noc18-mm14 Lecture 30-Exchange Current Density and Standard Hydrogen Electrode

[Music] [Music] let us start lecture 30 and we'll continue our discussion on kinetics of corrosion and we have been discussing on current density and we saw the concept of a zero also which is called exchange current density which appears when for a particular redox reaction for this particular reduction when it reaches equilibrium so that means then this is a kind of situation which is reversible as well as this is non corroding why it's non corroding because when a metal is dipped in us in an aqueous medium the metal ion comes out from the metal surface as well as the reverse reaction which is the metal ion gets reduced to metal these two processes happen at the same rate and that's the condition when we have the exchange current density the metal ion formation which is M goes to M n plus plus ne this is corrosion or we can take it as anodic reaction or oxidation reaction and then this backward reaction which is reduction or cathodic reaction or deposition reaction both this process happens happen at a very same rate and the rate is termed in terms of I which is the exchange current which is basically the current density since we have seen that I a divided by n F is nothing but the rate of metal dissolve in ground per unit area and per unit time where is nothing but atomic weight of that particular metal n is the number of electrons that participate for this reduction reaction or oxidation reaction F is 1 Faraday so this particular term we call it as IC and this one as I a I a is nothing but the corrosion current density and in order to indicate the direction of flow of these two currents we have put a negative sign in front of IC these negative sign does not indicate that the value of current is current density is negative it indicates that it flows opposite to the anodic current or the corrosion current density now when they reach equilibrium we come across a 0 which is the exchange current density and that is basically the reversible condition and non corroding situation because whatever metal ion forms that metal and goes and deposit on the on the metal surface now once we know this is zero we need to look at little bit carefully on this I 0 because this is 0 is a very very strong and fundamental kinetic parameters and this parameter would have a huge importance on the corrosion of a particular metal in a particular medium and that will see little later when we start discussing on mix potential here now if we see this eyes ear oh this eyes zero we have said that this is specific to a particular relaxed reaction it's not for the complete cell reaction this is for a half cell reaction I would say for example if we take this consideration like zinc plus HCl equal to zinc chloride plus h2 for this reaction it can be broken into two half cells one is zinc minus 2e equal to zinc plus plus and two h plus plus 2 e equal to h2 this is the gas that forms on the zinc surface so it's experimentally we have seen that when zinc is dipped in HCl let us say that HCl is pure HCl and it doesn't contain any oxygen so that means it's a de rated solution that time we have these two half cell reactions so for each half cell reaction we will have corresponding e 0 and in this case how do i how do i denote this it should be denoted like this this is i0 this is also a 0 for h plus h2 so for the reduction reaction we are considering this is the this is the notation good use and now question is this i's 0 indicate some equilibrium reaction of that particular metal metal ion disposition or dissolution or hydrogen ion formation or hydro gasps formation now those reactions happen on a particular surface and in order to indicate that this is taking place on that on a particular surface we have to also indicate on which surface this is taking place so in the bracket we have to mention zinc because on the zinc surface only that zinc dissolution takes place as well as hydrogen evolution also takes place on the zinc surface so zinc so now exchange current density I zero on a metal surface and then so this is the notation and now these particular redox reaction which is half-cell reactions before examples this is for a hub so this is for a half cell and they are combination they are making constituting one complete full cell full cell reaction now these particular reaction might change it could be metal dissolution or it could be any other cathodic or anodic reactions that are taking place on this metal surface for example in this zinc case the zinc dissolution is taking place on zinc so from the zinc surface and that's what we are considering eyes Eero for zinc dissolution on zinc as well as hydrogen evolution is also taking on taking place on the zinc surface that's what we are considering hydrogen evolution exchange current density on zinc surface also now when we consider this another important aspect is since we are considering the material aspect so of course this exchange current density would depend on materials similarly it would also depend on the surface condition so these are two important issues we would like to consider so one is metal surface and of course a kind of metal so let us put it in two ways one is metal which is type depending on zinc platinum or palladium or gold this icy road would change for a particular reaction second is metal surface so the metal surface condition would also change the i0 for a particular reaction so this is surface condition I would say surface morphology morphology so let's look at this two situations one case is metal soft metal surface depending on the type of metal let us say we consider this reaction this hydrogen evolution reaction state their state this we call it h e our hydrogen evolution reaction now depending on this reaction we can constitute a cell which is called standard hydrogen electrode which we in short we call it as is a chi and when we talk