2C5n0j2Ky1LZ - 26-4-2019 at 03:10
I am trying to bring the ph of my solution down from 11 to 7 with muriatic acid but I only have crappy ph paper and I always overshoot it.
I know I can try to bring it back up by adding sodium hydroxide but I then start yo-yoing too high too low and back
I'm not clear if a ph buffer can be used when I get close to my target to make it easier to hit 7 or if is the wrong application for a buffer/
I am saving up for a ph meter.
Metacelsus - 26-4-2019 at 03:24
What else is in your solution? It may be possible to precisely calculate the amount of HCl you need to bring the pH to 7.
Tsjerk - 26-4-2019 at 10:01
Using a buffer may help, but using less concentrated HCl/NaOH might also do the trick.
DraconicAcid - 26-4-2019 at 10:21
If you're using a strong acid and a strong base to try to get the pH to 7, of course you're going to yoyo back and forth between extremely basic and
extremely acidic. Use NaHCO3 to neutralize the excess acid- you may not get to exactly 7, but it will be a lot closer than what you've been doing.
Sulaiman - 27-4-2019 at 01:38
Calculations are messy when pH is near 7 ... very low concentrations ... because water itself dissociates.
Wikipedia https://en.wikipedia.org/wiki/PH#Calculations_of_pH
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In practical terms;
One drop (0.05 ml) of 36%w/w HCl (11.64M) is (0.05/1000) x 11.64 = 0.000582 moles HCl
If this one drop of HCl is added to 100ml (0.1 litre) of water, the molar concentration of HCl would be
0.000582 / 0.1 = 0.00582M
pH = -log10(0.00582) = 2.235
So, one drop of 36% HCl in 100ml of water would give a pH of about 2.24
This is why you need to use MUCH more dilute HCl (or NaOH) if you want to get near to pH7
... as noted above by Tsjerk
(e.g. dilute 1ml of your 36% acid to 100ml total (0.1164M)
take 1ml of your 0.1164M acid and dilute to 100ml total (0.001164M)
use the (11.64M) conc. acid to get near to pH7,
then the 0.1164M to get nearer,
and the 0.001164M to get very close to pH7 (at 25oC for 'neutral')
... or similar.
A concentrated solution of a weak base (or acid) can have higher ionic concentration than a dilute solution of a strong base (or acid)
P.S. I had the same problem for literally years when doing titrations,
but +/- one drop is my smallest measurement increment anyway,
(e.g. for a 50ml burette, 0.05ml is 0.1% accuracy - which is my limit for weight accuracy also, I try to work to 0.1% error and I am happy with
+/-0.25% final results errors.)
- so its not as critical as I initially thought to get to exactly pH7.
If you are concerned about accuracy to the degree of one drop vs. 50ml,
then you also need to accurately (to <0.1% error) weigh or measure your (>99.9%) pure reagents.
[Edited on 27-4-2019 by Sulaiman]
maldi-tof - 28-4-2019 at 01:21
I don´t know what kind of synthesis are you trying, but sometimes if you are using HCl, boiling the solution (and adding water if so much solution is
evaporated), HCl may leave the solution and the pH increases...
The same procedure is applicable if you add NH3 instead of NaOH.
As commented before, using dilute HCl and adding the acid with micropipette may be useful... (adding 10 uL, or so...).
fusso - 28-4-2019 at 01:50
Why not try add some ammonium acetate?Both NH4+ and AcO- are weak acid/base so they can help buffer the pH.