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Little_Ghost_again
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where did my Iodine go :{
I needed some Iodine crystals to help mark some spots on a TLC plate. Having none left I thought I would use some of my precious potassium iodide and
make some.
So I dissolved around 12g of Potassium Iodide in a small amount of water in a flask and added 98% sulphuric acid, this seemed to go well and got hot
and went a dark red colour, I could see crystals on the bottom and assume this was Iodine.
I then add water and decant off some of the liquid, but the crystal seemed to dissolve! So I ended up with no Iodine and just a red solution.
So where did I foof up? I only have around 10g left of potassium Iodide but need some Iodine, the Sulphuric acid is new and clear and states its 98%.
Judging by the way it eats through paper I am pretty sure its close to the 98%
Dont ask me, I only know enough to be dangerous
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fluorescence
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Strange that that happened with conc. Sulphuric Acid. I had that problem many years ago when I started
doing Chemistry and mostly people didn't really get what I was talking about. I didn't get much Iodine out
although I used quite a lot of Iodide. Today I think this is due to Iodide being present while you make it.
Iodine and Iodide will form Polyiodides and the slower you oxidize your Iodine the more should built up.
I compared the reaction with Hydrochloric Acid + Peroxide in comparison to Sulphuric Acid + Peroxide and
the Hydrochloric Acid produced only very little Iodine but a lot of a brown solution. If you add conc. Sulph Acd.
to Bromides it will form a lot of Bromine so the oxidizing power should be high enough to make some Iodine even
without using another oxidizer like Peroxide. Thus it quite wonders me ... Iodine should not really dissolve in water
or Acid, and no matter how bad the yield is there was always some Iodine flakes in there that could be
filtered off. The only reason I can think of is more Iodide but I dunno why that would happen if you only added water...
Take some of it and try adding some Starch to it to see if it's indeed a mixture of Iodine in Iodide. That might explain
why it dissolved.
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Little_Ghost_again
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Quote: Originally posted by fluorescence  | Strange that that happened with conc. Sulphuric Acid. I had that problem many years ago when I started
doing Chemistry and mostly people didn't really get what I was talking about. I didn't get much Iodine out
although I used quite a lot of Iodide. Today I think this is due to Iodide being present while you make it.
Iodine and Iodide will form Polyiodides and the slower you oxidize your Iodine the more should built up.
I compared the reaction with Hydrochloric Acid + Peroxide in comparison to Sulphuric Acid + Peroxide and
the Hydrochloric Acid produced only very little Iodine but a lot of a brown solution. If you add conc. Sulph Acd.
to Bromides it will form a lot of Bromine so the oxidizing power should be high enough to make some Iodine even
without using another oxidizer like Peroxide. Thus it quite wonders me ... Iodine should not really dissolve in water
or Acid, and no matter how bad the yield is there was always some Iodine flakes in there that could be
filtered off. The only reason I can think of is more Iodide but I dunno why that would happen if you only added water...
Take some of it and try adding some Starch to it to see if it's indeed a mixture of Iodine in Iodide. That might explain
why it dissolved. |
I flushed it as it was giving off a gas, having now seen a video I can see two mistakes I made.
1. I didnt wait long enough and probably didnt add enough acid, I saw some on the side of the flask go black and assumed (probably wrongly) I had
added too much acid. According to the video you add it slowly and keep the flask in iced water, I didnt do this!
2) add the solution with crystals to ice water I just added water and the crystals went.
So I assume that the first mistake was not letting it all go black, although I did get crystals, I then think adding water just heated what Iodine I
did have and boil it off. I will try again and use an ice bath, this time slowly adding acid until it goes like slush then add more until liquid.
I will then let it cool off in an ice bath before adding iced water to it. I will have another go in the morning, its a pain as both iodide and Iodine
are pretty expensive for what they are.
The plate I wanted to develop has now gone clear but hopefully there will be enough compound on the plate to detect it
Dont ask me, I only know enough to be dangerous
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fluorescence
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Dunno. I've been making Iodine for quite a while till I just bought a bottle. I had various attempts to do it, usually I would
mix the Iodide directely with an Acid ( usually diluted Sulph) and thenn add Hydrogen Peroxide to it so it would form Iodine. Then usually large lumps
of a black precepitate and thats it. I didn't wait, cool or did anything else, just filtered it off and recrystallized it ( if I even did that).
