Redox Chemistry inside wall

A discussion of chemical values is necessary to understand groundwater pollution problems. A variety of parameters space measured when a sample is accumulated for water analysis. The complying with is a brief compilation and discussion of some of the an ext common terms and principles provided in the Plume Busters conversation of chromium chemistry and groundwater.

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Water is consisted of of hydrogen (H) and also oxygen (O) atoms v a formula of: H2O. This method that because that every oxygen atom there space two hydrogen atoms bound come it. Each of this atoms has an electrical charge associated with it, either positive or negative. In this case the hydrogen atoms have actually a optimistic charge (H+) as soon as not linked with oxygen and also the oxygen has a an unfavorable charge (O-2). The superscripted number to the ideal of the chemistry symbol suggest the variety of electrons acquired or lost by chemical bonding. A confident number indicates that the hydrogen atom provided up an electron. A an adverse number suggests that the oxygen atom obtained two electrons, one from every hydrogen atom, in the chemical bonding process. The combination of atoms makes a molecule, i m sorry is bound with each other by the sharing of electrons with gained electrons under the influence of the oxygen atom many of the time. As soon as H2O is created the optimistic and an unfavorable charges are balanced out and also the fee on the molecule is neutral or zero.

Water, as soon as it dissociates, forms H+ and also OH-. A fee particle, such together (H+) or (OH-) is dubbed an ion. The charge, whether optimistic or negative, is referred to as the oxidation number, which to represent the number of electrons gained, lost, or common in a chemical reaction.

Oxidation-Reduction reaction

Redox reactions describe the motion of electron from one ion to another. The ax RedOx originates from the two procedures that occur during a reaction: REDuction and OXidation. These reactions happen together, lock cannot happen separately.

Oxidation is the term provided to the process of gain/addition of electron from one more compound, ion, or atom. Due to the fact that the oxidizing substance removes electrons from one more substance, which are then included to itself, the oxidizing substance i do not care “reduced” (more negative). And also because it “accepts” electron it is called an electron acceptor. The oxidation number becomes much more negative.

Reduction is the term offered to the procedure of loss/removal of electrons, i m sorry are provided to one more compound, atom, or ion. Because the reduce substance gives electrons to an additional substance it becomes “oxidized” itself (more positive). And also because it “donates” electrons it is referred to as an electron donor. The oxidation number becomes much more positive.

The an essential terms involved in redox have the right to be confusing. A mnemonic because that remembering what occurs is OIL RIG: Oxidation Is ns of electrons; palliation Is get of electrons. As an example, an facet that is oxidized loser electrons and becomes an ext positive; however, that aspect is described as the to reduce agent. If you desire to know, because that example, what one oxidizing agent does in regards to electrons:

Oxidizes something rather Oxidation is ns of electron (OIL RIG) That means that an oxidizing certified dealer takes electron from that various other substance and adds them come itself, for this reason the oxidation number becomes much more negative. For this reason an oxidizing certified dealer must get electrons.

In contrast, a reduce agent:

reduce something else Reduction is gain of elections (OIL RIG) This method that a to reduce agent provides electrons to one more substance and the oxidation number becomes more positive. A reducing certified dealer must lose electrons

The Figure listed below illustrates the process of oxidation reactions.


adapted from: http://depts.washington.edu/chemcrs/bulkdisk/chem162A_sum04/handout_Lecture_14.pdf

oxidization reactions are evaluated the equation in regards to which ions are lessened (gain electrons) and which space oxidized (lose electrons). This is excellent by writing half equations because that each reaction and also including the electrons in the equation. An example of a reaction is given below for iron.

Iron develops a variety of different ion for example, Fe+2 and Fe+3. If you think about how these can be produced from steel metal, the 2+ ion will be formed by oxidation the the metal by removing two electrons:

Fe+0 ® Fe+2 + 2e-

The iron is currently said to it is in in one oxidation state of +2. Remove of one more electron gives the Fe3+ ion: Fe+2 ® Fe+3 + e-. The iron now has actually an oxidation state of +3.


Batteries make use of redox reactions to develop electricity. What renders this possible?

Oxidation/Reduction Potential (Eh)

Metal ions have solid and weak oxidizing/reducing potential. In order to evaluate redox reactions, the oxidization potential that a systems is measured and also reported as Eh. The adhering to site http://www.chemguide.co.uk/physical/redoxeqia/introduction.html, shows how the Eh values are figured out experimentally. A hydrogen electrode is supplied as the recommendation to which every one of the experiments are compared and has a voltage of zero.

The outcomes are listed as voltage (or potential), which to represent the driving pressure for “moving” the electron from one place to another (for example, indigenous a reducing agent to an oxidizing agent). The higher the voltage the much more strongly the electrons will certainly be moved through a solution. A confident cell potential (voltage) indicates a spontaneous electrochemical reaction. A an unfavorable cell potential (voltage) shows a non-spontaneous reaction (the the contrary reaction will, therefore, be spontaneous!).

