CouchHatter - 27-2-2020 at 19:29
I just finished titrating my Klean-Strip hydrochloric acid and the results are not what I was expecting.
The SDS says that it's 31-35% acid, but my titration calculations say 18.2%. Granted my titration technique is not all there, but that cannot explain
the near halving of the concentration.
I have heard many say that the SDS conc. isn't to be trusted as far as this particular chemical goes, I just assumed that it would be closer than
this. It is about 2 years old. Maybe half of it has volatilized out of the HDPE bottle like my older bottle of acid, although this Klean-Strip jug did
not have visible bubbles on it like my Crown bottle developed. Also the Klean-Strip acid is clear. It was yellow when I bought it.
Has anyone else titrated their hardware store HCl?
[Edited on 28-2-2020 by CouchHatter]
j_sum1 - 27-2-2020 at 22:34
Not me. My current bottle of HCl is adulterated with Ti.
For this reason I have been distilling it. I have assumed a concentration aropund 30-32% and diluted appropriately to get just below the azeotrope.
Then with a short fore-run the azeotrope has come across at the right temperature. Without titration I have assumed that it is 20.2%. It won't be
far off. Based on my dilution and the very small fore-run I am prtety confident that the concentrate is over 30% which is what it is supposed to be.
That said, I did titrate some of the azeotrope a couple of weeks back as a student demonstration. I completely mucked up my standard solution. I
wanted to prepare some Na2CO3anh from NaHCO3 as a primary standard. It was a good technique and theory to demonstrate and discuss with students but
my practice was shot all to pieces. I just did not have enough time to complete the Na2CO3 properly and I thought it would be at least close when
there was little visible offgassing. I was wrong. My azeotrope clocked in at over 40%. Not a good look.
So. What are you titrating against? That could be part of the problem: although a conaminant here is likely to increase apparent acid concentration
not decrease it.
CouchHatter - 28-2-2020 at 09:42
0.5M NaOH, <1 month old. I could make a new solution, but age would also make the base solution weaker, and the acid register as more concentrated
right? I also realize I did not dry the NaOH I used to make the solution.
Should I use something other than NaOH? Is it very important to use fresh titrant?
morganbw - 28-2-2020 at 16:39
I do not know how expensive it would be for you but perhaps buy some standards.
In my past life we used 0.1N NaOH and 0.1N HCl for titrations and they were bought from some God like chemical vendor, Fisher/Alpha......
There are some lab techniques which allow a mere mortal to make a decent non carbonate NaOH solution but I am not able to help you here.
I do know that some decades ago I did some research on the subject and there are methods we can use to make a good titration (standard solution).
Without our proper/known exactly strength base or acid, how to know the actual results.
Suggestion is to buy known reagents or research and make your own.
CouchHatter - 28-2-2020 at 18:25
I had not considered that my degrees of inaccuracy might have compounded. Thank you for the suggestions, I will study more about this and then be able
to reach a more certain conclusion! Even if the conclusion is buying standards
Sulaiman - 28-2-2020 at 19:25
>99% anhydrous sodium carbonate is cheap via eBay etc.
this would put a limit on your final errors of <1% +/-(your experimental errors)
1% error is acceptable for all hobby chemistry other than precision metrology,
for mix-it-and-look wet chemistry +/- a few percent is ok.
Domestic baking soda (sodium bicarbonate, NaHCO3) is fairly cheap and pure,
(not baking powder - it contains acid and probably other stuff)
heating sodium bicarbonate (e.g. in an oven set to Max.) decomposes the sodium bicarbonate to sodium carbonate, water and carbon dioxide;
2.NaHCO3 -> Na2CO3 + H2O + CO2.
Cook for about 2 hours at max. temp. with occasional stirring.
(the inside of the oven will get covered in fine white dust - easily cleaned off)
Ideally you could do repeated weighings to determine when all bicarbonate has decomposed,
but I've not found a convenient method so just cook it reall well.
It works but I prefer to buy >99% purity sodium carbonate.
I once purchased >=99.9% sodium carbonate to practice titrations,
For me, working to <= 0.1% error is extremely difficult
overall errors of <=0.25% is practical, but tedious.
Even maintaining <=1% error consistently requires effort.
In practice, all of the chemicals that we buy are impure to some extent or other, and yields are never 100%,
so don't get too hung up on precision - unless that's what interests you.
____________________________________________________
It is easy to determine if the HCl gas escaped - look at nearby steel objects -
they will be very rusted
[Edited on 29-2-2020 by Sulaiman]
j_sum1 - 28-2-2020 at 19:29
Maybe time for a thread on standard solutions for titration. I don't recall seeing one, but I think it would be a good addition. I will start one.
And here it is. https://www.sciencemadness.org/whisper/viewthread.php?tid=15...
J.
[Edited on 29-2-2020 by j_sum1]
unionised - 29-2-2020 at 06:27
Did you try doing a density measurement?
Even a fairly rough one would tell you the difference between 18% and 33%.
It's interesting that your result is out by roughly a factor of two.
Tsjerk - 29-2-2020 at 08:18
A freshly opened and microwave dried cleaning soda should make a decent standard, if you have an appropriate scale to weigh the Na2CO3.
You could check the sodium carbonate for bicarbonate by titrating it against an strong acid of which you determined the strength by making a dilution
series.
unionised - 1-3-2020 at 01:55
In my experience, sodium carbonate sold for cleaning has some insoluble impurity- I think it's CaCO3- present at a level of about 2%.
To be fair, NaCO3 is easy to crystalise.
Heptylene - 1-3-2020 at 02:31
Reminds me of a time in a lab I was working at where the "32 %" HCl from a 5 liter jug wasn't behaving as expected. We used it to prepare aqua regia
to clean the glassware but the aqua regia was pale yellow instead of fuming dark orange. I took a density measurement and it turned out the HCl was
only 4 %! I assume this was due to evaporation but it still blew my mind it could be so dilute.
Lesson learned: check the density of your HCl regularly, it could be more dilute than you think. I guess ammonia is subject to the same problem
CouchHatter - 1-3-2020 at 14:31
I bought some new Crown HCl acid and titrated that with the same 0.5M NaOH solution, and got 32.75%. Leads me to the conclusion that the Kleen-Strip
evaporated through the jug over its lifetime. Going clear/not fuming should have been a dead giveaway! I'm going to store this stuff in glass and see
how it fares over time.
unionized, I did measure the density of each solution before titrating. 1.103g/mL and 1.153 g/mL, respectively.
Amos, my old acid was the version on the left in the attached photo. I don't think that what I had is the safe kind you mentioned.
[Edited on 1-3-2020 by CouchHatter]
Amos - 1-3-2020 at 14:34
New jugs of Klean-Strip or Jasco muriatic acid in the United States have been repeatedly tested by memebers of our forum to be 31.4%, but if your
bottle wasn't fuming in air something was definitely up. You didn't buy the "greener", low-fume variety, did you?
monolithic - 1-3-2020 at 19:27
I think HCl definitely evaporates through plastic jugs/caps regardless of the brand. I have some Duda Diesel brand HCl that was fuming when it was
new. Several months later and the plastic is shitty and yellow in appearance, and the acid no longer fumes when the cap is removed. Long term storage
should probably be in a glass bottle with an airtight cap.
[Edited on 3-2-2020 by monolithic]