Bezaleel
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Titration of ethylenediamine
A year ago, I did some experiments to synthesize Ni(en)3Cr2O7. This is said to be a light red compound, which dissolves quite well in water. A dark
brown complex Ni(en)2Cr2O7 also exists and is formed more easily. In the process, I also obtained two less soluble compounds, which are probably
regular chromates, and I would like to determine how much (en) is contained in these compounds.
Hence I'd like to know whether there is any way to determine the amount of (en) in a solution through titration. (Other methods which can be easily
carried out in an amateur setting are welcome too of course.)
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Bedlasky
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Amines can be determinated by titration with 0,1M HClO4 in glacial acetic acid. That 0,1M HClO4 MUST be water free, reagent is prepared from 72%
HClO4, glacial acetic acid and acetic anhydride (I will later find exact preparation of the reagent). Crystal violet in GAA is used as indicator.
Bromothymol blue should also work, but I never tried it (and it is not suitable for you, because transition is from yellow to colourless). However I
don't know how much dichromate will distort colour change of the indicator (dichromate is orange, acidic form of crystal violet is yellow). Instead of
indicator you can perform potentiometric titration. If you have pH electrode, just fill it with saturated KNO3 in GAA.
0,1M HClO4 needs to be standardize. Predried (120 °C) potassium hydrogenphtalate is often used as standard. I also have good experience with predried
glycine and I have pure cyclohexylamine. Cyclohexylamine have advantage that it quickly dissolves in GAA, potassium hydrogenphtalate and glycine need
cca 1 hour of stirring.
For less polar amines like dicyclohexylamine, mixed solvent (1:2 GAA:Chlorobenzene) is used. en is very polar, so use just GAA. Addition of small
amount of acetic anhydride in to your GAA solvent is advantageous in the case of potentiometric titration, reaction is sensitive to water and without
Ac2O potential tends to jump.
And don't forget blank. Crystal violet indicator in your solvent (GAA) and add HClO4 until indicator change colour (usually around 0,1 ml). This
volume subtract from volume from your titration of en.
Maybe addition of excess of 0,1M HCl and retitration by 0,1M NaOH using phenolphtalein as indicator would also work. But you must consider two
reactions - reaction of H2en2+ with NaOH and reaction of (Cr2O7)2- with NaOH. So you must know how much dichromate do you have in your complex
(ferrometric or iodometric titration is way to go).
[Edited on 5-4-2022 by Bedlasky]
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Bedlasky
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Here is preparation of 0,1M HClO4:
8,5 ml of 70-72% HClO4 is mixed with 500 ml of glacial acetic acid and 30 ml of acetic anhydride in 1l volumetric flask. The flask is next filled with
glacial acetic acid to the line. Solution is left standing for 24h.
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Bezaleel
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Thanks, Bedlasky!
I think I should have provided you some more information. I have already removed the dichromate from the nickel complex by dissolving it in acetic
acid and precipitating it as lead chromate using lead acetate. This yields the well know yellow precipitate, which is filtered off. Next, the excess
lead is precipitated with sulphate and after filtration, I'm left with an acid solution in water of Ni2+, acetate, sulphate and (en).
Is it maybe easier to start from this mixture than from the initial compounds? Your suggested method poses some challenges for me, since I have no
experience working with these chemicals in a water free way and I should first order GAA and HClO4.
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DraconicAcid
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Most amines can also be titrated as a back titation- add more than enough HCl to your nickel complex to neutralize all the ethylenediamine, then
titrate the excess hydrochloric acid with sodium hydroxide, using methyl red as an indicator. That works very well for determining ammonia in
Ni(NH3)xCl2 (common first year undergrad experiment). Ethylenediamine has a different Ka, of course, but it may still work.
I'm pretty sure I've read that [Ni(en)3]2+ can be quantitatively precipitated with thiosulphate(?)- I'll get back to you when I dig my copy of Vogel
out of the pile of papers it is currently lying in.
Please remember: "Filtrate" is not a verb.
Write up your lab reports the way your instructor wants them, not the way your ex-instructor wants them.
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Bedlasky
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en is fairly strong base, so it shouldn't be problem titrate it in aqueous media. But that dichromate somewhat complicate things. Another option that
comes to mind is mixing your complex with 40% NaOH and distilling en in to the HCl which is adjusted to certain pH. Solution would be than titrated to
its original pH with 0,1M HCl. This method is used for determination of NH3. Apparatus must be well sealed to prevent en from escaping.
In theory you can also use method which you described above. Instead of acetic acid use HCl of known concentration and excess of acid titrate with
NaOH.
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