Geochemistry quick tips: Activity, Molality, Normality calculations for most common water quality parameters.

Geochemistry quick tips: Activity, Molality, Normality calculations for most common water quality parameters.

Simple thing that might be useful! Just my tips to all young geochemist starting their job and looking at lots of water quality data with different water analysis!!

Unit conversion – Have a cheat – sheet ready for all common unit conversions! You will need it for almost any type of calculations.

Example: Say our example species is Fe+2. Fe+2 is reported as 4.25 mg/L in laboratory reports. Now lets calculate the activity of Fe+2.

Activity of Fe+2 = (weight in mg/L) / (Molecular weight x 1000) =  4.25 / (55.847*1000) = .0000761 = 10^ (-4.119)

always remember to convert your results to log units. It will make your calculations tremendously easier!!! Also remember, activity is unitless.

Well, here I am attaching a spreadsheet to calculate “activity”, “molality”, “normality”, “moles” for most important water quality parameters : Fe+2, Fe+3, Na+, K+, Mg++, HCO3-, CO3–, SO4–, Cl-. Just change the “Example value” to the actual number you have in your lab analysis sheet. For our purpose we used example concentration of 1 mg/L for all parameters. So, change 1 to the exact value of each parameter. Also note that we did not use “activity co-efficient” in our calculations assuming very dilute solution at low temperatures.

Activity, molality, normality calculations-Download Spreadsheet
Activity, Molality, Normality etc

This is just a screenshot of the spreadsheet I am attaching here. You only need to change the “orange” shade area to the actual numbers.

Well hope you can use the spreadsheet. Now lets go to the next step.

Note: Please let me know if you have any comments. Use the spreadsheet for free.




About Editor
Ankan Basu is a Certified Professional Geologist (CPG) with 10+ years of experience in the field of geology, hydrogeology and geochemistry.

3 Comments on Geochemistry quick tips: Activity, Molality, Normality calculations for most common water quality parameters.

  1. Your spreadsheet has “morality” as one of the column headers! Didn’t realise geochemists had an algorithm for assessing moral values!

  2. how do you convert to log units?

  3. on a related but separate subject….whats the best way to graph a variety of parameters with a wide range of concentrations (for example converting parameters from mg/L to ug/L results in a range of numbers too wide to graph). also how do you convert concentration differences that result in a negative value into a log unit?

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