Sniffity
Hazard to Self
Posts: 70
Registered: 27-12-2014
Member Is Offline
Mood: No Mood
|
|
Titrations and Systematization?
Hey,
I'm curious as to how you make a titration as systematic as possible.
Let me give an example of what I mean:
We're all familiar, one way or another, with the Mohr titration for Chloride in Water. Titrate problem solution with AgNO3, do the math, arrive at
chloride concentration.
My interest lies in the experimental part:
Given an aqueous sample to analyse for chloride content, I have to decide on two variables.
1.- Asuming I will use 50 mL total volume in my Erlenmeyer flask for the titration, how much of that 50 mL will be my aqueous sample and how much will
be further dilution by water, i.e. how much volume of alliquot do I take?
2.- What concentration of my titrating agent, AgNO3 should I use?
Now, these two questions both seem, at first, fairly simple to answer: Asuming I have no idea about the concentration, I could just play around until
I find an optimum point at which to analyse my solution... However, this poses several practical problems: It could be time consuming, I could not
have enough sample to play around with, and finally I am still playing with TWO variables that I need to set.
I'm curious as to how do professional labs do this? ASTM standards provide no information on this subject.
Once again, I emphasise that answering this questions by playing around is possible.
But something tells me there's a proper way to do it. What do you do if you're given a chloride sample in water, without any information and told to
analyse it? In what order do you procede?
Any information/input is welcome! If you know of any books/standards/anything which mentions this, I'd love to know!
|
|
|
brubei
Hazard to Others
Posts: 187
Registered: 8-3-2015
Location: France
Member Is Offline
Mood: No Mood
|
|
This is the method teach in my analytical chemistry classroom
If you have no way to know the concentration of your sample, prepare a little titrating solution which is very concentrated (like 1M) and do a rapid
and approximating analysis, then dilute your titrating solution by ten and repeat if required. The game is, you'll lose less time by starting by a too
concentrated titrating solution. A few drop should be necessary a each step until you are in a good range, don't work with higher than few mL of
solution à this stage.
When you have a approximating idea of the concentration of your sample, then you can do a proper analysis by measuring exactly the volume of solution
required. In a good experiment your equivalent volume should be 10mL to 50mL maxRemember than your sample and titrating agent should not be too
concentrated, 0.001M is a good aim when your prepare your dilution. Else there are other effect (described by the Debye-Hückel model) disturbing the
reactivity of the ionic species in your mixture.
|
|
|
Magpie
lab constructor
Posts: 5939
Registered: 1-11-2003
Location: USA
Member Is Offline
Mood: Chemistry: the subtle science.
|
|
Very nice, brubei.
My approach starts with assuming I will use a 0.1M titrant. Then I want to use 10-15 ml of that. I then normally always have an estimate of the
concentration of my unknown. So then I can calculate the sample size.
The single most important condition for a successful synthesis is good mixing - Nicodem
|
|
|
unionised
International Hazard
Posts: 5102
Registered: 1-11-2003
Location: UK
Member Is Offline
Mood: No Mood
|
|
If you dilute the solutions too far, the indicator error becomes significant.
|
|
|
Sniffity
Hazard to Self
Posts: 70
Registered: 27-12-2014
Member Is Offline
Mood: No Mood
|
|
Hey,
So, starting with a concentration of titrant of 0.1 M - 1 M (ASTM recommends 0.025 M) seems logical, we afterall have to start somewhere if we've got
no idea what we are doing.
But what about the second factor? Say we've agreed on a concentration of titrant, let's call it 0.1 M.
I now have a sample ready to be analysed. It would of course be illogical to use 50 mL of said sample for my analysis, I would be wasting precious
sample which is not infinite.
So: If I'm starting with 0.1 M titrant concentration, what sample dilution should I use?
I am thinking 1:5 (10 mL sample + 40 mL water), then 1:10 (5 mL sample + 45 mL water) then 1:50 (1 mL sample + 49 mL water). Doing one test run with
each of those concentrations should give you a good idea of what your concentration is, right?
Does that sound logical?
|
|
|
![[*]](images/xpblue/default_icon.gif){kind=icon}
