Am about to make up some solutions for a precipitation prac and to waste some more time (having a very very slow day today) I thought Id look it up in 'the laboratory' book, well I saw something that made me a bit unsure...
I looked at copper sulphate, with a molecular weight of 249.68 to make up 0.2M solution it tells you to do the following: dissolve 49.9g of copper sulphate in distilled water containing 15mL OF DILUTE (0.1m) SULPHURIC ACID AND MAKE UP TO 1 LITRE WITH DISTILLED WATER...
Its the sulfuric acid bit that got me confused as I have never done that before, so then I looked at other things like Iron sulphate and it also said to put 15mL OF DILUTE (0.1m) SULPHURIC ACID AND MAKE UP TO 1 LITRE WITH DISTILLED WATER...
And I am sure I NEVER did that before either!
So have I missed something fundamental here or what???
Hey Sassi!
I am with you, I have never included the dilute Sulfuric Acid part either. I have just referred to the table of inorganic compounds, determined the quantity of chemical for molarity required, and mixed with distilled water as appropriate. Well, you are certainly right about the Laboratory and the "recipe". Can any one explain if it is really necessary to add the acid and if so, why?
Boy! Am I really hoping to make it up the waiting list for CSIS course!
I feel such a novice...
Lou P.
Now, this a NON CHEMIST speaking, and I hope that I am not treading on Robb's toes.
If you do not put the H2SO4 in, then after a while (few days) the CuSO4 goes a bit murky. It seems to get a greenish sludge in it (CuCO3, perhaps?)
The H2SO4 seems to dissolve, of prevent the formation of the green grunge and keeps the CuSO4 pristeen and sparkley blue.
At least, that is my story, and I'm sticking to it
Hi Sassi,
I've never added anything to my Copper Sulphate solutions, but the teachers sometimes ask me to acidify Iron solutions.The reason, I think, is that the iron ones can change into something else if they are not acidified.(You can tell I'm NOT a chemist either!!) I'm sincerely hoping Robb will reply to this thread soon, and give us an explaination that makes sense!!
Cheers,
Lisa
I have always put sulfuric Acid in our Copper Sulfate, I believe it keeps it longer. Or perhaps I have got it mixed up a little. Copper Sulfate pentahydrate has a much longer shelf life. But putting Acid into must do some thing else.
Hurry up Robb we need you
Regards Labbie
Lab Manager/Lab Tech, mind reading etc etc
Now retired
Hi Sassi, Loopy, Ian, Labman, Pommy and our other colleagues...
It seems that you have unearthed a good question. This, you will need to include in your notes for future reference:
The Sulfuric Acid is indeed added to do something and Ian was almost there. The Sulfuric Acid is added to solutions to keep the Copper (II) from forming insoluble Copper (I) Ions within solution.
The Copper Sulfate also has a tendency to Crystallise out of solution at high concentrations (Saturated Solutions), and the addition of the Sulfuric Acid also keeps the crystals from forming as it keeps the copper in solution.
The Ksp values (or Solubility Constant) of Copper and particularly Copper (II) Sulfate substantially changes in acidic environments hence the effectiveness of the acid.
So all-in-all it is a two way process, the Sulfuric Acid prevents Copper (I) from forming the insoluble salt (Greenish Yellow material that coats the glass bottle) and also keeps the Copper (II) in solution so it doesn't crystallise out. Keep in mind though, approx 0.1M H2SO4 is all that is needed, as the solution just needs to be slightly acidic for this to take place.
I hope this has been informative for you.
By the way, this will only be true for Sulfuric Acid only
Cheers,
Robb.....
Last edited by Robb on 26 Oct 2007, 02:36, edited 1 time in total.
Reason:Include content
Dr Robert Crosdale. MRACI. NSS. NSSA. NASA.
Ph.D (Chem), Post Grad Ph.D (Physics), M.Ed, B.Sc (Hons), Dip. Appl. Sc. (Chem)
Lake Munmorah High School.
University of New England.
University of New South Wales.
University of Newcastle.
To understand the Universe from our perspective, we need to look towards our own backyard first for answers.
Thanks Robb!
You have removed the clouds that were hanging around my head. All makes sense now. I must go and check my copper sulfate solutions. Seems like housework is never done...
Lou P.
Thanks Robb,
Could you tell me if it is the same explaination for the the Iron solutions I mentioned earlier? I can't remember which experiment it is, but the teachers ask me to acidify the solution to prevent it changing to another Iron??(or something like that)
Cheers,
Lisa
Thanks Robb,
I was hoping that you would shed light on this one.
