Thursday
October 30, 2014

Homework Help: science (chem)

Posted by christina on Friday, June 22, 2007 at 7:27pm.

For the preparation and standardization of NaOH with KHP
im supposed to boil water for 1hr and 30 min to remove CO2....the problem is that if I don't boil it for that long and (30 min) b/c of not enough time but I put the water I boiled for 1/2 hr aproximately into a NaOH bottle with a CO2 absorber and stored it there for a few days. I would assume that I would have to boil the water again...but I am supposed to let it cool after i go and boil it...which comes to my question. Wouldn't CO2 gas go back into the water while cooling? and about how long does it take for the CO2 gas to go back into the water since it would affect the pH from what ive heard when I titrate with the dilute NaOH that I would have to dissolve with the CO2 free water.

My comments aside, follow instructions by your laboratory instructor.
First, and personally, I think 30 minutes is ample to prepare CO2 free water. I think 30 minutes will drive all the CO2 out of the solution. Second, and a direct answer to your question, some CO2 will be reabsorbed if the boiled water is allowed contact with air while it cools. You should consider boiling the water, placing it in a bottle with a CO2 absorber (I'm assuming the bottle will be plastic and not glass), and allowing it to cool to room temperature, then use the cooled water to prepare the NaOH solution. Third, you know that solid NaOH, fresh from a bottle, contains absorbed CO2 to produce Na2CO3; therefore, preparing and using CO2 free water does not necessarily produce a NaOH solution free of Na2CO3. You don't say in the question but you could be using a saturated solution of NaOH, which if prepared ahead of time, precipitates all of the Na2CO3, and it IS possible to pipet a small amount of this approximately 50% solution into CO2 free water and produce a Na2CO3 free solution of NaOH. Remember, of course, that the solution being titrated will absorb CO2 from the air, also; therefore, VIGOROUS stirring is discouraged. Finally, you may want to think about how the final results of an unknown acid titrated with "pure" NaOH vs NaOH containing some Na2CO3 will compare. Unless the NaOH titrant is VERY weak, AND if the standardization AND unknown titration are carried out using the same NaOH solution (CO2 free or not), my experience is that the results of an unknown may be obtained within 2-3 parts per thousand of the correct answer and that's quite good. Two parts per thousand is 0.2%. That means that an unknown containing exactly 50.0% acid might be titrated and calculated to contain 49.9%. That's good enough to get an A for the titration in my class.


First of all about the lab tech...If they actually knew what they were doing maybe I wouldn't get responses from them such as, "um" and during the titration experiment with the methyl orange indicator,"This is why I hate titrations and that kinda looks like it's starting to change colors"....which leads to the whole reason I end up posting questions here...anyways...
Why in the world would the manual say boil it for 1hr and 30 min if that is uneccessary? I'm starting to think they just want us to waste our time...
and when you say that it will absorb CO2 from the air if exposed while cooling, I assume I incorrectly allowed it to cool for 15 min before pouring it into the plastic bottle with the C02 absorber and I think I'm going to boil it for another 30min just to be sure that I really got the C02 out.

The lab manual says that I'm supposed to dissolve 5ml of 50% NaOH in 995ml of boiled water and let the solution cool to room temperature.(this is going to be a stupid statement but 5ml in 995ml water would be very dilute right? I really don't know how much water would you have to add to a solution to consider it VERY dilute) I guess that they prepared the saturated solution for us already and precipitated the Na2CO3 instead of us actually preparing it ourselves.
And also I assume that I am using the standardized solution of the NaOH for all my titrations since I have to find the Molarity first and then store it in the plastic bottle with the CO2 absorber. But the thing is that not only do I have to find the % of a KHP unknown but I also have to have enough solution to titrate vinegar and wine with the same solution I assume since they don't say to prepare some more NaOH.
And about the CO2 being absorbed from the air, it says that fresh NaOH should be added to the buret if the solution has been in the buret for more than 20 min.
When you say that, "vigorous stirring is discouranged" does that also apply to swirling the flask around while doing the titration?
When titrating with a unknown acid with with pure NaOH compared with NaOH containing some Na2CO3 I'm thinking that it would take more solution if it contained Na2CO3 if I'm not incorrect.