Redox potential (voltage) is a measure up of how easily a steel (or various other ion) will offer up electrons or maintain electrons, no the likelihood for a particular oxidation or reduction reaction occurring. The chemical equations are written together half-reactions through the most reducing reactions, donating electrons, having an Eh value that is an ext negative and the most oxidizing reactions, receiving electrons, having an Eh value that is the much more positive.

A perform of conventional electrode potentials for various reactions is given at: http://en.wikipedia.org/wiki/Standard_electrode_potential_%28data_page%29

This table illustrates the Eh worths that are really negative, such together for lithium (Li, Eh = -3.04), are solid reducing agents, that is, electron are given up quickly and also the oxidation number becomes more positive. Top top the other finish of the chart, fluoride (F, Eh = +2.87) is a solid oxidizing agent an interpretation that it takes electrons and the oxidation number becomes much more negative.

More an unfavorable values show reducing reactions developing (loss of electrons; more positive ion results) and oxidizing reactions (gain that electrons; more negative ion results). Higher Eh optimistic values imply that reaction is much more likely to happen spontaneously there is no the require of extra energy.

The Eh value gives an point out of exactly how likely a offered redox reaction will certainly occur. When the 2 equations room written in regards to electrons gift donated or obtained the Eh value will indicate whether the reaction is likely to proceed. In the situation of chromium the following reactions are provided in the typical electrode potential table described above:

Cr2O72−(aq) + 14H+ + 6e− → 2Cr3+(aq) + 7H2O +1.38 volts
Cr3+(aq) + 3e− → Cr(s) -0.74 volts
Cr3+(aq) + e− → Cr2+(aq) -0.42 volts

Based ~ above the voltages, the chromium(+6) (Cr2O7-2) value is a voluntarily reaction if it is in the visibility of a reducing agent (Eh = +1.38 v). The conversion of chromium(+3) to one of two people a solid step or come chromium(+2) is much much less likely come happen based on the an unfavorable Eh potentials.

This suggests that chromium(+6) will be preferentially lessened to Chromium(+3) and also then is not most likely to be more transformed into a mobile form.

Redox reactions space the managing chemistry because that chromium conversion from Cr+6 to Cr +3. Chromium (+6) is really mobile type and moves swiftly with ground water. Chromium is a an extremely toxic chemical, specifically in the Cr+6 form, to both human and environmental health. Reduction of chromium native its Cr+6 state come Cr +3 calls for a substance the will give electrons to the chromium for this reason it will become more stable and remain in the sediment fairly than move with the groundwater.

In the In Situ oxidization remediation method discussed in the Remediation ar of Plume Busters, a reagent is injected right into the aquifer with injection/withdrawal wells in ~ a rate and duration important to act the wanted volume of aquifer sediments. This treatment volume plus the amount of available iron in the sediments determines the quantity of reductive capacity generated in the barrier and the barrier’s productivity. The redox-altering reagent generally used is salt dithionite (Na2S2O4). The dithionite ion is frequently known as hydrosulfite and also is a strong reducing agent, an especially in strongly an easy solutions.


1. Sodium dithionite is a reducing agent. What is a reducing agent?

2. What happens as soon as chromium (+6) is reduced?

Measuring pH

Another parameter measured in groundwater is pH. This term way the “power that hydrogen” and is a measure up of the quantity of hydrogen ion (H+) in a solution. PH is a measure of the mountain of a solution in terms of the amount of hydrogen (H+) existing (http://en.wikipedia.org/wiki/PH). A solution has an acid pH if the value measured occurs between 0 and also 7 meaning there is an ext H+ present. A solution has a straightforward pH if the value measured occurs between 7 and also 14 definition there are much more OH- ions 보다 H+ ions present in the solution. A neutral pH is taken into consideration 7 where there are equal number of H+ and OH- ions. A ph that 7.00 is typically the worth measured for uncontaminated water.

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The values of pH are on a log in scale and represent a ten-fold diminish in H+ ion concentration measurement as one goes increase the scale. This method that a shift in pH from 2 to 3 to represent a 10-fold diminish in H+ concentration, and a shift from 2 come 4 to represent a 100 (10 × 10)-fold diminish in H+ concentration. The figure listed below shows a color scale of the pH worths of common substances listed to the left. One deserve to see indigenous the diagram that coffee is much more acidic 보다 bleach. This means that over there are much more H+ ion in coffee 보다 in bleach.

This also way there are much more OH- ions in bleach 보다 in coffee so bleach is more basic than coffee.

The pH of groundwater is useful to assist determine if contamination has actually occurred. Since water typically has a pH around 6-8 a number greater or reduced than the is a signal that there is something different about the water sample. The may imply a contamination problem if the number is a low pH (3-5) indicating the system is more acidic and potentially has much more dissolved metals or contaminants. The pH additionally helps to determine the type of reagents that may be required to treat a details contamination plume.