about standard electrode standard hydrogen electrode we need to also see the how it how what is the importance of the standard hydrogen electrode as well as its constitution so it looks like this somatically you have acid solution where you can make it as h2so4 where H+ ion concentration is maintained at one molar one more now this is a glass simple a glass electrode where this is a metal wire which is connecting a platinum plate which is dipped in acid medium and through this route we send hydrogen gas and that pressure is maintained as one atmosphere and when you do that that time we get potential would be e zero plus RT 2 F Ln ox red which is e zero and if we consider 25 degree Celsius and 1 atmosphere pressure so then this quantity would become 0.0591 divided by 2 and that time this quantity this is B in terms of log instead of lon and then ox is nothing but h plus ion concentration square and ph 2 the reaction is h2 now this is maintained at one atmosphere so it will become this is zero minus 0.0591 pH so these two they will get canceled so this is the equation so finally we are getting e equal to minus like this and I would like to put this zero value now when this is maintained that one mole one molar solution so then this goes to zero so e equal to zero volt so now once we maintain this easy row so if it is connected to let's say another cell with a salt bridge this is the salt bridge and if we have a zinc rod here see if we connect like this this is zinc sulfate we would see some voltage here and this voltage is the difference between the electrode voltage between this one and this two so here we have electrode voltage which is e 2 now if we maintain this situation 1 molar as well as one atmosphere hydrogen pressure then e 2 would be always zero and that is what the e 1 is nothing but e 1 minus e 2 which is the difference between these two electrodes is equal to e 1 so now this becomes my unknown electrode potential isn't it so that's the use of the standard hydrogen electrode and why we are calling standard because we are maintaining one atmosphere pressure and 25 degree Celsius and the activity or the concentration of hydrogen ion is also maintained that one so now in order to maintain so we have to make sure that this particular cell this particular cell stays at 0 volt all the time that means that standard reduction potential for this reaction in this particular hydrogen electrode is to be maintained at zero volt in order to do that we have to maintain those concentration as well as pressure now if we and this is taken as platinum now if we take some metal and why we take platinum because the on platinum surface it is around 10 to the power minus one to 10 to the power rather 10 to the minus 2 2 minus 3 ampere per centimeter square which is considered to be very very high current density now what is why we should choose rather we should choose some particular metal where this hydrogen reduction exchange current density should be very very high now what do you mean by very very high and what is the advantage of very very high high value of i0 for this particular hydrogen reduction reaction or hydrogen reaction which is the half-cell reaction now when you have this then if we see the rate part let us say this is the time and this is the rate once a metal is introduced in a solution so this metal ion would form so or other way around if we have the solution maintained at with the metal ion concentration M maintain with some metal ion presence there so then there could be reduction reaction as well as oxidation reaction so reduction reaction is nothing but M n plus plus any equal to M or oxidation reaction a minus any equal to MN plus so these two reaction would happen now let us say initially the reduction reaction would be very high because we have already the presence of MN plus depending on its reduction potential MN plus would try to reduce and then form M and as this reaction is starting this reaction would also start but they will be sluggish in the beginning but as the time passes on they will pick up the rate will pick up so and the forward reaction which is this reaction the rate would reduce so M n plus plus any this rate initially would be very high because already they are mn+ aren't presence and then initially this particular reverse reaction which is M going to MN plus would be less because initially if I assume that the first the MN plus odd ion ion would get reduced first and then metal would release electron and then for metal line so this is rate one and this is rate too so rate 1 is nothing but I a sorry I see because this is the cathodic reaction and rate 2 is ia in terms of current density if we like would like to express them so that I see the first rate is this one and then second rate this is rate - this is rate one now at some time after some time they will reach equal and then they maintain if we don't disturb the equilibrium because they would reach equilibrium so this is the condition which is basically the equilibrium rate and or eyes zero this is I see this is I a and now interestingly you see this eyes ero is arriving at this time this is the time let's say T zero and this of course happens in a metal surface that's per se metal surface so this is for this particular redox reaction so similarly if we can consider this hydrogen reaction here also this forward reaction and backward reaction they will reach equilibrium after some time on a particular metal surface let us say metal one now this on metal - this is i0 on metal one eyes ero on metal - and when we talkin siddur about eyes zero metal - if I zero metal 1 is less than i0 metal - it means that equilibrium on metal - for this reduction reaction or that this particular half cell reaction would actually derive at a much faster time so means it will arrive here so that means this is my new eyes ero you see the i0 this is on metal one this is metal - so that means i