[pyrogallol]

Hello...

I am new here. I have a problem with this kind of titration thing. We use Kr2Cr2O7/potassium dichromate as titrant to determine total iron and metallic iron of iron ore and reduced iron ore sample. Normal practise, we usually check dicromate prepared by using NIST standards/ SRM (standard reference material).

My question is, when i do the testing, using SRM that has known value of 84.6-85.2% of metallic, I always got high and absolutely out of range, normally 85.6-86.3%. But, when my colleague and my technicians do the things, they got within the range. When I ask them , they told me to titrate as fast as possible during titration. I did refer to some net and books, I havent found any data or theory that the titration must be fast or so, because I am also afraid if we titrate too fast, mabe we gonna have excess of titrant, then resulting high value of mettallic iron. Right now, we are using digital burette. Same conditions, solutions, methonds and procedure, but when I do the titration, I always got high compared to them.

So, anybody out there can give some idea on this or how to overcome this or what is a proper to do this kind of titration.

Thanks a lot

[Borek]

You have to compare your procedures, try do observe how they determine iron and note what they do differently, then ask them to observe and note the differences while you perform the analysis.

How do you dissolve samples? Reduce iron to Fe(II)? IIRC if there is an excess of reducing agent left it can react with dichromate.

[pyrogallol]

Actually, we use same procedure, same method, same reagents, same solutions. I did try once, I prepare sample and I ask my technician to titrate for me  and he got kinda low. So, I suspect, there must be something wrong with titration. But, personally I think, it is not practical to titrate damn fast, as we might excess the titrant and we cannot see obviously the color changes. It is my opinion, so what do u think?

Then, my technician also told me that, I was not as fast as him while titrating the samples. Means, nothing to do with preparing samples. Personnally, I want to know, which one is a recommended one or how?

Not sure whether becos of me cannot get the value of result within the range, then I blame their method of titration. So, my question is, is it practical to titrate and stirr the samples as fast as we can?

[Borek]

In general speed of titration shouldn't matter. In some cases, where there are possible side reactions, you should hurry. However, it shouldn't matter in the case of dichromate and iron.

You have not answered my questions about dissolution of the sample and iron reduction. I'll repeat: if there is an excess of reducing agent used, it can slowly react with dichromate, and then the final result can depend on titration speed.

[pyrogallol]

Hello Borek,

Sorry not answering your question. I use FeCl3.6H2O to dissolve this sample, so cold cold digestion as we just dissolve the sample wiith this reducing agent and stir it for 45 min in a conical flask with stopper to prevent o2 get in to the analysis. After 45 mins, we mix with 50ml of boiled distilled water + NH4Cl + acid mixture (H3PO4/H2So4) with addition of diphenylamine as indicator. last one, we titrate with dicromate, end point is purple.

May i know, what kind of side reaction you refferring to?

I undersyand that, If such titration needs hurry and fast one, why dont we go to back titration as when the titration is done in hurry one, sure the end point is not so abvious and thus, back titration is good to practise. Please do correct me Iif I am wrong.

Thanks a lot.

[Borek]

Hm, right now I feel like I know less than before...

I use FeCl3.6H2O to dissolve this sample

Never heard about such method. My books suggest using HCl to dissolve iron ores. Could be the general idea is that Fe(0) is oxidized while Fe(III) is reduced

Fe + 2Fe³⁺ -> 3Fe²⁺

but that'll require sample that doesn't contain Fe(III), which as far as I can tell is present in all ores but siderite.

so cold cold digestion as we just dissolve the sample wiith this reducing agent

This is an oxidizing agent, if anything.

and stir it for 45 min in a conical flask with stopper to prevent o2 get in to the analysis. After 45 mins, we mix with 50ml of boiled distilled water + NH4Cl + acid mixture (H3PO4/H2So4) with addition of diphenylamine as indicator. last one, we titrate with dicromate, end point is purple.

At least this part sounds reasonable.

May i know, what kind of side reaction you refferring to?

Most methods I am aware of don't bother whether iron has been oxidized to III or not, instead they call for a reduction just before titration. However, there is a risk that reducing agent will later react with th dichromate, so you need to use correct reducing agent and remove it from the solution afterwards. That's not a difiicult procedure, but if not done properly, excess reducing agent can give positive errors.

I undersyand that, If such titration needs hurry and fast one, why dont we go to back titration as when the titration is done in hurry one, sure the end point is not so abvious and thus, back titration is good to practise. Please do correct me Iif I am wrong.

Sounds logical.

[pyrogallol]

Hello..

