Sciencemadness Discussion Board

pH help

botanistbee - 18-5-2016 at 06:28

Hey SM, Long time lurker here, finally decided to make an account.
I have a ph problem that I was wondering if you all could help me with. I am trying to neutralize some very alkali mud with a 1 molar HCl solution so I get a specific final volume. I know the pH of the mud slurry which is consistently 9.2 in a 60% wt/wt slurry with water which has a pH of 7.8. I know the dirt here contains some calcium carbonates and some phosphates.

I want to neutralize a specific dirt sample size (say 2.0kg) to a pH of 7.0 while ensuring a 60%wt/wt slurry. I realize this will be a system of equations of some kind but I am having trouble setting it up.
Again here is what I know:

pH of 60%wt/wt slurry is 9.2
pH of water used to make slurry is 7.8
Desired pH is 7.0
Using 1 molar HCl as modifier
Final slurry concentration must be 60%
dirt sample size is 2.0 kg

I appreciate any help in advanced, I have spent several days on this problem and have gotten nowhere.

Metacelsus - 18-5-2016 at 07:03

Without knowing the precise composition of the mud, this will be impossible to calculate. Instead, I suggest doing a small-scale test. For example, take 3 gram portions of mud, and to each portion add 2 grams of hydrochloric acid solution of varying concentration, and measure the final pH. This will enable you to experimentally determine the required acid concentration.

blogfast25 - 18-5-2016 at 07:39

Metacelsus is right.

Alternatively, dry the mud to determine its water content. The amount of HCl needed could then be estimated but would still have to be experimentally verified.

aga - 18-5-2016 at 08:31

Does the Mud start off as a 60w% mud/water mixture ?

If so, then HCl (max 37w% concentration) might be the wrong thing to be using, because you'll be adding water along with the acid.

Something cheap and powdered, maybe vitamin C would be more practical.

blogfast25 - 18-5-2016 at 12:17

Quote: Originally posted by aga  
Does the Mud start off as a 60w% mud/water mixture ?

If so, then HCl (max 37w% concentration) might be the wrong thing to be using, because you'll be adding water along with the acid.

Something cheap and powdered, maybe vitamin C would be more practical.


Quote:
I am trying to neutralize some very alkali mud with a 1 molar HCl solution so I get a specific final volume.


At pH = 9, [OH-] = 10-5 mol/L

Even if the mud was 100 % water, to neutralise 1 L of mud would take only 0.01 ml (millilitre) of 1 M HCl.

[Edited on 18-5-2016 by blogfast25]

aga - 18-5-2016 at 13:07

Such is the Power of maths.

Metacelsus - 18-5-2016 at 13:44

Quote: Originally posted by blogfast25  

At pH = 9, [OH-] = 10-5 mol/L

Even if the mud was 100 % water, to neutralise 1 L of mud would take only 0.01 ml (millilitre) of 1 M HCl.


The mud is almost certainly a buffered system, so it will take much more acid than that to bring it to pH 7.

aga - 18-5-2016 at 14:29

Why would it be buffered ?

AJKOER - 18-5-2016 at 14:33

Here is an idea in place of HCl, used a powdered metal (like Al or possibly Zn), which may avoid some of the carbonate buffering issue.

Another choice is powdered FeO working with air and water producing Fe2O3.xH2O and H+. This is more of a green solution ( actually reddish brown). The equation concerning the action of O2 on a ferrous salt in water is given by:

O2(aq) + 4Fe2+ + 6H2O ↔ 4FeOOH(s) + 8H+

Source: "Air Oxidation of Ferrous Iron in Water" by Ahmet Alıcılar, Göksel Meriç, Fatih Akkurt and Olcay Şendil, link: https://www.google.com/url?sa=t&source=web&rct=j&...

[Edited on 18-5-2016 by AJKOER]

Metacelsus - 18-5-2016 at 16:06

Quote: Originally posted by aga  
Why would it be buffered ?


Because:
Quote: Originally posted by botanistbee  
the dirt here contains some calcium carbonates and some phosphates.

blogfast25 - 18-5-2016 at 16:33

Quote: Originally posted by Metacelsus  


The mud is almost certainly a buffered system, so it will take much more acid than that to bring it to pH 7.


My point was not an exact calculation (that's not possible w/o more compositional data), rather it was to show how little alkaline pH = 9 really is.

Unless it's high ionic strength buffer you won't need much acid to correct that bit of alkalinity.

Ultimately only experimentation can show, of course...

[Edited on 19-5-2016 by blogfast25]

aga - 19-5-2016 at 08:56

Quote: Originally posted by blogfast25  

At pH = 9, [OH-] = 10-5 mol/L

Even if the mud was 100 % water, to neutralise 1 L of mud would take only 0.01 ml (millilitre) of 1 M HCl.

Just so i can be sure i got this straight ...

pH + pOH = 14, so at pH 9, pOH = 5

p being -log()

So if there's a pH of 9 and he wants it at 7, there's a difference of 9.9*10<sup>-6</sup> mols of OH<sup>-</sup> to react.

Certainly helps to know that even a Drop of 1 [M] HCl would be a LARGE amount instead of starting off with a bucket full of acid !

blogfast25 - 19-5-2016 at 09:23

Quote: Originally posted by aga  

Just so i can be sure i got this straight ...

pH + pOH = 14, so at pH 9, pOH = 5

p being -log()

So if there's a pH of 9 and he wants it at 7, there's a difference of 9.9*10<sup>-6</sup> mols of OH<sup>-</sup> to react.

Certainly helps to know that even a Drop of 1 [M] HCl would be a LARGE amount instead of starting off with a bucket full of acid !


Yes. 10-5 - 10-7

My point was to show just how dilute in OH- a pH = 9 really is.

So when someone says, "OMG, pH = 11, sooooooo alkaline!", that person is almost certainly not a chemist: 1011-14 = 0.001 M of OH-.

Even pH = 14 is only 1 M OH-.

[Edited on 19-5-2016 by blogfast25]

aga - 19-5-2016 at 11:03

So, how would a really realy basic or acidic solution be measured ?

Presumably not on the pH scale ?