From what you say, should we NOT add the acid if we are making a sat sol for crystal growing?
When it comes to crystal growing don't add the acid to the solution.. The addition of Sulfuric is there only if you are storing the solution of long periods. This solution should only be slightly acidic anyway.
For crystal growing a fresh Saturated or Super Saturated solution should be made fresh and stored knowing that it will be used for crystal growing. Otherwise the crystals will take longer to grow and the size of the individaul crystals will be some what smaller than normal.
As this may be confusing as to when and when not to add Sulfuric to Copper Sulfate solutions. A good rule of thumb is to weigh up whether you are going to store the Copper Solution over long periods and whether adding the acid will compromise other reactions or materials for what it may be reacting or used with.
As for Iron compounds it is a little different. The Acid only prevents the formation of Higher Ion Charges within solution and vice versa. For example: If Iron (I) is used in solution it prevents Iron (II) from forming, and the same in reverse Iron (II) into Iron (I). Very important....
Cheers,
Robb.....
Dr Robert Crosdale. MRACI. NSS. NSSA. NASA.
Ph.D (Chem), Post Grad Ph.D (Physics), M.Ed, B.Sc (Hons), Dip. Appl. Sc. (Chem)
Lake Munmorah High School.
University of New England.
University of New South Wales.
University of Newcastle.
To understand the Universe from our perspective, we need to look towards our own backyard first for answers.
Hi all
I have been very quiet on the chat site so far this term.Although Yr.12 are doing their HSC exams.Yr.11 who are our new Yr.12s are full on with doing pracs. Yr. 10 are also busy preparing for their exams(doing pracs.)Not enough hours in the day!!!!
Anyway it is Saturday and I was up at 5.30am and decided to check out what was going on with all of you.
Interesting about the copper sulfate,ay??? We have just done a prac. which required Potassium Permanagate to be acidified with Sulfuric Acid,what would this do!! I always thought that adding sulfuric acid to certain solutions was to act as a preservative!!!
Cheers
Mother
Good question. Keep in mind that Acidifying solutions is not the be-all-of-end-all particularly when it is mentioned as a preservative. (This is where a lot of confusion lays).
For some solutions it is a completely different story, Potassium Permanganate can be acidified because certain chemical reaction require an Acidic Environment for a reaction to successfully take part.
So sometimes this may be the reason behind acidifying solutions. (Oh and by the way when you acidify Potassium Permanganate it reduces it shelf life, this is because the acid begins to oxidise the solution and in a matter of hours the solution will turn brown. If not the glass storage bottle will be!!). Certainly one for the reference book....
As a Scientist I would say, to save any argument, :that when acidifying solutions it is for a good reason, and it is just fortunate that slightly acidifying Copper Sulfate increases it's shelf life and prevents the green-brown deposts inside storage bottles". "Very fortunate"...
Cheers,
Robb.....
Dr Robert Crosdale. MRACI. NSS. NSSA. NASA.
Ph.D (Chem), Post Grad Ph.D (Physics), M.Ed, B.Sc (Hons), Dip. Appl. Sc. (Chem)
Lake Munmorah High School.
University of New England.
University of New South Wales.
University of Newcastle.
To understand the Universe from our perspective, we need to look towards our own backyard first for answers.
Question: how much sulphuric acid I need to add if I need less than a L of Iron sulphate? Is any formula that I can use to calcutated this?
Thank you all.
From "The Laboratory". Dissolve 27.8g of iron(II) sulphate in distilled water containing 15mL of 0.1M H2SO4 and make up to 1L. This will give you a strength of 0.1M iron sulphate. Obviously just change the amount of iron sulphate for a different strength.
Hi smcm13,
Can I assume that you are starting of with Copper Sulfate (pentahydrate) crystals? (the pretty blue ones), with a molecular weight of 249.68 grams per mole? (generally written on the contaner). To make a 1M solution of Copper Sulfate, you put 249.68 grams of blue crystals into a 1000mL beaker and fill it up to 1000mL and stir until it all dissolves. Warming it on a hot plate will speed up the dissolving. Stir gently so as not to overflow the beaker (or use a larger beaker).
To make a 0.5M solution, use half as much copper sulfate crystals in the litre of water. You won't make much more than a 1M solution, as I molar is just about saturated for Copper Sulfate. To make less than 1L, scale down the amount of crystals accordingly
If you are planning for the solution to last a long while on the shelf, then just add a couple of mLs of sulfuric acid. It stops the Copper sulfate breaking down to form a brown sludge in the bottom of the container.
I hope this helps
Ian
Seems like housework is never done... 
