogen-bonded water clusters are formed before all the single hydroxyls are formed. A fresh exposed surface of quartz consist of very reactive silicon and oxygen bonds, the surfaces are highly unstable and quickly hydoxylate. The water molecues are rapidly separated at the surface with protons bonding with oxygen and hydroxyl with Si atoms.

Silica is one of the most important adsorbents used in chemical technology, chromatography, filler for polymer material, etc.

Calcium Chloride

Calcium chloride is a salt of calcium and chlorine. It is solid at room temperature and is widely used for desiccation because of its hygroscopic nature. It is known to be a deliquescent rather than a desiccant because it transforms from a solid state into a liquid state when it has absorbed enough water. [4] Calcium chloride can be found in commercially sold products such as 'Thirsty Hippo', which is used in closets, wardrobes or vehicles to eliminate moisture and bad smell in its vicinity. Calcium Chloride usually comes in a powder or granular form with a sieve to allow the aqueous calcium hydroxide to be contained or with a dry sponge to absorb the calcium hydroxide.


Zeolites are volcanic rock composed of hydrated alumina silicates of the alkali earth metals. Sizes of the zeolite structure range from 2.5 to 4.3 angstroms in diameter with respect to the type of zeolite. Specific channel size enables zeolites to act as molecular gas sieves. Zeolites have a high affinity for water and have the capability of adsorbing and desorbing it without damage to the crystal structure. This property makes them useful as desiccants. When the zeolite is placed in an air stream it will adsorb moisture up to its saturation point and will not adsorb any more moisture after it has reached saturation point. [5]

Zeolites are crystalline aluminosilicates with the general formula M2/n0.Al2O3.ySiO2, n represents the the valaence of the cation M and y may vary from two to infinite. Zeolites are basically crystalline polymer with three-dimensional arrangement of TO4 tetrahedra (SiO4 or AlO4 ) connected to their oxygen atoms to form subunits and the lattice structure with repeating identical building blocks.

Pore Structure

Diffusion of molecues takes place within the zeolite during the adsorption and catalytic process. Zeolites are generally classified into three categories, small pore zeolites, medium pore zeolites and large pore zeolites, with eight, ten and twelve membered-ring pore apertures respectively.

Each AlO4 of negative charge is balanced by an extra-framework cation. The framework structure contains intracrystalline channels or interconnected voids that are occupied by cations and water molecules. The cations are mobile and ordinarily undergo ion exchange. Through application of heat, water can be removed reversibly, leaving intact a crystalline host structure permeated by micropores and voids. The preffered type of intracrystalline channels would be the two or three dimension which allows for easy intracrystalline diffusion in adsorption and catalytic application. In general, most zeolite structures, the primary structures consist of either AlO4 or SiO4 tetrahedra assembled into secondary building units which can be simple polyhedral such as cubes or hexagonal prisms. The framework of zeolite is to some extent flexible as the size and shape of framework and pores responds to changes in temperature.

Some of the common zeolites used in commercial applications types include mordenite, chabazite, erionite and clinoptile. Small amount of mordenite and clinoptilotile are used in absorbent applications such as air separation, drying and purification. Natural zeolites are also used in bulk applications like fillers in papers, fertilizers and soil conditioners and in cements and concrete. The other applications of zeolites are ion exchangers in the nuclear industry for radioactive storage and cleanup.

Zeolites for Cleaner Technologies Vol. 3, 2002, Gilson, Jean-Pierre, Michel Guisnet

Zeolites in Industrial Separation and Catalysis, Edited by Santi Kulprathipanja 2010


Activated charcoal, also known as activated carbon, is formed by increasing the porosity and surface area of the charcoal in order for it to be an adsorbent. The surface area can be increased significantly from 10m2/g to about 1500m2/g through the activation process. [6] Adsorption is achieved when the adsorbate is held onto the surface of the activated carbon by Van Der Waal's forces.

Activated Carbon and Its Surface Structure, Roop Chand Bansal and Meenakshi Goyal, 2005

The primary composition of an active carbon is 88% C, 0.5% H, 0.5% N, 1% S, and 6 to 7% O, with the remaining representing inorganic ash constituents. The most widely used carbon adsorbents have a specific surface area of 800 to 1500 m2/g, a relatively high surface area which makes it a good absorbent and a pore volume of 0.20 to 0.60 cm3g-1.

