science( chem)

Basically what color is yellow when they say "Yellow"?

The titrations I have done are light yellow when approaching the end point but whatever color you see is what you get, I suppose. So titrate to a very light color.

Would the extra 5ml of starch indicator have any effect on the solution?

No.

What does adding urea to the solution do?

Urea is added as a scavenger for thre nitrogen oxides to make sure they are gone just in case the boiling didn't get rid of all of it (them).

Why does the addition of NH4OH cause the solution to turn that intense blue color and while adding it the solution starts to have ppt floating around before it actually turns bright blue?
But the addition of H2SO4 (3M) causes the solution to revert to it's original color?

Cu(II) forms a complex with aq NH3 that is Cu(NH3)4^+2. This is the INTENSE blue color you see and it is the basis in qual to prove the presence of copper(II) ion. The addition of H2SO4 neutralizes the NH3 and the copper solution reverts to Cu(II). Note that I didn't write NH4OH since that has been shown not to exist. NH3 + H2O ==> NH4^+ + OH^- but NH4OH is not an intermediate in that equation. Aqueous ammonia or NH3(aq) is the proper way to write it.

What is happening when you swirl the flask for 30 sec? Mine solution turned white but is it or is it not supposed to have a color change? (I still have 2 flasks I didn't titrate yet so I really would like to know what is this yellow they're describing and also what the solution's supposed to look like after swirling)

Upon the addition of KI, the copper(II) is reduced to copper(I) and the ppt you see is CuI, copper(I) iodide or cuprous iodide. That has kind of a coffee color. That color is quite dark at the beginning when large amount of liberated iodine are present. Titrate to the light light color of that material. Then add the starch and you get the blue starch/iodine color although it may be muddied with the coffee color of the ppt of CuI. Continue titrating with thiosulfate until the muddied blue color disappears. But you aren't at the end point yet BECAUSE CuI adsorbs (on the surface of the ppt) free I2. Since the I2 is not available to react with the starch, the end point is fleeting but eventually more I2 is released from the ppt and the blue color returns. This could go on forever; technically, the end point is never reached. SO, we dump in KSCN, which displaces the CuI and forms CuSCN in its place. The CuSCN is white and it doesn't adsorb I2 on its surface. The blue color returns because the adsorbed I2 is released to the solution and you continue titrating with thiosulfate (dropwise) until the blue color disappears for about 30 seconds. That is the end point. The reason the blue color may come back after 30 seconds is because air oxidizes iodide ion to free I2. Frankly, I always thought 30 seconds was a little long and if it disappeared for 15 or 20 seconds I was satisfied I had reached the end point.

Now, I have a question. Shouldn't all of this be discussed in your lab prep class? And isn't all of this information available in your text? I looked in my 6th edition and all of it is there. Don't get me wrong. We like for you to post your questions here, and in detail; however, it would appear to me that class discussion and reading the text would take care of most of these and you could concentrate on the why did this happen, and the other nuances you see, with your posts.

The calculations I was told, could be calculated since I had standardized the sodium thiosulfate with KIO3 right before this and found that the solution was 0.1 (my results had the average of 0.09952M RSD= 0.002)

I plugged my values in and got :

0.2437g Cu (1molCu/63.55g)(1mol Na2S2O3/1molCu)= (3.834e-3/0.03086L)= 0.1242M Na2S2O3

When I went and calculated about how much it would take to go and titrate with by using the g of Cu and the standardization with the KIO3 (0.09952M Na2S2O3) I got 38.52ml needed

Upon the addition of KI, the copper(II) is reduced to copper(I) and the ppt you see is CuI, copper(I) iodide or cuprous iodide. That has kind of a coffee color. That color is quite dark at the beginning when large amount of liberated iodine are present. Titrate to the light light color of that material. Then add the starch and you get the blue starch/iodine color although it may be muddied with the coffee color of the ppt of CuI. Continue titrating with thiosulfate until the muddied blue color disappears. But you aren't at the end point yet BECAUSE CuI adsorbs (on the surface of the ppt) free I2. Since the I2 is not available to react with the starch, the end point is fleeting but eventually more I2 is released from the ppt and the blue color returns. This could go on forever; technically, the end point is never reached. SO, we dump in KSCN, which displaces the CuI and forms CuSCN in its place. The CuSCN is white and it doesn't adsorb I2 on its surface. The blue color returns because the adsorbed I2 is released to the solution and you continue titrating with thiosulfate (dropwise) until the blue color disappears for about 30 seconds. That is the end point. The reason the blue color may come back after 30 seconds is because air oxidizes iodide ion to free I2. Frankly, I always thought 30 seconds was a little long and if it disappeared for 15 or 20 seconds I was satisfied I had reached the end point.

