[felitix]

 Hello,

   I'd like to ask if anyone would know what is the potential cause the pH of the solution did not drop as expected.

I have a solution of 1.8% Urea, 3.44% 50% NaOH and 14.2% DDBSA. The pH of this solution is expected to be between 6.0 and 8.0.
It is 13.1
In order to drop it about 8% of DDBSA was added, but pH dropped only to 12.8.

Is there any way I can theoretically calculate how much of DDBSA should be added in order to drop the pH to the expected range of 6.0 - 8.0?

[felitix]

sorry forgot to add
the ingredients from my post above are in the solution where water takes 58.5%

[fledarmus]

Can you identify which compounds are acids and which are bases? Are they strong or weak acids or bases?

Can you calculate the molar ratio of your compounds?

Can you use this information to calculate the expected pH of your solution?

[Borek]

What are other components?

DDBSA means dodecylbenzenesulfonic acid?

Are these w/w percents?

Not that I have any idea, but without knowing these things for sure there is no way anyone will be able to help you.

[felitix]

I have gathered some more information. So the solution is:
Urea - 158 lbs (60.07 g/mol M.W.) 158 lbs = 71667.6 grams = 1193.07 moles
DDBSA (dodecylbenzenesulfonic acid) - 1245 lbs (326.49 g/mol M.W.) 1245 lbs = 564722.5 grams = 1729.8 moles
NaOH (50%) - 302 lbs = (40 g/mol M.W.) 302 lbs = 136985 grams = 3424.6 moles
Water - 5127 lbs (18.02 g/mol M.W.) 5127 lbs = 2325568 grams = 129054.83 moles
The expected pH is 6.0 - 8.0. It came 13.1

Is there any way to theoretically calculate the expected pH which should be 6.0 - 8.0? Is there any way to calculate what was in excess (probably NaOH) to cause the high pH? Urea pH  is 7.2 at 100g/L of water. However can it change the pH when mixed with the NaOH, DDBSA and water?

[Arkcon]

With those quantities, the problem almost belongs in the Chemical Engineering sub-forum.  Are you certain you mixed thoroughly prior to sampling for pH?

[fledarmus]

Good, you've got the moles for each of your components. (The water probably isn't necessary since it is a solvent, but still good!) Now, first question, are you sure your sodium hydroxide was 50%? Because that isn't the way you've calculated your moles.

Second question - can you identify which components are strong or weak acids? What would this imply about the concentrations of ionic species in your mix, especially H⁺ and OH^-?

[felitix]

yes, I'm sure NaOH was 50%. So I divide the number of moles by 2 giving:

NaOH (50%) = 3424.6/2 moles = 1712.3 moles
Urea = 1193.07 moles
DDBSA (dodecylbenzenesulfonic acid) = 564722.5 grams = 1729.8 moles
(Water - 5127 lbs (18.02 g/mol M.W.) 5127 lbs = 2325568 grams = 129054.83 moles)

As Urea pH is 7.2 at 100g/L of water it should not contribute significantly to pH changes, unless something happens when mixed with the rest of the ingredients.
I assume DDBSA is a strong acid, NaOH is a strong base so if H+ and OH- will dissociate completely (amount of moles used was almost 1:1) we should get pH about 7. (the range defined 6.0 - 8.0 allows for some changes in the quantities taken). Am I right so far?
Anyway, I've got pH 13.1 and the weight of the solution was as expected assuming all ingredients were added in the correct amounts. So how it happened? Is there any way Urea contributed to the pH change? The order of adding ingredients is water, urea, 50%NaOH and DDBSA. If the order will be changed does it matter?

[fledarmus]

Keep doing the math. Assuming the urea doesn't contribute, how much excess sodium hydroxide do you have? Convert your grams of water to liters instead of moles - what molar solution of hydroxide ions would that be? What is the pH of the solution?

[felitix]

So 2325568 grams of water will convert to 2325.6 liters, if I have 1712.3 moles of NaOH I have 0.74M solution.
The pH of such solution would be:
pOH = -log(0.74)= 0.13
pH = 14-0.13 = 13.87
Am I right?
I'm a bit lost now. So if I neutralized the pH 13.87 with 1729.8 moles of DDBSA I should get pH about 7. I've got 13.1. If the excess of NaOH has been used how do I know how much was in excess? (pH 13.1 compared to 13.87 looks like almost double amount of NaOH was used and DDBA did only neutralized it to 13.1 - right?)

[Borek]

Two random thoughts.

I have a feeling problem is not with the acid-base equilibrium. I would start checking reagents - for example, are you sure it is DDBSAcid and not sodium dodecylbenzenesulfonate?

Also I doubt you have used exactly 1245 lbs of the DDBSA - somehow I don't believe in four significant digits in this context.

[felitix]

the raw material used was for sure DDBSAcid. The amount which was supposed to be used to make the solution was 1245 lbs of DDBSA.
What was really done I have no idea as I didn't make the solution. I received a sample to test. So I wonder what happened that it came with so high pH, especially it happened twice in a quite short period of time.

[DrCMS]

My two thoughts are:
1) Are you sure 50% NaOH solution was used and not pearl caustic?
2) What temperature did it get to during the neutralisation?

[AWK]

1.8% Urea, 3.44% 50% NaOH and 14.2% DDBSA

% if not indicated means in this case w/w. 40-50 % solution of NaOH is often used in lab.
3.44% of 50 % NaOH = 1.72 g/100g = 0.0430 mole
14.2 % DDBSA = 14.2g/100 g = 0.0435 mole
Since DDBSA is a relatively strong acid - the acid and base are nearly in 1:1 molar ratio then pH should be close to neutral.
There should be an error in the preparation of the solution.

[felitix]

the order of adding ingredients is as follows:
- soft cold water
- urea
- 50% NaOH
- DDBSA

If the order will change does it matter for pH?