ORP calibration solutions

[simonh]

Hi Randy,

I've just performed the tests on some ORP calibration solutions. Here's the story:

Over night I cleaned my ORP probe using dilute bleach to remove organic deposits followed by dilute Hcl to remove calcium deposits. I rinsed the probe well in DI water.

Today, I carried out some tests of my ORP calibration solutions. First, I mixed the YSI Zobell Solution (3682) using 125ml of de-ionized water (Hanna) as instructed. At 25C (the weather has been nice in England yesterday and today ) the Zobell reference solution claims 231mV +/- 10mV.

For each solution I rinsed the ORP probe in de-ionized water and then in the calibration solution, before placing the probe in a clean 10ml vial filled with the 230mV calibration solution.

I calibrated the ORP probe of my IKS aqua-computer using the fresh Zobell solution as a reference standard. After calibration I took a reading and the meter indicated 231mV.

I then tested the following samples of IKS 230mV redox calibration solution:

Sample 1
Date of purchase: 4/8/03
Opened: Never
Valid to: 08/03
Reading: 215mV

Sample 2
Date of purchase: 10/10/02
Opened: Once
Valid to: Not given on bottle
Reading: 236mV

Sample 3
Date of purchase: 10/10/02
Opened: Twice
Valid to: Not given on bottle
Reading: 212mV

Given the precision of +/- 10mV on the Zobells I freshly mixed and also the high temperature dependence of the Zobell I think the measurements were all within the margins of error.

I feel much more confident of these solutions now!

 

[Randy Holmes-Farley]

I agree, that is very good!

Where did you buy them?

 

[simonh]

Hi Randy,

The YSI Zobell's solution was purchased from Cole Parmer (part# EW-05478-60)

IKS 230mV calibration solutions are manufactured by IKS Computersystem GmbH (www.iks-aqua.com). Those I purchased on 10/10/02 were from www.aquaristic.net. The one I got on 4/8/03 was from www.sewatec.de.

HTH.

 

[Randy Holmes-Farley]

Thanks. I'll be putting together an article on ORP, I'll want to discuss available reference solutions.

Do you have any recollection how far off your ORP probe was before it's first reference calibration? A lot of folks seem use them with no tests at all.

 

[simonh]

I have no idea. The IKS works a little different from what I have seen of most ORP meters. You buy the ORP modules as an add-on to the base computer unit. It is not factory calibrated and requires you to calibrate it before use. To do so requires a 0mV and 230mV reference. The 0mV is done using a BNC terminator plug, then you plug in the electrode and use the 230mV calibration solution as the second point.

Due to it needing calibration before it will give a reading was my main reason behind wanting to check the aquarium ORP calibration solutions against fresh YSI Zobell's.

 

[Randy Holmes-Farley]

OK, that is very different than the usual plug and read.

 

[Boomer]

Simonh

Nice job

:thumbsup:


Randy


I'll be putting together an article on ORP

Well it is about time

:thumbsup:

 

[Randy Holmes-Farley]

Yes, Boomer, I know. I mentioned that you'd been after me to do it when John suggested I write one to replace the ORP article that we discussed a week ago.

 

[Boomer]

:thumbsup:

 

[simonh]

Hi Randy,

I did a few more measurments today. I found an old Pinpoint ORP monitor in the cupboard (I had stored it as I couln't be bothered getting a new probe for it when the old one expired). Pinpoint factory calibrate the machines before shipping. I checked the calibration using a pH/ORP simulator (those little meters where you dial in an ORP or pH to check the instrument is functioning correctly) at 0mV and 230mV and also a few other values often mentioned for aquariums suchas 350 and 400mV. It was spot on.

So, I decided to clean down the IKS ORP probe as before and plugged it into the Pinpoint. I did a test in the fresh Zobell's 230mV +/-10mV @ 25C I mixed the other day. I got a reading of 160mV, so 70mV difference between "true 230mV" and the reading from the probe in calibration solution.