And I wish I could get a letter grade in lab...we get scored on a grading scale of 1-5 for all our unknowns.




Much to comment on so I will just copy your response and place my comments in the appropriate spot with bold face print.
First of all about the lab tech...If they actually knew what they were doing maybe I wouldn't get responses from them such as, "um" and during the titration experiment with the methyl orange indicator,"This is why I hate titrations and that kinda looks like it's starting to change colors"....which leads to the whole reason I end up posting questions here...anyways... We are glad to have you post here. And don't get discouraged with titrations. They are dependable, reproducible, and easy to do once you get the hang of it. Each indicator has its own "color change" problem but once you know what you are looking for usually there is no problem. Some students catch the color change before others but I never had a student who couldn't do it. I always demonstrated to my students when we started using a new indicator so they had a chance to see the correct color change first hand.
Why in the world would the manual say boil it for 1hr and 30 min if that is uneccessary? I'm starting to think they just want us to waste our time...I don't know why your lab manual would suggest boiling for 1 hr and 30 min. I just opened my latest copy of Skoog, West, & Holler (A quant text --6th edition--1992 so its SLIGHTLY dated) and they suggest boiling for a few minutes. Frankly, I think 10-15 minutes is sufficient but 30 minutes surely is adequate.
and when you say that it will absorb CO2 from the air if exposed while cooling, I assume I incorrectly allowed it to cool for 15 min before pouring it into the plastic bottle with the C02 absorber and I think I'm going to boil it for another 30min just to be sure that I really got the C02 out. I think that is wise.

The lab manual says that I'm supposed to dissolve 5ml of 50% NaOH in 995ml of boiled water and let the solution cool to room temperature.(this is going to be a stupid statement but 5ml in 995ml water would be very dilute right? I really don't know how much water would you have to add to a solution to consider it VERY dilute) I guess that they prepared the saturated solution for us already and precipitated the Na2CO3 instead of us actually preparing it ourselves. I like to work with approximately 0.1 M NaOH but I notice that many of the newer quant books use about half that for acid and base titrations. VERY dilute would refer to less than 0.05 M and I don't think you will have that so don't worry about that part of my first response. Yes, someone either prepared the 50% solution of NaOH earlier (or that can be purchased now made to order) so you don't need to take the time for that step.
And also I assume that I am using the standardized solution of the NaOH for all my titrations since I have to find the Molarity first and then store it in the plastic bottle with the CO2 absorber. But the thing is that not only do I have to find the % of a KHP unknown but I also have to have enough solution to titrate vinegar and wine with the same solution I assume since they don't say to prepare some more NaOH. I ALWAYS told my students to make up more than 1 L of approximately 0.1 M NaOH so that some of it could be squandered while looking for an unfamiliar end point with some indicator. Nothing is more disheartening to a student than to learn that they have just finished standardizing their NaOH but there is not enough of it remaining to do their titration of the unknown(s). Pity. :-)). You may have rules that prohibit that or you may not have containers to hold 1.5 or 2 L but I would make more if I had the equipment and the rules allowed it.
And about the CO2 being absorbed from the air, it says that fresh NaOH should be added to the buret if the solution has been in the buret for more than 20 min. That's ok. That just means that you want to get with it and finish the titrations as quickly as possible. And I wouldn't try to save any unused NaOH from the buret when I'm through. I don't like to add ANYTHING back to the NaOH I'm standardizing for fear of contaminating the entire bottle.
When you say that, "vigorous stirring is discouranged" does that also apply to swirling the flask around while doing the titration? Good point and I'm glad you brought it up. Most labs now have magnetic stirring mechanisms, something I didn't have when I was a student, and many students get the idea that if a little stirring is good then a lot is better. So they turn the motor to higher and higher speeds and the magnet inside the titration flask is going 90 miles an hour. That is vigorous stirring. About swirling. That's the way I did ALL my titrations in quant and I developed a good technique with it. I swirled with one hand and handled the stopcock on the buret with the other. Another thing--that salmon color I mentioned with methyl orange end point is BEST SEEN while swirling the titration flask. I always looked at the thin part of the swirl near the top of the swirling liquid and that salmon color jumped out at me.
When titrating with a unknown acid with with pure NaOH compared with NaOH containing some Na2CO3 I'm thinking that it would take more solution if it contained Na2CO3 if I'm not incorrect.
IF you standardize the NaOH with the SAME INDICATOR used to titrate the unknown, then CO2 in the water and Na2CO3 in the NaOH makes little difference in the final results. So why prepare CO2 free water and Na2CO3 free NaOH? For the practice AND because it's good procedure AND you may not want to standardize with the same indicator used for the unknown.