well the blue color never came back...I left the solution on the table while titrating..I was told to go and titrate till the coffee color disapeared and it did upon the addition of 1 drop of titrant. And the solutions today did get darker upon the addition of KSCN. The 1st solution did turn blue upon adding the KSCN as well and upon swirling it went and promptly turned milk white, but not today's solutions. I was told that the solution's endpoint was when the dark color brown disapeared and was replaced by a color change of light brown which happened after adding 1 drop of titrant

Now, I have a question. Shouldn't all of this be discussed in your lab prep class? And isn't all of this information available in your text? I looked in my 6th edition and all of it is there. Don't get me wrong. We like for you to post your questions here, and in detail; however, it would appear to me that class discussion and reading the text would take care of most of these and you could concentrate on the why did this happen, and the other nuances you see, with your posts.

lab prep class?? There's a lab prep class that exists?? (I would like to join this class) REALLY...I hope that isn't the pep talk that the lab tech's give at the beginning of lab b/c I had that for gen chem but It doesn't exist in my class. You just go to lab and ...do the lab..And we DO NOT talk about this in class AT ALL.(we only discussed the gravimetric determination of chloride)
And YES this info is in this text that I have but when it's a week before finals AND my third test is this thursday while the final is next tuesday this = NO time to look for why each step happens in lab...
My appologies if I'm getting annoying...
P.S: this was my next to last lab which was determination % of Copper in a sample of brass and I promise I won't ask about that lab at all even though I did that today..So there will be no more lab questions..happy?

But thanks anyways Dr.Bob for answering my questions =|

For the standardization of Sodium Thiosulfate vs. Copper Wire

I'll just list the steps...

1) cut copper wire with scissors and clean with filter paper
2) weigh 0.200g-0.2500g of copper wire into each erlenmeyer flask
3) add 5ml 6M HNO3
4) cover with watch glass and warm until the metal dissolves
5) 10ml of 5% urea
6) boil to eliminate the nitrogen oxide
7) cool the 3 flasks to room temp
8) add concentrated NH4OH drop by drop and mix to produce a intense blue color
9) make dropwise additions of 3M H2SO4 until the color of the complex disapears
10) add 2.00ml of 85% H3PO4
11) cool to room temp
12) add 4.00g of KI and titrate with Na2S2O3 until pale yellow
13) Add 5ml of starch indicator and continue to titrate until blue color becomes faint
14) Add 2.00g of KSCN
15) swirl for 30 sec
16) complete titration using the disappearenc eof the blue starch/I2 as the endpoint

I went up to step 12, then I got confused as to the color and no one would help... After adding the KI(4g)the aqua color solution turned brown. Then I titrated the solution with the sodium thiosulfate. The solution got lighter but I over titrated b/c I didn't know what was yellow. The solution turned from being brown=>coffee with milk brown=> light light lavenderish brown( had a whitish tint to it).

So then I was told I overtitrated (very helpful). I thought, well now I have to discard the solution. But, I was told to go on with the experiment as if I didn't go over the endpoint. So I did and then added 5ml of starch indicator. Nothing happened. Then I forgot I added 5ml of starch indicator and added anothe 5ml. Nothing happened again. I then added 2.00g of KSCN to the flask. Solution then turned bluish white. I swirled for 30 sec and the blue tint disappeared and the solution stayed milk white.

Basically what color is yellow when they say "Yellow"?

Would the extra 5ml of starch indicator have any effect on the solution?

What does adding urea to the solution do?

