5 replies to this topic

[danleeper]

At our water plant, I’m trying to calculate the cost of running our the process at a lower pH than in the past (to help increase sulfide removal in a degasifier).

In order to lower the pH, we dose with 93% sulfuric acid using an LMI metering pump.

The entire plant volume of 5200 cubic meters per day of drinking water is dosed with acid which drops the pH of the drinking water from 6.1 to 3.9. There are several pipe-elbows and an improvised static mixer downstream of the dosing point which aid in mixing the acid and water volume before the pH sensor can record the pH value.

What is the volume of sulfuric acid required to make this (or any) change?

Alternately, what is the volume of 50% sodium hydroxide required to bring the pH back up to 6.1 after reducing it to 3.9?

I have used some calculations using the molarity of sulfuric acid and the pH change (and its effect on hydrogen ion concentration) that I found on the internet but I don’t think the calculation was correct since it doesn’t match the real-world numbers too closely.

I'm new to this forum and I appreciate your help in advance on this one.

-Dan

[Zauberberg]

Are there any other ionic species present in the system that can contribute to overall reaction chemistry? I'm surprised that you are getting significant deviations from calculated values since it is diluted system. Perhaps you can upload your calculation spreadsheet so that we can give further comments.

[danleeper]

Are there any other ionic species present in the system that can contribute to overall reaction chemistry? I'm surprised that you are getting significant deviations from calculated values since it is diluted system. Perhaps you can upload your calculation spreadsheet so that we can give further comments.

See attached pdf of the calculations. It was adapted from this forum: http://www.physicsfo...ad.php?t=217922

The amount of acid actually being used was on the order of 1/4 the amount calculated on that sheet. That sheet is determining the acid required to drop from 5.5 to 4.

Let me know what else I can provide.  pH Reduction to 4.pdf   14.56KB   602 downloads

[danleeper]

Are there any other ionic species present in the system that can contribute to overall reaction chemistry?

See below:

Parts Per Million
NH4: 0
K: 8.71
Na: 113.05
Mg: 3.39
Ca: 0.85
Sr: 0.02
Ba: 0
CO3: 0
HCO3: 2.09
NO3: 0
Cl: 190.69
F: 0
SO4: 2.43
SiO2: 0.02
Boron: 1.13
CO2: 2.48
Total Disolved Solids: 327.69

That is the hypothetical makeup of the water that is being injected with 93% sulfuric acid.

[Zauberberg]

It is a simple molarity balance, and the link you have provided shows the correct way (in my opinion) how the pH is calculated for strong acids in diluted systems. Make sure that you take into account whether the acid you use is bi-protic, or monoprotic.

It was a long time ago since I was doing pH calculations but here's a spreadsheet I made for your case. If I plug the same figures like those used in the web example you referred to, the same result is obtained. Something similar can be prepared for NaOH addition as well.

Good luck,

Attached Files

•  Acid Calcs.xls   24KB   964 downloads

[danleeper]

Thank you very much for taking the time to do this.