And I wish I could get a letter grade in lab...we get scored on a grading scale of 1-5 for all our unknowns.
Good luck to you




Most of the above response looks ok to me but I forgot to remove the bold on some of the last part so I will redo that portion.

And about the CO2 being absorbed from the air, it says that fresh NaOH should be added to the buret if the solution has been in the buret for more than 20 min. That's ok. That just means that you want to get with it and finish the titrations as quickly as possible. And I wouldn't try to save any unused NaOH from the buret when I'm through. I don't like to add ANYTHING back to the NaOH I'm standardizing for fear of contaminating the entire bottle.

When you say that, "vigorous stirring is discouranged" does that also apply to swirling the flask around while doing the titration?
Good point and I'm glad you brought it up. Most labs now have magnetic stirring mechanisms, something I didn't have when I was a student, and many students get the idea that if a little stirring is good then a lot is better. So they turn the motor to higher and higher speeds and the magnet inside the titration flask is going 90 miles an hour. That is vigorous stirring. About swirling. That's the way I did ALL my titrations in quant and I developed a good technique with it. I swirled with one hand and handled the stopcock on the buret with the other. Another thing--that salmon color I mentioned with methyl orange end point is BEST SEEN while swirling the titration flask. I always looked at the thin part of the swirl near the top of the swirling liquid and that salmon color jumped out at me.

When titrating with a unknown acid with with pure NaOH compared with NaOH containing some Na2CO3 I'm thinking that it would take more solution if it contained Na2CO3 if I'm not incorrect.
IF you standardize the NaOH with the SAME INDICATOR used to titrate the unknown, then CO2 in the water and Na2CO3 in the NaOH makes little difference in the final results. So why prepare CO2 free water and Na2CO3 free NaOH? For the practice AND because it's good procedure AND you may not want to standardize with the same indicator used for the unknown.


And I wish I could get a letter grade in lab...we get scored on a grading scale of 1-5 for all our unknowns.
Good luck to you


I forgot to mention that deionized water often is essentially CO2 free as it comes from the pipe.

Thank you very much for spending your time to answer my questions =D

Just more thing concerning the deionized water, I'm not sure but I was using distilled water (for everything including to boil with) that they prepared so that would be deionized right ?