Why does the addition of NH4OH cause the solution to turn that intense blue color and while adding it the solution starts to have ppt floating around before it actually turns bright blue?
But the addition of H2SO4 (3M) causes the solution to revert to it's original color?

What is happening when you swirl the flask for 30 sec? Mine solution turned white but is it or is it not supposed to have a color change? (I still have 2 flasks I didn't titrate yet so I really would like to know what is this yellow they're describing and also what the solution's supposed to look like after swirling)

The calculations I was told, could be calculated since I had standardized the sodium thiosulfate with KIO3 right before this and found that the solution was 0.1 (my results had the average of 0.09952M RSD= 0.002)

I plugged my values in and got :

0.2437g Cu (1molCu/63.55g)(1mol Na2S2O3/1molCu)= (3.834e-3/0.03086L)= 0.1242M Na2S2O3

When I went and calculated about how much it would take to go and titrate with by using the g of Cu and the standardization with the KIO3 (0.09952M Na2S2O3) I got 38.52ml needed.

I need help evaluating this...

Thanks


Upon rereading your question and my response, perhaps I should add the following:
Upon adding the KSCN, a white ppt forms that replaces the coffee colored (with a lot of I2 there it is almost black) material. Basically, CuI is being replaced with CuSCN. The reason you swirl the flask is to make sure all of the CuI is mixed thoroughly with the KSCN so you have complete conversion to CuSCN. So your swirling and the formation of the white material is ok and that should happen. However, the solution SHOULD turn blue due to the assorbed I2 being liberated from what was CuI ppt and reacting with starch. A slight over titration at this point is ok because the adsorbed I2 hasn't been titrated yet. If it stayed white whithout first turning blue upon the addition of KSCN,however, you were, indeed, past the end point and I would not trust the buret number except as an indicator of approximately what I would need for the other samples. In other words, a slight over titration before adding the KSCN is ok but if the solution doesn't turn blue when KSCN is added you know you have more than a slight over titration; that is, you have added enough thiosulfate to titrate all of the free I2 in the solution as well as the I2 that is adsorbed on the CuI, and then some extra. At this point, there is no returning and the sample should be discarded. Another good reason for the KSCN, in addition to forming CuSCN, is the color. The blue color of the starch is much easier to see from blue/black to white than to a coffee colored color.

I must admit I am preplexed about the Cu titration. What I wrote above is what I have experienced and the end point for me is when the blue color (usually so blue it is DARK blue to DARK black) turns to the white of the CuSCN.
No apologies are necessary and you aren't getting annoying. We are happy to have you post here. My point was that the information for much of what you asked was available in the text and I thought you could read the text to get the first thinking process started, then use these posts to get questions not answered in the book taken care of. I can see that you are, no doubt, harried somewhat for time. Keep in mind, however, that looking in the text HAS to be faster than posting here and waiting several hours for an answer. Good luck!

Well it did turn dark (the dark coffee color) after adding the starch indicator but upon swirling the dark color it just turned the solution a..darker shade of coffee. I titrated that dark shade until the color simply turned a light milky white shade of beige. I was then told to record that amount that I added, then if I added more and the color didn't change then that was the endpoint. And I did the same thing for the Det of Cu in brass lab today, HOPEFULLY that was the right thing to do. [but even if it wasn't correct it's only worth 1 point in comparison to the unknowns which are worth 5 points]

Maybe next month when I have time to myself I'll go and look through the book and the labs I've done and analyze what was happening in each step. (But seriously I think that at least I try to understand what's happening in each step while other people just mix the chemicals without a second thought as to what actually goes on in the rxns) If I don't get anything I'll post that here.

And my Q's from now on will be on actual science related math questions since that's whats on my test is of course..

Thanks again Dr.Bob for all your help =D


And I do agree that looking through the book would be faster O___O::

I think you did the right thing. And hopefully things will work out for you. Of course, there are many who just mix and match with no thought as to what goes on. (And some don't care what goes on.) So it is good you are thinking of the chemistry behind the mixing. Most students don't realize the amount of CHEMISTRY that goes on in these determinations. Most of it we know but some of it is still conjecture; we just know the procedure works but not why it works as it does. Good luck!

Thanks again Dr.Bob =D

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