[enahs]

Ok, somebody asked me to help them with a lab. They where to determine the Ca²⁺ concentration using a Ca²⁺ Ion Selective Electrode.

The numbers are not coming out all that great.
But I have a problem. The procedure they had and used called for
"10 mL of the unknown into a 100mL volumetric flask, add 1 mL of the ammonium chloride-ammonia buffer, 10mL of 4 M NaCl, dilute to mark."

10 mL of 4 M NaCl? Ummm, Why?
That gives the unknown solution a 0.4M Na+ concentration.
By reference book says that Na⁺ interferes with Ca²⁺  ion selective electrode in high concentration (I would consider that high)?


They where to do the 3 basic methods. Calibration Curve, Electrode Calibration, and Standard Addition.

The Calibration Curve and Electrode Calibration work out to with agreement with each other.
The benefit of the standard addition method is that it is to help compensate for the effects of diverse ions in the unknown in which the electrode responds to.

The Equation for the standard edition they are using is (and I solved for it my self to verify it is correct) :

C_unknown =                       C_stdV_std               
                (V_Sample + V_std) x 10^-nΔE/0.0592-V_sample

And it is coming up with a negative number!!

Any insight? Why in the hell did it have him add so much Na⁺?

I feel like such a tard for not being able to help my friend. I even did the experiment years ago too (but I do not recall adding NaCl!)...


Data:
Average Potential for 50mL of the unknown (prepared solution, 10 mL of unknown + 1 mL buffer + 10mL of 4M NaCl diluted to 100 mL): -7.35 mV
Average Potential for 50mL of the unknown (prepared solution as above, same exact solution that was measured) + 5mL of 0.10 M Ca²⁺ standard: 28.3 mV

Either I am missing something obvious or my friend did something wrong. But, the same exact problem is coming out for the Ca²⁺ determination of Milk. Calibration Curve and Electrode Calibration agree, Standard Addition gets a crazy number.

*edit* I looked up interference for Ca2+ ion selective electrode in my Instrumental Book, Na+ does start to interfere at concentrations at or over 0.2M. So why did my friends procedure call for making .4M NaCl in the solution?!

[Borek]

As long as you do calibration in the same solution effect of Na⁺ presence should be always identical (see Nicolsky-Eisenman equation at http://www.ph-meter.info/pH-electrode-selectivity - it is listed in the pH context, but it works for other ion specific electrodes as well). But I agree with you that it is not the best approach.

The only situation where addition of controlled excess of Na⁺ could make sense is when you know that your sample contains unknown amount of Na⁺. It will be masked by known excess then.

It also could be they wanted to keep ionic strength constant.