I had a search around on the internet for more information on ORP probes. I found this document interesting - http://www.fondriest.com/sensortech/orp.htm. Brand new YSI ORP probes had a stated accuracy of +/- 20mV in Zobell's. However, there is some interesting data on 5 new probes where 1 probe read 50mV different to the others in a water sample (unfortunatley not tested in saltwater). After 18 hours the difference came down to around 25mV difference. It was interesting that this probe was considered acceptable "The sensor's reading in natural water is not radically different (>100 mV) from the other sensors and becomes closer after extended exposure to this medium". I wonder how close even new aquarium grade probes would read in the same saltwater?

It is also interesting how the 70mV difference between "true 230mV" and the 230mV calibration standard arises. How much does the probe degrade in use? (mine is around 10 months in continuous use). I think for the next few months when I clean / calibrate the probe I will plug it into the Pinpoint and try track the difference between the "true" and "calibration" reading (hopefully the ORP calibration solution error won't cloud this issue too much). Maybe, when I replace the probe I will do likewise.

I think given the errors due to the probes I would take any absolute readings / recommendations with a bucket of salt. Also, when I previously used the Pinpoint on an old tank I noted over time the ORP in the aquarium slowly went down. I always assumed it was because the aquarium was getting older. After today's measurements I wonder how much the effect was due to an ageing probe (I see recommendations of 1-2 years life but nothing specific to continuous use in an saltwater environment). My list of useful uses is left at sudden drops in ORP.

 

[Randy Holmes-Farley]

Thanks, Simon. A lot of interesting info there! I've always wondered how good ORP measurements could be without true calibration of the probe in fluid. Now I know it may not be so good.

Let us know what happens over time!

 

[Boomer]

Simonh

That was an interesting link. I noticed something there I have posted a number of times

Sensorex
11661 Seaboard Circle
Stanton, CA 90680
Telephone: 714-895-4344
Fax: 714-894-4839

The kit contains solid quinhydrone which, when added to the supplied buffers, yields two solutions with well-defined, but different, ORP values..

I have two very expensive Senorex Probes, custom built for me by Mike Ross. I don't know if they are still good yet, been boxed for 4 years. If you or Randy have any questions contact Mike Ross, Director of Research and Development at Sensorex, a great guy. I always used their "Q" test kit to check probes

Here is their website ( just redone and new), click on products. Also look at Technical Articles and Technical Education, The also sell Zobel, to include 450 mV, 260 mV 228 mV and 90 mV solutions. I actually don't like the new site, allot of technical articles have been left out, on ORP, pH etc. and cleaning, calibration and even the instructions for the "Q " test. There use to be some elaborate stuff on ORP, like half reactions. The O2 is new and nicely done. He gives the exponential equation but it would have been nice if he popped in barometric pressure correction, rather than working it out and rearranging the "p" equation separately (yah, I know you have to work it out seperately in my old posd also )

 

[simonh]

Hi Boomer,

I've been trying to locate someone in the UK selling the Q kit so I can see what the slope is like on my probe. I have been on Sensorex old and new sites and read all there information. I agree that the old site seemed to have more techinical information. I may contact them to see if they have anyone who supplies in the UK.

I've also read most other ORP meter manufacturers websites, manuals, and technical information and it is amazing the differences of opinion. Some say that the solutions are just check the electrode, if the electrode doesn't read within spec then clean the probe and try again, otherwise replace the probe. Other manufacturers say that the offset should be used for matching ORP of an existing system, e.g. in process control situations when when changing electrodes. Others use the Zobbel's calibration solution to apply an offset to the electrode readings (apparently no need to alter the slope as it doesn't deteriate like pH probes as long as the probe is clean according to many). Some of the process control ORP systems seem to use the Q calibration and alter both a slope and offset!

Pinpoint have an offset calibration screw that allows approximatley +/- 10 mV offset. My IKS seems to set a true 0 mV and then alter the slope to hit the 230mV when in Zobell's !

 

[Boomer]

Simonh

:thumbsup:

I forgot this

"The sensor's reading in natural water is not radically different (>100 mV) from the other sensors and becomes closer after extended exposure to this medium".