Actually there isn't that much that you can do wrong. Perhaps add an Alkane like Heptane or Hexane something nonpolar you can buy in a drugstore and
add that while you make the Iodine under stirring. That could help to extract some of the iodine into a layer away from the Iodide but usually you
dont have to do that.
That reaction is something that hasn't ever failed. For Bromine you might add a little heat till it starts but Iodine ?
[Edited on 28-9-2015 by fluorescence]
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woelen
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Adding H2SO4 to KI leads to formation of I2, but also to formation of a reduced species (mainly SO2 and H2S). On addition of water, however the
reverse reaction occurs. The iodine is reduced to iodide again. Some iodine remains, because some of the SO2 escapes and some of the H2S is not
converted all the way back to H2SO4 but to elemental S.
If you want to make I2 from KI, then you have to add an oxidizer to the acid as well. A good method is to add excess dilute H2SO4 (e.g. 20%) to a
solution of KI and then add an excess amount of solution of 6% H2O2. You will get a lot of solid I2, which can be rinsed with water to get rid of the
H2O2 and acid.
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Little_Ghost_again
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| Quote: Originally posted by woelen | Adding H2SO4 to KI leads to formation of I2, but also to formation of a reduced species (mainly SO2 and H2S). On addition of water, however the
reverse reaction occurs. The iodine is reduced to iodide again. Some iodine remains, because some of the SO2 escapes and some of the H2S is not
converted all the way back to H2SO4 but to elemental S.
If you want to make I2 from KI, then you have to add an oxidizer to the acid as well. A good method is to add excess dilute H2SO4 (e.g. 20%) to a
solution of KI and then add an excess amount of solution of 6% H2O2. You will get a lot of solid I2, which can be rinsed with water to get rid of the
H2O2 and acid. |
Thanks Woelen, I dont have hydrogen peroxide at the moment so will order some. watched the nurdrage video and he didnt seem to use any with sulphuric
acid, but then again I didnt let mine go black and I didnt loose that much gas (flask was stoppered).
Oh well I will have to wait to develop the plate untill I get some peroxide.
Iodine is pretty expensive and seems to get out, so I would prefer to by iodide as its about the same price and I can make Iodine as I need it, plus I
two chemicals for one  .
I dont suppose any other oxidizer would work? At this point telling me permanganate would make me very happy lol but I cant see it.
It took ages to isolate the plant material but it seems very volatile, I might try and use a plate that glows with UV and try and isolate some that
way.
Thanks alot for the info I was feeling a bit stupid for messing up something so simple
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aga
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You definitely have an air bubbler.
May as well give that a try.
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Little_Ghost_again
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Yeah but being a Muppet I threw the solution out! When I can afford more Iodide I might try the air stone, the peroxide has cleaned me out! well that
and the gram stain kit.
My tomato died by the way  , it was doing well but I messed up with setting the dose
pump, by the time I noticed it was too late! I gave it sodium chloride for a couple of days!
The pump was in there for some marine algae and I connected the wrong pump up.. Man its been a week of cock ups, I do have an update to an old thread
with copper nitrate, I will take some pics and reveal the final crystals , I am
well pleased with the results, but still have no idea what the green powder is lol
Dont ask me, I only know enough to be dangerous
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woelen
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Permanganate is not really suitable. It does oxidize iodide, but you need an excess amount to get all iodide oxidized and your solution is very dark
with excess permanganate so that isolating the iodine becomes harder. Permanganate also may lead to formation of MnO2, which may be hard to separate
from the iodine (it looks nearly the same).
You need a colorless oxidizer, which produces (preferrably) colorless end-products, which are soluble.
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fluorescence
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Seperating the Manganese from Iodine wouldn't be too problematic. He could just add Hexane or Heptane and get it into the nonpolar layer. But it's
kinda annoying. I think you just need a certain minium level of chemicals in your lab. Iodine is something you could have although I see no reason why
as long as you have the stuff to make it. But Peroxide and the standart Acids should be somewhere around. Otherwise it's quite hard to make even the
simple chemicals. Some people tried to extract it from commercial Iodine-Solutions but that is not really easy and it only produces little yield.