I'm gonna have to look up where to find the boiling info in my copy of the Fundamentals of Analytical Chemistry 8th edition by Skoog, West, & Holler if you said that it's in your edition b/c I really didn't notice that anywhere. (school goes and makes there own little lab manual which is literally 1pg long for every single lab which is very uninformative)

Thanks again

Look in the index of your Skoog, West, and Holler under "sodium hydroxide, standardization of." There is a discussion of the effects of CO2 and some about the effects of using it with both phenolphathalein (a slightly basic type indicator) as well as with more acid type indicators (such as methyl orange) and or methyl red (although methyl red is not mentioned in my edition). As for the swirling versus comparison with a white piece of paper--I say what ever works for you is the one to use. For me, yes, the entire flask (I titrate in an Erlenmeyer flask) has changed color BUT the first faint salmon color I see is the change at the top of the swirl and I know I'm there. So I quit. I have had students tell me they just don't see that color and they add another drop or two or three until the see a shade they can reproduce. One isn't right and the other wrong. The one thing you need to remember about titrating is to find the end point that is reproducible for you. You may titrate to a couple of drops beyond what I see as the end point; however, as long as you titrate standards and samples alike, there is no error. That statement doesn't hold if you are titrating 20 drops past the end point but for a drop or two or evern three, it does hold. Incidentally, while you are reading Skoog et al., note what they say about titrating with an acid indicator (such as methyl orange or methyl red). Titration with methyl orange (methyl red is a little better) TITRATES THE Na2CO3 in the NaOH as if it were NaOH so standardizing with methyl orange and titrating samples with methyl orange intoduces no error due to CO2 absorption. That is because CO2 + 2NaOH ==> Na2CO3 + H2O. Thus, CO2 effectively neutralizes 2 mol NaOH (which is why you remove the CO2) BUT titration with methyl orange or methyl red titates all the Na2CO3 by CO3^-2 + 2H^+ ==> H2CO3. That is, all of the "neutralized" NaOH is recovered in the titration of the carbonate. The existance of H2CO3 in the solution at the end point makes the end point fuzzy but using methyl red AND boiling the solution to help decompose all of the H2CO3 to H2O + CO2 makes the end point sharp. Such is not the case with phenolphathalein indicator, for that indicator turns when only ONE hydrogen ion has been added to the CO3^-2. Even then, however, standardization using phenolphthalein and titrating an unknown using phenolphthalein (assuming phenolphthalein can be used for the end point), produces no error.

I forgot to ask about the methyl orange indicator... you say that it's on the top of the swirl? so you don't mean to actually let it stop swirling and then compare the color to a white piece of paper? (so the whole thing isn't supposed to be salmon colored?)I'm kinda confused with that and really have to clear that up before I continue my titration and decide whether or not to add some more HCl to my "peach" colored solution

No, distilled water is prepared by just that; i.e., distilling water. Many labs have an automatic distilling apparatus that adds more water as needed and distills constantly, usually until some reservoir has been filled. Deionized water, which has become more common since the late 70s and early 80s, is passed through a resin that takes out the anions and cations (thus the term DE-ionized). The stuff that comes out the pipe is VERY good water; it is extraordinarily low in contaminants, including CO2. Both faculty and students often refer to deionized water as distilled water but they are not the same. If you are starting with distilled water, then you are making CO2 free distilled water. Just as a side note, I started using deionized water for the entire chemistry department in the middle 70s because I had a local supplier who wanted to say the "the local university uses my deionized water" as a sales/marketing tool. We put a meter on the pipe and I paid 10 cents/gallon for all the deionized water we wanted to use with a maximum payment of $100/year. Good deal for me and good deal for the supplier. I had them install an in-line light that burned when the water had a resistance of 1,000,000 ohms at which time I called the company and they came out and changed the bags of resin (free, of course). So water we used in the department was 1,000,000-ohm water or better. That was a lot more convenient that trying to keep a still going. And personally, I think it is better than distilled water; however, others may not have that opinion.


I looked at the text and it mentions that, "With more dilute reagents (< 0.05 M), however, the water used as a solvent for the analyte and in the preparation of reagents must be freed of carbonic acid by boiling for a brief period. Water that has been purified by distillation rather than by deionization is often supersaturated with carbon dioxide and may thus contain sufficient acid to affect the results of an analysis.”
They just say boil for a brief period but if it was supersaturated with CO2 as they mentioned, would that be part of the reason that the lab manual says to boil for 1hr 30min? or it wouldn’t make a difference to boil it for 30 min? And since I have to find the concentration of the NaOH how do I know if it isn’t (<0.05M)?