zero in case of metal - arrives at a much faster time this is let say T 1 now what it indicates it indicates that the equilibrium achieves or is arrived at much quickly in case of higher exchange current density so higher exchange current density indicates hourly now early equilibrium also indicates this particular our Linda K equilibrium also indicates love let's say somehow on this metal to surface we disturbs the equilibrium because of some situation and since it has got a higher i0 like the situation what we have seen higher i0 so the equilibrium will be reestablished much quickly now interestingly when equilibrium reestablishes quickly it indicates if he is disturbed so higher exchange current density it indicates hourly equilibrium and early equilibrium indicates that if the equilibrium is disturbed by some means it will reestablish quickly what is the importance of that part now this particular cell is basically a reference cell which is the hydrogen reproduction hydrogen standard hydrogen electrode and in case of standard hydrogen electrode we have to maintain this voltage which should be known beforehand otherwise we cannot know the potential of the unknown electrode so in order to know that in order to maintain this particular potential all the time similar potential and known potential we have to also see that that potential is stable and in order to make it stable we have to also see that the equilibrium is reached quickly because these potential is actually condition of indicating a condition of equilibrium this also indicates a condition of equilibrium so what and that means indirectly it also indicates that it has to have a non corroding reversible condition and when it is a non corroding reversible condition that means it is also related to i0 so higher rise zero means the condition equilibrium condition of equilibrium is established and if there is a disturbance then higher Iser also indicates that condition of equilibrium will be reestablished quickly so all the time we can maintain this standard reduction or standard potential or standard reduction potential to be de to be at the known potential so it will help us in determining the reduction potential or the electrode potential of some unknown cell there could be possibilities that that equilibrium is disturbed so by some means during the experimentation during measurement of this unknown electrode potential measurement and since platinum has got a very very high deduction exchange current density of hydrogen reaction it will reestablish the equilibrium and so we are will be okay with this reference electrode now if I try to see platinum as well as zinc now in that case on zinc for this particular reaction Isay Row is of the order of 10 to the power minus 11 to 10 to the minus 13 ampere per centimeter square and here it is as we have mentioned is equal to 10 to the power minus 2 2 to the power minus 3 and also if I try to see other values of Phi 0 on top of let us say iron it is of the order of 10 to the power minus 6 ampere per centimeter square a 0 of hydrogen reaction on nickel is of the order of 10 to the power minus 7 ampere per centimeter square so these are the values that means on platinum surface hydrogen reaction would achieve equilibrium quickly compared to sink surface it will take much longer time and when it takes much longer time that means if we make some standard reduction standard for cell with by using zinc it will not be a good practice because it will take long long time to reestablish to establish equilibrium and then if somehow equilibrium is this is to stop disturb it will also take long time to reestablish that equally so that's what the Platinum is very much preferred electrode for hydrogen cell hydrogen standard electrode now instead sometimes we use platinum platinum and in case of platinum platinum so I zero for this hydrogen even reaction is much higher than i0 on platinum see the when it is platanus on platinum platinum is splitted so when platinum is plated it's actually forms a very rough surface and it means that if we have a particular area of platinum and if it is platinum platinum so then the actual area increases if we see this one from cross-section so it would be look like this so this is the cross-section from the side if you look at so actual area increases this so here actual area increases which leads to much higher hydrogen reaction or a zero increases it suggests that if we use platinum patina in place of platinum standard hydrogen electrode would be much better compared to if we use platinum because in case of Platina is patina is zero is higher so the equilibrium would be much stable equilibrium or if we disturb this it will come back to the equilibrium quickly so that helps us to maintain the standard potential of that hydrogen electrode so that we can measure the another potential of an electrode with certainty or purity so this is about the effect of different electrodes different metals as well as the surface conditions so now there are other factors on which this exchange current density depends it depends on as we have said metals type morphology of the surface then we have temperature environment why environment because sometimes we have a situation like in one place we have higher salt concentration salt content or in other place we have less salt content then for example in seashore area salt content is very high and if we go to him Allah and belt the salt content would definitely be less so in those two cases even in use the same temperature and pressure and the same metal and solution we would have a difference in exchange current density of a particular reaction and then we have solution concentration this would definitely change the exchange current density also so these are the some of the factors on which density so we'll talk about exchange current density more in our next lecture so let's stop here thank you [Music] [Music]