# Dewatering: Thickening, Filtering, CCD, Water Treatment & Tailings Disposal

Dewatering: Thickening, Filtering, CCD, Water Treatment & Tailings Disposal 2017-04-04T06:57:46+00:00
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## Compare Measured Acidity VS Calculated Acidity (5 replies)

Sturmbann
2 years ago
Sturmbann 2 years ago

I’ve been doing some acidity measurements recently (following the APHA 2310 method, including the hot peroxide pre-treatment). Then, I tried to compare my measured values with the total calculated acidity values (using the usual equation, taking into account dissolved amounts of Fe2+, Fe3+, Al, Cu, Ni, Zn, Mn and pH) and surprisingly my measured values are higher than my calculated values. I know that these two values don’t necessarily match very precisely, but usually the calculated acidity (which corresponds to the total potential acidity) should be higher than the measured acidity…

Any thoughts?

Sugar Watkins
2 years ago
Sugar Watkins 2 years ago

How are you calculating the acidity? Are you taking into account all the complexion, especially of Fe3+ by OH- as Fe(OH)2+ which dominates over Fe3+ in many cases? See Kirby and Cravotta, 2005, Applied Geochemistry 20, p. 1920-1964. Are your analyses for dissolved species only?

Tony Verdeschi
2 years ago
Tony Verdeschi 2 years ago

If you are interested I have some publication from a PhD student who did work on using titrations to determine metal concentrations. This may be helpful.

Sturmbann
2 years ago
Sturmbann 2 years ago

I used the following equation to calculate the acidity:

Total Calculated Acidity (mg/L as CaCO3) = 50(2Fe2+/55.85 + 3Fe3+/55.85 + 3Al/26.98 + 2Cu/63.55 + 2Ni/58.71 + 2Zn/65.38 + 2Mn/54.94 + 1000(10-pH))

Metal concentrations are dissolved species, and so far I did not take the speciation into account. I'll try to use PHREEQC to get an idea of the speciation. I also have the total metal concentrations, so I'll try this as well. And thank you for the Kirby and Cravotta reference, both articles (part 1 and 2) seem very interesting.

Gruppen
2 years ago
Gruppen 2 years ago

How much of difference do you have? 10%? 300%? How high are the metal and concentrations? The correlation is not very good at low concentrations.

Sturmbann
2 years ago
Sturmbann 2 years ago

The differences are about 25% - the calculated acidity is approx. 75% of the measured acidity.

Regarding the metal concentrations, Fe is between 15 and 50 mg/L; Al is 15-20mg/L; Mn is 12-18mg/L; Zn is - 3mg/L; all other metals are <1mg/L.

I haven't had time yet to check the speciation as recommended by Arthur Rose. I'll work on this a bit more, redo some analysis and try to sort that out 🙂

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