This conforms also to seawater. It has often been seen and noticed that an ORP probe in natural water, to include sewater, needs to be "broken in" before it beecomes stable and reliable. Usaully around two weeks in the solution. This "break in" period has been mentioned many times in this hobby, books, articles, forums and NG's. My two ORP probes, Sensorex and HACH 1, never read the same and were usually off about 25 mV. The HACH 1 is an odd probe, as it has a depsenser pump, that allows flush out the ref. solution, in the probe and replace it at the touch of a button

 

[Randy Holmes-Farley]

I've got an idea: why don't you two coauthor an aritcle on ORP. You both know a lot more about it than I do.

 

[Boomer]

:rollface: :rollface: :rollface: :rollface:

Randy "young " buddy, knowing and uderrstanding are two different things . I am clueless, as to why a probe needs to be broken in, in NW, but on many "lab solutions" it does not, other wise it would be a usless measurement in the lab.

I have a much better idea, why don't Randy and Mike Ross ( sells many probes to the aquarium industry) get together and do an artilce. Simonh and I will be your "sideliners". I will even call Mike and put in a good word for Randy and to see if he would do it

 

[simonh]

I've got an idea: why don't you two coauthor an aritcle on ORP. You both know a lot more about it than I do.

'cause you write articles that are easy for the reader to understand. I just write techno-babble

 

[simonh]

Hi Randy,

I've just done a few more testsââ"šÂ¬Ã‚¦

First, I cleaned down my IKS probe using the same procedure as on the 8/9/03 . I connected it to the Pinpoint meter which is calibrated to read absolute mV (no offset or conversion). I then poured a sample of the YSI Zobell 231mV solution (the same bottled solution as used on 8/9, shelf life says 6 months) into a clean sample cup and took a measurement at 25C

IKS probe (YSI Zobell) = 173mV
on 8/9 the reading was = 160mV

In addition I have just purchased a new double-junction ORP probe from Omega. It appears to be an unbranded Sensorex S500CD-ORP. I also purchased some individual use glass ampoules of Schott 220mV and 470mV (Pt Ag/AgCl) at 25C redox calibration solutions. I beleive the difference in the mV listed on the Zobellââ"šÂ¬Ã¢"žÂ¢s is due to the differences in KCl strength used by the various manufacturers probes or maybe some of the values listed are dependent on pH of the solutions? It seems there are a few numbers floating around on the web for both Zobellââ"šÂ¬Ã¢"žÂ¢s and Lights solution but no big differences.

Anyway, I tested my new ORP probe using ampoules of Zobell's at 25C and also of Lights at 25C in clean sample containers.

New probe (Schott Zobells) = 214mV
New probe (Schott Lights) = 447mV

I then tested the old IKS probe (purchased 10/10/02) in the solutions.

IKS probe (Schott Zobells) = 170mV
IKS probe (Schott Lights) = 374mV

So, it would appear that my old probe is reading lower than my new probe and the calibration standard. Just to note that from my research it appears the IKS probe uses 3M KCl whereas the Omega/Sensorex probe uses 3.5M KCl so this would account for a small difference (5mV?).

Finally, I re-did the original test of the available aquarium calibration solutions in my first post. Looking back at the original post I realised the values may not be quite as good as I would like due to the way the IKS was calibrated at the time. I took all these measurements with the Pinpoint meter so readings are absolute mV.

Zobell solution.....IKS probe.....New probe
YSI.......................173..............216
Schott..................170..............214
IKS Sample 1.......153..............203
IKS Sample 2.......168..............218
IKS Sample 3.......153..............203

WOW! Thatââ"šÂ¬Ã¢"žÂ¢s about 3 hours of the evening gone! Hopefully, Iââ"šÂ¬Ã¢"žÂ¢ve managed to put the information into a readable form.

 

[Boomer]

:thumbsup:

It appears to be an unbranded Sensorex S500CD-ORP

Up, it is . Simon, you now are beyound me with this Zobell and Lights stuff:rollface: :rollface:

 

[reevs]

Hello Simonh,
I havent found yet 'llink for IKS user in RC'
do you about that?