The 8th edition contains lab instructions on a cd that came with the textbook said when I read the part on the storage and titration using the NaOH to cover the buret with a small beaker or test tube (I don’t have test tubes so it would be the beaker) to minimize exposure to the air while titrating. Would I still have to change the solution every 20 min if I did this?

For the standardization of the NaOH and also for the titration of the unknown, vinegar, and wine my lab manual says to use phenolphthalein as the indicator. Phenolphthalein would turn when 1 hydrogen ion is added to the carbonate as both you and the text says. But I don’t get why you say that it wouldn’t produce any error when the text says that there would be systematic carbonate error resulting from such a reaction when there is carbon dioxide involved.


See bold face type in appropriate places.
I looked at the text and it mentions that, "With more dilute reagents (< 0.05 M), however, the water used as a solvent for the analyte and in the preparation of reagents must be freed of carbonic acid by boiling for a brief period. Water that has been purified by distillation rather than by deionization is often supersaturated with carbon dioxide and may thus contain sufficient acid to affect the results of an analysis.”
They just say boil for a brief period but if it was supersaturated with CO2 as they mentioned, would that be part of the reason that the lab manual says to boil for 1hr 30min? or it wouldn’t make a difference to boil it for 30 min?

I think 30 minutes is more than a brief period. Water that is even supersaturated with CO2 contains only a little CO2. The authors of your procedure sheet may know something Skoog et al. don't know but I agree with Skoog et al.
And since I have to find the concentration of the NaOH how do I know if it isn’t (<0.05M)?
When you standardize the NaOH I expect you will use KHP. That will tell you what the molarity is. And I suspect that taking 5 mL of the 50% NaOH solution and mixing with 995 mL water will produce a molarity of between 0.05 and 0.1 M.

The 8th edition contains lab instructions on a cd that came with the textbook said when I read the part on the storage and titration using the NaOH to cover the buret with a small beaker or test tube (I don’t have test tubes so it would be the beaker) to minimize exposure to the air while titrating. Would I still have to change the solution every 20 min if I did this?
Covering the buret with a test tube or beaker is a good idea. I usually had my students do that. Yes, I think the 20 min rule still stands.

For the standardization of the NaOH and also for the titration of the unknown, vinegar, and wine my lab manual says to use phenolphthalein as the indicator. Phenolphthalein would turn when 1 hydrogen ion is added to the carbonate as both you and the text says. But I don’t get why you say that it wouldn’t produce any error when the text says that there would be systematic carbonate error resulting from such a reaction when there is carbon dioxide involved.
If you titrate the standard KHP sample with NaOH using phenolphthalein, then, it's true, only one of the H ions is attached to the carbonate; however, if you also titrate the unknown with the same procedure (using phenolphthalein), the same amount of error made in the standardization will be made in the unknown titration, and the error cancels. This assumes that about the same amount of titrant is used in both standardization and unknown titrations. From your previous posts, however, I take it that you are standardizing using methyl orange indicator (which titrates BOTH hydrogens onto the carbonate ion). Now if you titrate with phenolphthalein, there is an error. The book is correct; there is a carbonate error. I am correct, also, that the error cancels (and the book says so if you read far enough) so there is no discrepancy.

