My laboratory is looking to buy a new probe for ammonia for wastewater samples.  We are now using the Hach Ammonia Gas Sensing Combination Electrode (Model 51927-00).  I find that this Probe is very hard to calibrate; slopes are most of the time not within range.

What type of probes is everyone using and which ones do you prefer?

Any help would be wonderful. 

We also have found the old Orion probe to be best but you have to use the loose membranes. The bonded caps are too erratic for consistent readings.

Recently I noticed Orion came out with a new type probe that was touted as more sensitive. I tried it out and it only lasted two days. I have not been able to figure out why. It did have some interesting features like a translucent body so you could see the fill level.

Another company, VWR, came out with their own branded ammonia probe that looks just like the old Orion probes but uses different filling solution.

I could not get documentation on that filling solution so I have not been able to use it.

Hi,

We have been using the new Orion High Performance Ammonia probe for almost a year.  Have not had a single problem with it.  Remember that the HP Ammonia probe has its own filling solution.  You cannot use the old type.  I agree that the loose membranes work better than the bonded ones.

Ann

One problem with the electrode method is the ISA solution. Standard methods calls for a 10 N NaOH solution with 0.1 M EDTA solution for pH adjustment. It calls for enough solution to reach a pH of 11 for analysis. I find this solution hard to make and we use a 5.0 N NaOH and .05 M EDTA and just use about 2 ml for pH adjustment instead of 1 ml. The 20th edition of Standard should have been written with 5 N but some how it was not changed even though the problem was pointed out.

If you use an Orion electrode their ISA will also include methanol to eliminate grease covering the membrane allong with the EDTA to eliminate the hardness plugging the membrane. I have been using this solution for years and the ammonia 9512 electrode works well. We analyze H2SO4 preserved samples of influent, effluent, and streams.

How would you comment to the efficacy of the 9512 probe membranes.  It seems several people have had issues with said membranes.  Perhaps the greater normality ISA solution is part of the problem? 

We have an online probe that might benefit from the addition of methanol to prevent oil fouling, but the samples in the lab we have conducted our studies on don't typically have this issue.

I appreciate your input, as I would really like to use this as a solution to our ammonia testing.

Thank you.

We have used this electrode and membranes for some time now and we have had no problems. I prefer to add the ISA while mixing and then put the probe in the pH 11 solution for analysis so the probe will only be in pH 11 for the analysis. The lab techs prefer to put the probe in the acid preserved sample and then add the ISA solution. They feel that the probe recovers faster from one ammonia level sample to the next sample. Both was seem to work good.

The higher caustic strength might not mix as easy so the pH hitting the membrane might be higher and cause damage, I do not know. My problem was getting the EDTA in solution with the 10 N NaOH. We do use a pipet despenser so the higher strength ISA would destroy it a lot quicker than the 5 N ISA.