I can't come up with a fitting oxidizer, too. The only thing I could think of was to not dilute your Acid beforehand nor dissolve the Iodide in Water.
Might be quite ugly result but when I added a bit of bromide to my conc. Sulph. Acd. it produced fumes of Bromine and a really dark red solution. If
you do this with 38% Battery Acid + Oxidizer it needs quite a long time and you have to heat it and won't get even close to the conc. Acid. Since it's
an Oxidizer, too you coul try it and dilute it afterwards although adding water to acid is dangerous and not that easy to pull of. So I can't
recomment you that.
Mh.. okay I'll go in the lab in a minute and just try what happens before you waste more of your Iodide and the thing just spills everywhere.
Edit:
Okay don't try that one either. It produces only very little Iodine but a lot of sulphur gases that smell horribly. I guess dilute Acid + Oxidizer is
still the best method.
[Edited on 29-9-2015 by fluorescence]
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woelen
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Iodide and conc. H2SO4 gives a horrible mess. You get iodine, but the sulphuric acid is reduced to a mix of SO2, H2S and S. The mix of H2S and SO2
gives a horrible smell, due to formation of Wackenroder's compounds.
You definitely need diluted acid (at most 20%) and you need a suitable oxidizer. H2O2 is perfect, bleach also works, but with bleach you need to use
the precise stoichiometric amount. Too little and a lot of iodine remains in solution with not converted iodide as I3(-). Too much bleach and your
iodine is converted to tetrachloroiodate(III) and iodic acid, which also remain in solution. With H2O2 you have the nice advantage that if you use
excess amount, then all iodide is converted to iodine and any excess H2O2 does not further react. Iodine, without iodide present, is nearly insoluble
in water and hence all of it settles as a solid and the liquid only becomes fairly pale brown.
The iodine mud can be filtered preferrably not on paper, but on sintered glass. If this is not available then decanting as much as possible of the
water, rinsing with clean water and then decanting again and then putting the mud on a double-folded coffee filter put on a pile of paper tissue can
be done succesfully. Final drying can be done in conc. H2SO4 and gentle heating. The iodine will melt and form a dark liquid under the acid. Allow
this liquid to solidify, take out of the acid and rinse very well in water and then dry with a paper tissue and only after that crunch it into smaller
parts and store in a vial.
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macckone
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You really need an oxidizer when making iodine from potassium iodide.
KI + H2SO4 -> KHSO4 + HI
4HI + O2 -> 2H2O + I2
Bubbling air through is slow but sufficient.
Hydrogen peroxide is the better way.
Hydrogen Iodide will reduce sulfuric acid to some extent
which is where the iodine comes from if you don't add an
oxidizer but then you need more sulfuric acid.
Note that you can also get potassium sulfate so you need a total
excess of sulfuric acid.
Also note that although chlorine bleach can be used as an
oxidizer, you will likely wind up with chlorine contaminated iodine
which is usually fine for developing plates.
[Edited on 29-9-2015 by macckone]
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Texium
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I once tried using nitric acid to oxidize iodide to iodine, and the iodine formed briefly, but then redissolved forming a somewhat sweet, clean
smelling colorless solution, leading me to believe that iodic acid had formed. Adding ascorbic acid caused some of the iodine to precipitate again.
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Hawkguy
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If you didn't add enough acid, the Iodine could have simply dissolved. Iodine is soluble in Alkali Iodide solutions.
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aga
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Measurement.
Only the starting weight of KI is roughly measured.
You should also measure the water, the acid - everything.
For starters, titrate your sulphuric acid to determine what the actual w% is.
No, i do not do it all Right like that either, but when stuff goes wrong, there isn't much choice but to start again and at least collect accurate
data so you can work out how to do it right.
I could easily be wrong, but i believe that's what they call Science.
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fluorescence
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| Quote: |
You really need an oxidizer when making iodine from potassium iodide.