About the methyl orange indicator, I was refering to the first part of the lab 5 in my manual which includes the preparation and standardization of HCl with sodium carbonate under the whole topic of Determination of postassium hydrogen phtalate by acid/base titration.
For that first part with the standardization of HCl it refers to the rxn with the sodium carbonate and the acid by Na2CO3+ 2HCl => H2CO3 + 2NaCl and also says to use methyl orange or bromcresol green. I as you know used methyl orange and that refers to the Q I had about the color before and have to continue that during my next lab session. But it says to heat the solution after reaching the endpoint for the first titration and then titrating it again with about 1 drop of HCl. I guess the heating is to go and boil off the CO2 right because that would lead to an error? Then they say that the volume added after heating is called the "indicator/blank/titration error" and when I looked that up online it says that this volume is subtracted from the original amount used. Assuming that is correct this error subtraction would or wouldn't be applied to the rest of the titrations where I find the unknown/vinegar/wine%?
Another thing is I don't get why they tell us to do this under the Determination of KHP by acid/base titration if I'm not going to be using the methyl orange indicator for the vinegar or wine titration but rather use phenolphthalein.
And when you say that the error cancels out are you refering to the textbook statement that says "The silid reagents used to prepare standard solutions of base are always contaminated by significant amounts of carbonate ion. The presence of this contaminant does not cause a carbonate error provided that the same indicator is used for both standardization and analysis. It does, however lead to less sharp endpoints."? What does it mean by less sharp endpoints??

About the methyl orange indicator, I was refering to the first part of the lab 5 in my manual which includes the preparation and standardization of HCl with sodium carbonate under the whole topic of Determination of postassium hydrogen phtalate by acid/base titration.
For that first part with the standardization of HCl it refers to the rxn with the sodium carbonate and the acid by Na2CO3+ 2HCl => H2CO3 + 2NaCl and also says to use methyl orange or bromcresol green. I as you know used methyl orange and that refers to the Q I had about the color before and have to continue that during my next lab session. But it says to heat the solution after reaching the endpoint for the first titration and then titrating it again with about 1 drop of HCl. I guess the heating is to go and boil off the CO2 right because that would lead to an error?
The presence of dissolved CO2 leads to fuzzy end points. Boiling the solution rids it of CO2 (raising the pH at the same time), then adding HCl again (usually a drop to several drops) but then the end point is quite sharp.
Then they say that the volume added after heating is called the "indicator/blank/titration error" and when I looked that up online it says that this volume is subtracted from the original amount used.

I am confused by this statement.

Assuming that is correct this error subtraction would or wouldn't be applied to the rest of the titrations where I find the unknown/vinegar/wine%?
I wouldn't subtract but since I'm confused by the earlier statement, my advice may not be pertinent.
Another thing is I don't get why they tell us to do this under the Determination of KHP by acid/base titration if I'm not going to be using the methyl orange indicator for the vinegar or wine titration but rather use phenolphthalein.

The most probably answer is that many labs have the student determine the molarity of HCl as well as the molarity of NaOH, then if you overtitrate with the KHP standardization of NaOH, some standared HCl can be added and you can try the end point again on the same sample instead of having to discard that sample and use another one. In my earlier discussion about the "no error if we use the same indicator", note that standardizing the NaOH with KHP using phenolphthalein and using HCl standardized using methyl orange WILL lead to a carbonate error if the HCl is used to back titrate the unknown.

And when you say that the error cancels out are you refering to the textbook statement that says "The silid reagents used to prepare standard solutions of base are always contaminated by significant amounts of carbonate ion. The presence of this contaminant does not cause a carbonate error provided that the same indicator is used for both standardization and analysis. It does, however lead to less sharp endpoints."?
Yes, that's the stataement.
What does it mean by less sharp endpoints??
Answered above. By the way, I find from personal experience, that methyl red works a little better than methyl orange for the Na2CO3/HCl titration but that information won't do you much good since your instructions are to use M.O. Just file that somewhere in the back of your mind for future reference.

I was reading the cd that came with my text and in the lab instructions it clearly stated to go and wait for the water to cool to room temperature before mixing in the NaOH with GOOD stirring to make the solution dissolved in the water (you can dissolve a solution? doesn't that only apply to solids) but, In my lab manual it says to go and mix in boiled water with the NaOH and wait for the solution to cool ( I guess in the bottle with the NaOH with the CO2 absorber) Would the water being hot instead of at room temperature be better to incorporate the NaOH into the water and would result in not having to stir that much?.


to explain about the heating the solution...I'll just copy what it says.
1-add 8ml of concentrated HCL to 992ml of distilled water in a volumetric flask....mix thoroughly and store...

