I'm using "Intellical ISENH4181 Ammonium (NH₄⁺) Ion-Selective Electrode (ISE)" for measuring ammonium in wastewater samples. I realised that the variation in sample temperature leads to major changes NH₄⁺. Such that decreasing 2 C will reduce around 10 mg/l the ammonium (that electrode shows). Does anybody know the reason for that?
Hi,
you have got two different temperature dependecies for your measurement. The first one is in the procedure of measurement. The potential mesured by using the ammonia electrode is depending on temperature and concentrations given by nernst equation. This effects should normally be considered by your controller of the electrode.
The second temperature dependence of the concentration is given though the temperature dependence of the NH3/NH4+ equilibrium. The higher the temperature is, the higher is the ratio of NH3. The relation is DG = DGref + RTln(K). The constants for that equilibrium should be listed in literature e.g. in the handbook of chemistry and physics. (K is the equilibrium constant and could be e.g. obtained by using the definition of acid constant K = Ka c(H2O).)
For comparable results you should work at one temperature or you refine your results by estimating the NH3/NH4+ equilibrium for the measurement temperature. That the decreasing of temperature is resulting in a reduced ammonium concentration leeds to the first case. A problem with the temperature compensation of the controller or imperfect calibration of the electrode might be the reason.
Kind regards
A critical information is missing in your post: the main value of the measured concentration. How much is 10 mg/L in percent (i.e., as a relative error).
For a even better evaluation of the relative error I suggest you to compare values measured as pX (i.e., pNH4+).
Dear Cornel Radu thanks for your response.
Actually, It is a real sample from the wastewater treatment plant and I'm not sure the main value of the ammonium concentration. NH4+ was 63.3, 56.2 and 45.2 mg/L at temperature 14, 16 and 18 C respectively. Since I should calculate and add other chemicals (that I need for the experiments) based on the amount of NH4+ I think it's not an acceptable error.
I agree with you. The temperature compensation or its measurement is faulty.
If you can read the basic measurements (in mV) , you may try a manual calibration, by standard addition method.
Show me these potential values (in mV), maybe I can add a more precise diagnostic.
Thanks a lot, Cornel Radu
I made an ammonium 70 mg/L solution (by adding 207.57 mg of NH4Cl to Milli-Q water). Then, I changed the temperature of the solution and measured the value of ammonium by using "Intellical ISENH4181 Ammonium (NH₄⁺) Ion-Selective Electrode (ISE)". I should note that I didn't adjust the pH during measurement but by changing the temperature it changed a little bit (about 1 unit), I calibrated the probe at room temperature and, room temperature is around 20 C. You can find the data in the attached excel file.
As you can see in the data, I didn't get the real value of ammonium concentration at room temperature. In my experiment, I should work with real wastewater that I don't know the real amount of ammonium in it and, I should calculate and add other chemicals (that I need for the experiments) based on the amount of NH4+ and because of this, I cant trust on my calculation.
Do you calibrate the sensor? If so, do you use an Ionic Strength Adjuster (ISA) ?
Choose/set to calibrate with 10 and 100 mg/L standards.The same ISA is used for routine measurements of samples.
Cornel Radu
Yes, I calibrated the probe with 10 and 100 mg/L standards (that bought from Hach) at room temperature (around 20 C). And I could not understand why the probe doesn't show the real amount of ammonium in the solution at room temperature.
Cornel Radu Yes, I added as explained in probe instruction. I used 1 liquid pillow "that I got with the probe from HACH" for 25 ml sample, mixed them and then tried to measure ammonium with the probe.
Prepare a 1:1 mixture of the two standard solutions (10 and 100 mg/L) and try to measure it: you must have a reading very close to 55 mg/L. Use also the two standards as samples (you have to obtain values close to 10 and 100 mg/mL, and a difference of about 57 mV for potential readings).
Try to measure at room temperature., even better at 25 C.
Cornel Radu
The room temperature of the lab is 20 C and I will do all my main experiments at this temperature. Please see the attached table.
I should note that I sent this number to HACH Tech. service and he doesn't believe there is a hardware fault here.
Excellent results: a slope of 56.5 mV/decade (i.e., 251-194.5) is a good one . Your experimental practice is good and the standards are in their best condition. The instrument itself and the electrochemical sensor (as NH4+ ISE) are in good operational state. The Hach expert is right if he have seen only this last table.
But you have a problem with the temperature compensation. The Intellical ISENH4181 ISE measures the sample/standard temperature and compensates in such a way that the result is reported as it is measured at 25 oC. Look again at the Excel file you sent 6 days ago. I suppose the reported temperatures are what you see on the instrument display and they are correct (you may verify with an external thermometer if the temperature probe of Intellical ISENH4181 ISE is working fine). It remains to comment on the temperature compensation itself. Now look at the Nernst equation for any ISE. In RT/zF factor, T is measured in K (Kelvin). A 25 oC value means about 300 K. A variation of 10 oC (i.e., also 10 K) means only 3% for T in RT/zF. In the same proportion must be the variation in potential you read for the same sample at 15 oC and 25 oC, ie., the variation must be only 3% of 56,5 mV decade, thus only 1.7 mV. But in the table you sent 6 days ago , the variation (246.8 - 232.4 = 14.4 mV ) is 9 times higher if you compare measurement at 15 oC and 25 oC.
If the temperature compensation table/electronics is inside Intellical ISENH4181 ISE it must be replaced. If it is inside the instrument it is a little bit more complicated.
Send the file with the temperature variation and my comment to your Hach expert. Ignore the absolute values of concentration. Observe only the potential vs temperature values.
Notes:
How do you measure pH in this table?
Try to verify what was wrong with your 70 mg/L sample. 207.57 mg of NH4Cl in 1000 mL solution is correct. Other reason is to find.
About NH3/NH4+ equilibrium: at pH
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