KI + H2SO4 -> KHSO4 + HI
4HI + O2 -> 2H2O + I2 |
Really ? Does it happen that way ? I'd say since HI is way stronger than H2SO4 sulphuric acid shouldn't usually be able to
protonate an Iodide. I first thought it would work like that, too that the Acid produces HI and Peroxide would oxidize it but when I look at the
Redox-Potentials:
2I- -> I2 + 2e- => +054
H2O2 + 2 H3O+ + 2e- -> 4 H2O => + 1,78
Or the one you were referring to
O2 + 4 H3O+ + 4e- -> 6 H2O => 1,23
So both of these Systems would be strong enough to oxidize Iodide to Elemental Iodine. Ehm...if I remember correctely Redox Systems with Acids or
Bases can be calculated with the Nernst Equation, there was one that deals with pH, too. So for these Systems the conc. of Acid is important, too.
Stochiometrically from the equations above it needs quite some acids to produce these quite high Redox Potentials. And that might be the reason why
Acids are needed.
I might be totally mistaking here but for me it just sounds better to produce Iodine via a Redox Reaction than protonating a stronger Acid with a
weaker one.
Does anyone see the mistake I'm making here, it's been a while since I had to calculate out stuff like this (my high school finals to be honest xD ).
[Edited on 29-9-2015 by fluorescence]
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aga
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That's the Spirit : Knowledge, Science, Facts, Equations.
Very interesting to see the eV used as a result of a reaction, and to be able to use that to guage what will happen.
Not seen that before. So much to learn !
[Edited on 29-9-2015 by aga]
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fluorescence
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That's Inorganic Chemistry, the things we love is not Salts or Crystals, it's Equations, Solubility Products, Complexation Constants, Dissociation ...
so much stuff to calculate but so many possibilties if you can do it quite well.
I remember in school we did that even more advanced than in the University. We used to calculate the amount of water produced, too and calculated the
dilution generated from it into let's say neutralisations and pH Values and stuff like that.
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Rhodanide
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| Quote: Originally posted by Little_Ghost_again | I needed some Iodine crystals to help mark some spots on a TLC plate. Having none left I thought I would use some of my precious potassium iodide and
make some.
So I dissolved around 12g of Potassium Iodide in a small amount of water in a flask and added 98% sulphuric acid, this seemed to go well and got hot
and went a dark red colour, I could see crystals on the bottom and assume this was Iodine.
I then add water and decant off some of the liquid, but the crystal seemed to dissolve! So I ended up with no Iodine and just a red solution.
So where did I foof up? I only have around 10g left of potassium Iodide but need some Iodine, the Sulphuric acid is new and clear and states its 98%.
Judging by the way it eats through paper I am pretty sure its close to the 98% |
Same happened to me but w/ HCl+H2O2. I have no idea what happened. Hope your luck changes, though. 
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Little_Ghost_again
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My acid is bang on 98% sulphuric, I think the big mistake was not using enough. I stopped adding it as soon as a small portion went black went black,
looking at the video you actually want it to go black.
I have peroxide ordered but just for the hell of it I will try this later with half of what I have left. I will report back with the measured amounts
.
aga get off my case about measuring stuff, I still remember the post about deckchairs etc! As far as I am aware 3 deckchairs is not a recognized SI
unit of measurement lol.
Just because you have got all scientific lately dosnt mean I cant go through the initial bung it all in a pot route that you also started with  .
TO BLOGFAST
Stop teaching aga to be a real scientist he is becoming a pain in the arse with his proper procedures!! I want him back to pyrolising deckchairs over
BBQ.s immediately please
[Edited on 30-9-2015 by Little_Ghost_again]
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macckone
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| Quote: Originally posted by fluorescence |
| Quote: |
You really need an oxidizer when making iodine from potassium iodide.
KI + H2SO4 -> KHSO4 + HI
4HI + O2 -> 2H2O + I2 |
Really ? Does it happen that way ? I'd say since HI is way stronger than H2SO4 sulphuric acid shouldn't usually be able to
protonate an Iodide. I first thought it would work like that, too that the Acid produces HI and Peroxide would oxidize it but when I look at the
Redox-Potentials:
2I- -> I2 + 2e- => +054
H2O2 + 2 H3O+ + 2e- -> 4 H2O => + 1,78
Or the one you were referring to
O2 + 4 H3O+ + 4e- -> 6 H2O => 1,23
So both of these Systems would be strong enough to oxidize Iodide to Elemental Iodine. Ehm...if I remember correctely Redox Systems with Acids or
Bases can be calculated with the Nernst Equation, there was one that deals with pH, too. So for these Systems the conc. of Acid is important, too.
