In my assignment I've been given two water samples from the hinze dam. The first sample has been untouched, however the second has been contaminated with an unknown ionic compound. To identify the unknown contaminant, multiple tests were done, namely ISE tests and a TDS test.

The results were as follows


Nitrate test – Nitrate ISE with an accuracy of ±10% full scale (calibrated 1 to 100 mg/L) and a range of 1mg/L to 14,000 mg/L

Pre-contamination sample – 3mg/L

Post-contamination sample – 2000mg/L

Ammonium test – Ammonium ISE with an accuracy of ±10% full scale (calibrated 1 to 100 mg/L) and a range of 1mg/L to 18,000 mg/L

Pre-contamination sample – 0.1mg/L

Post-contamination sample – 0.2mg/L

Calcium test – Calcium ISE with an accuracy of ±10% full scale (calibrated 10 to 1000 mg/L) and a range of 1mg/L to 40,000 mg/L

Pre-contamination sample – 10.8mg/L

Post-contamination sample 2.8mg/L

Chloride test – Chloride ISE with an accuracy of ±10% full scale (calibrated 10 to 1000 mg/L) and a range of 2mg/L to 35,000 mg/L

Pre-contamination sample – 67mg/L

Post-contamination sample – 316mg/L

Phosphate test – colorimeter test using a reagent that turns the solution blue using a 10mg/L standard and a 0mg/L standard (distilled water)

Pre-contamination sample – 2.3mg/L

Post-contamination sample – 0.56mg/L

TDS test with 200mL of post and pre contamination solutions. Water was poured through filter paper into beakers and the beakers baked in an oven, then the weight of the new solids in the beakers weighed.

Pre-contamination sample – 0.02g

Post-contamination sample – 3.89g


Conducting a flame test on the post-contamination sample's TDS provided an orange flame, which indicated that part of the compound was sodium.


Since the TDS indicated an excess of sodium and the ISE tests indicated an excess of nitrate, the pollutant is likely sodium nitrate.


If the pollutant is sodium nitrate, why did the ions ammonium (although negligible)and chloride increase after contamination, when the other ions calcium and phosphate decreased?

So far I believe that the calcium decreased as it bonded with the dissociated nitrate.

Also, i think that the phosphate decreased as the dissociated increased sodium levels bonded with it.

However, I believe the decrease in calcium should cause the chloride to increase as the presence of calcium chloride would have its equilibrium shift in a way as to create for chloride as calcium is reduced. Which isn’t supported by the data?

Does solubility affect how the ions change?

  • 2
    $\begingroup$ Nitrate won't form an insoluble salt with calcium. You have an analytical problem. You are changing ionic strength, which changes activity. Yet, you don't mention how you measured the individual ions. Your ions won't increase/decrease simply by spiking in sodium nitrate. So, again, as mentioned in your previous post, you have an analytical problem - ie this is a poor analysis causing a confusing conclusion. $\endgroup$ – prof.kvothe May 25 '18 at 21:06
  • $\begingroup$ I've edited my post to add the methods for how I measured the other ions. But if I understand correctly you believe my data is erroneous because the only ions that should change are the contaminant's ions (nitrate)? Does this also mean there can't be any other explanation for why the ions change? $\endgroup$ – cjeccjec May 26 '18 at 5:48
  • 1
    $\begingroup$ Just about every one of your samples are out of the calibration range and therefore inaccurate and cannot be believed. You WILL NOT form a calcium nitrate salt - look up the solubility. You cannot magically form chloride - your analysis is wrong or the "contaminant" is contaminated. Your ammonia didn't change, it is within analytical error. ISEs are easily interfered with by other ions, so you need to read the instructions first to determine what those are. "Does solubility affect how ions change" Of course, that's what solubility is. $\endgroup$ – prof.kvothe May 27 '18 at 20:21

Ion selective electrodes are known to have two major limitations:

Ion selective electrodes are not ion-specific

Most ion selective electrodes are known to have interfering ions. For example, the fluoride ion selective electrode has interference by hydroxide.

This website lists technical specification sheets for common ISE methods, including known interferences. Check out the interferences on the ISEs that you used. That might explain some of your issues.


The response of ion selective electrodes is dependent on ionic strength

The total ionic strength changes the activity of the ions, and ion selective electrodes measure equilibrium interactions across the membrane. These equilibria depend on the activities of the species.

If your two solutions were not equal ionic strength, your measurements could be off.

Here's a link to a guide on dealing with these two issues. http://www.nico2000.net/Book/Guide7.html

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