The arrangement of the aromatic sheets is irregular and, therefore, leaves free tiny gaps between the sheets, which may become filled with the residue matter or the products of decomposition and blocked partially by disorganized carbon. These spaces give rise to pores that make active carbons excellent adsorbents. Pore structure is further enhanced during the activation process when the spaces between the aromatic sheets are cleared of various carbonaceous compounds and disorganized carbon. The activation process converts the carbonized char into a form that contains the largest possible number of randomly distributed pores of various shapes and sizes, giving rise to a product with an extended and extremely high surface area.

In general, active carbons have a strongly developed internal surface and they are usually characterized by a polydispersed capillary structure comprising pores of different sizes and shapes.

Active carbons are have pores starting from less than a nanometer to several thousand nanometers. The pores are classified into three groups: the micropores, the mesopores (transitional pores), and the macropores.

Micropores have molecular dimensions, the effective radii being less than 2 nm. The adsorption in these pores occurs through volume fillings, and there is no capillary condensation taking place. The adsorption energy in these pores is much larger compared to larger mesopores or to the nonporous surface because of the overlapping adsorption forces from the opposite walls of the micropores.

Mesopores, also called transitional pores, have effective dimensions in the 2 to 50 nm range, and their volume usually varies between 0.1 and 0.2 cm3/g.

Macropores are not of considerable importance to the process of adsorption in active carbons because the contribution to the surface area of the adsorbate is very small. They act as transport channels for the adsorbate into the micro- and mesopores. Macropores are not filled by capillary condensation and are characterized by mercury porosimetry. Thus, the porous structure of active carbons is tridisperse, consisting of micro-, meso-, and macropores. Each of these groups of pores plays a specific role in the adsorption process.

Besides the crystalline and porous structure, an active carbon surface has a chemical structure as well. The adsorption capacity of active carbons is determined by their physical or porous structure but is strongly influenced by the chemical structure. The determining component of adsorption forces in a ordered carbon surface is the dispersive component of Van der Walls forces. Carbon can also adsorb certain molecular species such as amines, nitrobenzene, phenols, and several other cationic species.

The adsorption depends on the type of forces involved. There are two types of adsorption involved, physical or Van der Walls adsorption and chemical. In physical or Van der Walls adsorption, the adsorbate is bound to the surface by relatively weak van der Walls identical to molecular forces of cohesion that are involved in the condensation of vapours into liquids. While for chemisorption, involves the exchanges or sharing of electrons between the adsorbate molecules and the surface of the adsorbent, resulting in a chemical reaction. The bond formed between the absorbate and the absorbent is essentially a chemical bond and much stronger than physical adsorption. However, physical sorption do not require any activation energy hence the adsorption rate is high, even at low ambient temperature. For our experiment conducted at room temperature, it is predominantly physical adsorption.

When a solid surface is exposed, the molecules of gas strike the surface of the solid. Some of the molecules strike the surface becomes absorbed but the rate of absorption decreases as the more and more of the surface is covered by molecules.

Activated carbons are unique and versatile adsorbents. Their adsorbent properties are essentially attributed to their large interparticulate surface area, universal adsorption effect, high adsorption capacity, a high degree of surface reactivity and a favourable pore size which makes the internal surface accessible and enhances the adsorption rate. Most of the adsorption on active carbon takes place in micropores and only small amount in mesopores, the macropores acting only as conduits for the passage of the adsorbate into the interior mesopores and the micropore surface.

Because the active carbon application for the treatment of waste water is picking up, the production of active carbons has been increasing. The consumption of active carbons per capita a year is 0.5 kg in Japan, 0.4 kg in U.S.

[1] Written by S.E. Smith, Edited by O. Wallace - What are desiccants ? http://www.wisegeek.com/what-are-desiccants.htm

[2] 2006 SorbentSystems Desiccant types


[3] Silica Gel


[4] Calcium Chloride Desiccant, April 2010


[5] Written By Stevan Brown, CAFS - The Truth About Zeolites Used as Desiccants


[6] Carbochem (2010, 26th October) - Activated Carbon 101