Stochiometrically from the equations above it needs quite some acids to produce these quite high Redox Potentials. And that might be the reason why
Acids are needed.
I might be totally mistaking here but for me it just sounds better to produce Iodine via a Redox Reaction than protonating a stronger Acid with a
weaker one.
Does anyone see the mistake I'm making here, it's been a while since I had to calculate out stuff like this (my high school finals to be honest xD ).
[Edited on 29-9-2015 by fluorescence] |
I was over simplifying it to some extent to make it clear.
The reality is that the various species exists mainly as ions
in solution. You do need an acid to balance the hydroxide
ions in the solution as the iodide ions are oxidized to elemental
state. The net final products are going to be potassium bisulfate
and iodide and water with some sulfuric acid being consumed
as an oxidizer if no other oxidizer like hydrogen peroxide is
available.
Acid strength is misleading as it means it is more readily
converted to ions. For example hydrofluoric acid is a very
weak acid but it will attack almost anything and add fluorine
to the compound attacked. On the other hand, it is mainly
produced by the addition of sulfuric acid to calcium fluoride.
Other factors play in as well such as rather you are dealing
with an aqueous solution or an anhydrous environment or
an organic solvent (polar or non-polar). And generally these
things are in an equilibrium, not all on one side of the equation
or the other.
The exact path for oxidation is going to depend on the
oxidizing agent. A large number of species of ions are
possible there. I mean how many compounds are there
that can be used as oxidizing agents?
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Little_Ghost_again
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Ok I got it to work!! But what alot of agro!!
The stuff is horrible to make and then try and get into a container after purifying.
First the bad news, blue air stones and conc sulphuric acid dont mix! the blue stone dissolved into blue sand immediate and I was left with a little
plastic tube.
Next I just dissolved some iodide into water and kept adding acid until it was black, i also kept putting the flask into a bowl of cold water or the
iodine vapor simply left the flask as the solution got very hot when adding acid.
I used a shaker machine for 20 mins and that seemed to do the trick, but when I tried a small amount in another flask and added water the solution
went white, no amount of acid changed it back.
Anyway I poured off the solution (Ive kept that) and washed the crystals with very cold water, I had to do it kind of quickly or I found it started to
change. It was still acidic so in the end I stuck the beaker on a hotplate and sublimed it onto a round flask bottom (flask filled with cold water),
the stuff set like rock to the flask!
I got covered in the stuff trying to get it into a bottle, my yield was frankly appalling to the point I didnt even bother to weigh it. But at least I
have some iodine now.
Next job I guess is to try and distill back some of the 500ml of conc sulphuric acid I used!! There is both a white milk like colour and a pinky
colour in the solution and a brown. But I am assuming I should be able to distill azeotropic sulphuric from it. i will do this tomorrow, anything I
need to watch out for? and what temperature should I expect the acid to come over at?
I havnt got a manometer but I could use a vacuum pump if its needed. Thanks for the help I needed some iodine quickly or I would have ordered some,
the plate came up positive so at least I know I can extract the compound I am after .
In case anyone is interested (unlikely) I am trying to isolate the compound in Rhododendrons that stops other plants growing near it, I am hoping it
might be a good environmentally friendly herbicide for Ragwort.
Dont ask me, I only know enough to be dangerous
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macckone
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That is actually a pretty cool project.
An environmentally friendly herbicide.
It might be viewed negatively by some very large agribusinesses
like monsanto and chemical companies like dow.
It would seriously cut into their business.
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hyfalcon
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Try spraying your ragwort with a 20% solution of acetic acid. Best herbicide that leaves no long lasting residue. Plant in the same spot you sprayed
today, tomorrow.
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macckone
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Acetic acid and any number of other chemicals are fine for pre-planting.
Concentrated phosphoric acid would also work and actually have soil benefits.
A neutralizer like calcium ammonium nitrate would be great.
Pre-emergent is also a lot less difficult as you can use contact poisons.
Post-emergent you need something selective that will only effect
the plants that you don't want in the field.
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