I am not so sure about FeCl3. As my boss told me, that is nothing to do with reducing or oxidizing agent. It is just acting as buffer to prevent futher oxidation of the iron sample as iron is known as not so stable components.

Regarding HCl that u mention earlier on, we use it to determine total iron. It is different compared to Metallic iron, Please, do correct me if I am wrong. As whole things taht I told you is the things that we have been doing in our lab.

[Borek]

I don't get it. Procedure that you describe doesn't make chemical sense.

You are trying to determine iron content in iron ore.

Iron ores are not water soluble.

How do you dissolve the ore?

What kind of ore it is?

Dichromate oxidizes Fe(II) to Fe(III).

How do you make sure that all iron from ore is Fe(II) after it has been dissolved?

[pyrogallol]

Hmm...sounds complicated..

Ok...about excess reducing agent, i think that one is commonly used to detertime total iron.me also got problem with one..not sure how to make sure taht the reducing agent used is completely used or got excess or how. Its all about color changes.. and i am kind of poor in this case. I do not how to settle this problem...Please help me sir..

Thanks in advance

[Borek]

You are not using reducing agent, so that's a moot at this stage. So far procedure you have described doesn't sound correct, or your description is missing some important information about sample or aim.

[pyrogallol]

Hmmm...

Actually, at present, we have got two testing on iron in iron ore and reduced iron:
 1. Total iron in iron ore
 2. Mettalic iron in iron ore.

1. Total iron in iron ore:

We use HCl to diegst, then reduce it using SnCl2, indigo carmine, TiCl3 and cool it first. Then, we mix with acid mixture (H3PO4/H2SO4) and titrate with dicromate with addition of diphenylamine as indicator.

2. Mettalic iron
We use FeCl3, stirr for 1 hour on magnetic stirrer. Then mix with NH4Cl, acid mixture, boiled distilled water and diphenylamine as indicator and titrate with dichromate.

There got a lot of error on this kind of testings, but I am not so sure, may be my technicians and colleague not report the correct data or what. I do not know.

Observation of color changers also play an important role in this case, as people might see the changes differently..

The rest, I do not know..

[Borek]

So these are completely different procedures, and in the iron analysis FeCl₃ acts as an oxidizer. Looks like your boss is wrong.

No idea why speed of titration matters though, unless atmospheric oxygen oxidizes sample that fast.

When is the distilled water boiled? Is it added hot? How is the NH₄Cl added - in the solution, or as solid? Acid solution? Boiling removes dissolved gases.

[pyrogallol]

So these are completely different procedures, and in the iron analysis FeCl₃ acts as an oxidizer. Looks like your boss is wrong.

Yes, you so true. Because when I ask my boss about the role of FeCl3, sounds not logic, but I have no idea as my boss have been doing this forthe last 20 years as he is going to retire soon.So, could you please tell me, if FeCl3 act as oxidixer, what is oxidizer do? and why in this case, FeCl3 is chosen as oxidiser and what do you think, Is it necessary to oxidise the sample through this method or how?

No idea why speed of titration matters though, unless atmospheric oxygen oxidizes sample that fast.

Yes, I agree with you. I look trough books and net, i havent seen any data oe statemnt that titration must be too fast unless rapid and can observe the changes. So, do you think that atmospheric oxygen can oxidise the analysis done only for a few seconds?

When is the distilled water boiled? Is it added hot? How is the NH₄Cl added - in the solution, or as solid? Acid solution? Boiling removes dissolved gases.

We use boiled distilled water, we boil first, cool it down and keep in a bottle and use it. While for NH4Cl, normaly i prepare, 10g/1L, I use it in solution.

[Borek]

Because when I ask my boss about the role of FeCl3, sounds not logic, but I have no idea as my boss have been doing this forthe last 20 years as he is going to retire soon.So, could you please tell me, if FeCl3 act as oxidixer, what is oxidizer do? and why in this case, FeCl3 is chosen as oxidiser and what do you think, Is it necessary to oxidise the sample through this method or how?

I have already explained how it may work earlier.

2FeCl₃ + Fe -> 3FeCl₂

or net ionic:

2Fe³⁺ + Fe -> 3Fe²⁺

Amount of dichromate necessary to oxidize Fe(II) to Fe(III) is identical to the amount of dichromate that will be necessary to use if the Fe(0) was dissolved in HCl (and oxidized by HCl only to Fe(II)).

We use boiled distilled water, we boil first, cool it down and keep in a bottle and use it. While for NH4Cl, normaly i prepare, 10g/1L, I use it in solution.

Boil distilled water AND NH₄Cl solution just before using them, if they were standing waiting they are both already saturated with oxygen. No idea if it will help.