Estimating pH 2

[athomas236]

Hello,

I have two process streams both are water, one is rain water from a coal storage area with an estimated pH of 3 and the other is rain water from paved areas such as roads with an estimated pH of 5.6.

My question is if I know the flow rates of each stream, is there any way of estimating the pH of the combined streams.

Regards,

athomas236

 

[siretb]

You have to be a bit careful, and the situation may be a bit more complicated than what I describe.
However, you deal with almost pure water, so the buffer capacity of each stream should be low, and I think that, for an estimate, the following is OK.
The major assumption is that you have no components buffering.
A buffer is some component or mix that prevents pH from changing quickly.

at pH 3 you have 1.e-3 mol/l H+ (0.001)
at ph 5.6 you have 1.e-5.6 mol/l H+ (2.5e-6)

mixing Q1 of pH3 with Q2 of pH 5.6 yield a concentration of (Q1&Q2 same unit, both are mostly water)

C=(Q1*1.e-3 + Q2*1.e-5.6)/(Q1+Q2)
and the pH is -log10(C)

ex 50% 50% Q1=1 Q2=1 m3
C=0.00501 mol/l H+ pH=3.3=-log10(0.005)

 

[siretb]

Ooops one zero missing C=0.0005 !
PH=3.3 was correct sorry

 

[athomas236]

siretb,

Thank you very much for your assistance.

Best Regards,

athomas236

 

[25362]

As explained by siretb, assuming no buffering effects, a pH[≅]5.6 in rain water at ambient temperatures, points to the possible presence of CO₂ that partly dissociates to form both weak carbonic acid and bicarbonate ion in a ratio of about 5.5 to 1.

At pH<4,0 there is practically no bicarbonate ion.
A pH[&cong;]3 indicates the possible presence of other stronger mineral acid moieties as well.

Mixing both water streams may free some CO₂ releasing it from the higher-pH water resulting in a somewhat higher-pH mixture than would be obtained just by applying the above-mentioned mixing rule.

IMHO, measuring the pH of samples of various mixtures might give you a better picture of the expected pH.