pH vs % caustic solution 3

[brainstorming]

I'd like to know the pH value corresponding to caustic solution of 2% NaOH and remaining water. The solution is withdrawn from a caustic tower at about 50 oC.
If any body knows any idea, please let me know.

Regards

 

[25362]

A quick estimate. 2% is 0.51 mol/L, totally hydrolysed.

pOH = -log(0.51) = 0.29. Thus, pH @ 50^oC = (*)13.3-0.29 ~ 13

At 25^oC, pH = 14-0.29 ~ 13.7

(*) I assumed the water's autoionization Kw at 50^oC to be 5[×]10^-14 => pKw = 13.3. At 25^oC, Kw = 1[×]10^-14, and pKw = 14.

 

[brainstorming]

Well done 25362
I got it and it is similar to the value I have doubt about.
Just back to the basics of chemistry, how did you get 0.51 mol/g for 2%caustic solution?

Regards

 

[NeedAHoliday]

brainstorming

Would have thought this would be elementary for a chemical based professional to determine?? Or am I being to harsh??

Think you mean mol/L not mol/g. Anyway...

Density of 2% NaOH at 50C is 1009.2 kg/m3. If we have 1L then we have 1.0092 kg of solution.

m(NaOH) = 0.02 x m(Solution) = 0.020184 kg = 20.184 g NaOH
MW(NaOH) = ~40g/gmol
n(NaOH) = 20.184/40 = 0.5046 mol

As I used a basis of 1L then the concentration is:

[NaOH] = 0.5046mol/L = 0.51mol/L (based on being totally hydrolysed)

Cheers.

 

[25362]

A quicker method, if you have it nearby, would be to look for the value in the CRC handbook.

 

[25362]

I confess having used a density at 20 deg C of 1.0207 kg/L for my quick estimate.

2% = 20g/kg => 20[×]1.0207 = 20.4 g/L => 20.4[÷]40 = 0.51 mol/L

 

[nbucska]

Iwonder if density wouldn't have less temperature dependency than the pH ?


<nbucska@pcperipherals DOT com> subj: eng-tips
read FAQ240-1032

 

[25362]

As an interesting anomaly let's say that the pH of "neutral" (pH=pOH) water, as a supercritical fluid, increases with temperature, the opposite to the liquid's behaviour.

So, we may find a pH >11 at 250 bar and temperatures above 500^oC. Whilst a pH <5 can be attained at temperatures above 200^oC and pressures above 5000 bar.