The iodine number is a measure used to assess the effectiveness of activated carbons in the form of granular (or pelleted) form. Activated carbon is typically employed to adsorb molecules with a high percentage of covalent bonds, such as the carbon-carbon or carbon-hydrogen bonds found in organic compounds.
What is the activated carbons iodine number?
One of the most widespread uses of activated carbon is in the de-chlorination of water, but it can also be employed as a reagent or a catalyst. The following describes the de-chlorination reaction, which eliminates free chlorine and produces chloride ions:
CO + H+(l) + Cl- -> HOCl (l) + C(s)
Where, C(s) stands for activated carbon, HOCl (l) for hypochlorous acid, and CO for a carbon oxide, which may be attached to the activated carbon's surface or that may dissolve in solution as carbonic acid. When water is decanted with activated carbon, further reactions take place in which the carbon takes part as a reagent or a catalyst.
How to determine activated carbons iodine number?
Let's assume that an activated carbon's "operation capacity" is the volume of fluid (gas or liquid), which it can handle to produce a specific grade. Chemical Oxygen Demand (COD), colour, odour, free chlorine, or a particular chemical or family of compounds, can all be used to quantify this attribute.
The "breakpoint" is reached when the quality of the effluent coming from the device, using activated carbon reaches an undesirable level. The carbon at this time needs to be replaced because it is past the end of its useful life. It is necessary to recycle or dispose of the used carbon.
The only method to precisely determine a granular activated carbon's operational capability is thus in the field. Granular activated carbon manufacturers and consumers have been searching for easy-to-measure physicochemical indicators, which show the carbon's functional potential. Iodine, phenol, methylene blue, molasses, carbon tetrachloride, butane, and other substances have adsorption capacities that are among these factors.
Why is iodine considered as activated carbon’s measuring index?
The parameter that has received the most commercial acceptance is the iodine number (iodine index), which measures the amount of iodine that one gram of carbon may adsorb when the residual iodine concentration in the solution is 0.02 N.
Why do coals with greater iodine counts typically cost more?
The value of iodine has been so widely recognised that it heavily influences the pricing, of the majority of commonly available activated carbons.
Iodine number has been widely used as a factor in carbon evaluation for two reasons:
1: The iodine number has been demonstrated to be relatively proportional, to the surface area of the carbon for specific types of carbons that are activated under specific conditions, i.e., surface area determined by nitrogen adsorption; and
2: The material and reagents for measuring iodine number are inexpensive, and the analysis time is reasonably brief.
Unfortunately, the truth is that neither the activated carbon's surface area, nor its usable capacity are accurately reflected by the iodine number, so much so that the iodine number is not employed as a variable for carbon assessment in the scientific community.
Does the iodine number affect the activated carbon's ability to adsorb substances?
1. The surface area of the few activated carbons is the only factor that influences the iodine number.
The tri-iodide ion, or I3-1, is the molecule that is adsorbed on the carbon during the calculation of the iodine number. The surface oxides in the activated carbon reject it because it is an anion. As a result, even though an activated carbon's surface area remains constant, the amount of surface oxides it has will affect its iodine number.
2. The adsorption capacity of iodine is not related to the adsorption capacity of other molecules, even if the iodine number is proportional to the surface area of any activated carbon.
There are two causes for this:
· The link between the size-shape of the molecule to be maintained, and the carbon's pore size distribution determines an activated carbon's ability to adsorb molecules. Larger molecules cannot fit through a pore because of its diameter. Additionally, molecules that are significantly smaller than a pore's diameter, are adsorbed with less force and consequently less successfully.
· The chemical structure of the molecule to be retained and the surface chemistry of the carbon, both affect an activated carbon's ability for adsorption. Because of this, the adsorption capacity of the molecule (or family of molecules) that we are trying to adsorb, does not always correlate with the amount of iodine present.
3. In the majority of applications for granular activated carbons, their operational capacity is influenced by both their kinetics (speed of action), as well as their adsorption capacity, more correctly known as "equilibrium adsorption capacity".
A granular activated carbon's kinetics of operation depends on:
· Its major pore diameter and pore size distribution, respectively.
· The kinetics with which activated carbon operates increases with pore diameter.
· The distribution of granular activated carbon's particle sizes.
· The kinetics of the granular activated carbon is higher, the smaller the particle size.
When the "mass transfer zone," or MTZ, or depth of the carbon bed where the adsorption process occurs is measured, it is discovered that the MTZ's height decreases with decreasing particle size. Additionally, the break point will occur later for the carbon with smaller particle sizes, since the MTZ is lower and this carbon will have a higher operational capacity.
Conclusion
The capacity of a carbon to adsorb a molecule other than the tri-iodide ion, cannot be directly assessed using the iodine number. It is also considerably less likely to be a factor in predicting, the operating capacity of granular activated carbon.
If the same commercial granular activated carbon is used and the composition of the solution being treated, measuring the iodine number at the breakpoint and using this data to try to predict the approximate time of the breakpoint for future operating cycles, may be of some use in predicting the remaining operating capacity of an activated carbon in use.
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