2-weigh out Na2CO3 (3 samples) (0.2-0.25g) ([3] 250ml flasks)and dissolve in 50ml water

3-add 3 drops Methyl orange ind and titrate with HCl until solution begins to change from yellow to red

4-boil solution for 2-3 min, cool to room temp and complete titration

5- To be sure that you can determine a reproducible endpoint, prepare 100ml of 0.05M NaCl in 250ml flask

6- add 3 drops of indicator (assuming they mean methyl orange?) boil briefly, cool and titrate with HCl to the endpoint

7- ...volume will be as small as 1 drop

8-This volume is called the "indicator correction", "blank correction", or "titration error"

and that is all it says about that...not what to do with this error or nothing...

You see about back titrating with the HCl is that I didn't get any indication from the lab tech that we would be able to back titrate so that's why I was confused as to why they told us to standardize the HCl before standardizing the NaOH.
But would I have been able to back titrate the titration of the Na2CO3 with the HCl because I overtitrated the Na2CO3 with HCl (methyl orange indicator) and got a pink. I didn't record the amount and when I asked the lab tech about it she said that I could have back titrated it by heating as that was the instruction after titrating it in the manual but that statement sounds fishy to me or is she right?

what do you make of all this?

OK, so if the 1 or drops is the indicator error, then WHEN YOU TITRATE WITH M.O. as the indicator, you will subtract the indicator blank from all titrations in which M.O. is used. However, I believe you are to use phenolphthalein indicator for standardizing the NaOH, as well as the KHP unknown, and wine unknown, and vinegar unknown; therefore, that statement doesn't apply because M.O. is not being used. So they have given you the definition of an indicator blank and told you how to determine its value, but there is nothing in the experiment that requires you to use it if I understand these responses for the last day or two.

With regard to the "back titrating" with Na2CO3 and HCl, the lab tech is correct but I think the two or you are on a different wavelength. What the lab tech is talking about is not back titrating. When Na2CO3 is titrated with HCl using M.O. as indicator, the end point is fuzzy due to the dissolved CO2 at the end point. So you boil the solution in order to decompose the H2CO3 into H2O and CO2. The pH will be ABOUT 3.5 or so (on the acid side of M.O.) and boiling out the CO2 (decomposing the H2CO3) raises the pH to a point about 5 (perhaps 5.5) or so (on the basic side of M.O.), THEN you can finish the titration with a few drops of HCl that is being standardized. That second end point is not fuzzy. Technically, back titrating in this case would be one of two things; (a) adding some standard NaOH to move the solution back to the basic side, then finish the titration with HCl (after boiling, of course), or (b) adding a standard solution of Na2CO3 to move the M.O. indicator from the acid side (which has been overtitrated) to the basic side, THEN retitrating with HCl.

You want GOOD stirring. Whether it is cool or hot makes little difference in my opinion. I wouldn't want the boiled water to be too too hot for the dilution step of conc NaOH with water may be somewhat exothermic and you don't want the boiled water to be so hot that it would start to boil (and spatter all over you and the lab top). I would cool somewhat, add the conc NaOH, stir very well, place in the absorber bottle, and let it finish cooling.

Actually I specifically asked her if heating it was back titrating and she said YES....
Hm...well I discarded that flask with the overtitrated solution so I'm gonna do 2 more samples of the Na2CO3 for the HCl standardization to have a total of 3 since I left 1 in the drawer, tomorrow

Hopefully with all the advice you've given me I'll do fine
Thanks Dr.Bob

Yep that answered my question

I'll see how that goes tommorow

Thanks again Dr.Bob

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