Sciencemadness Discussion Board

Scott BlueLine pH probe woe's

dann2 - 15-11-2009 at 14:32

Hello,

I have a pH probe as described here:

http://www.wenk-labtec.com/katalog_pdf_uk/analytical_measure...

page 203, probe No. 3. It's a 'BlueLine 14 pH' probe.
The probe has two connections. A DIN and a four mm straight plug. The probe has a temperature sensor built into it.

The pH meter I have (Schott CG 842) has three holes at the back.
Electrode (DIN socket)
Temp. (4mm hole)
Ref. Elect. (4 mm hole)

The probe is described in the catalogue thus:
_______________________________________
pH-combination electrode with glass shaft, Ag/AgCl reference system and
1 metre x 5 mm dia. fixed cable, DIN plug connection and 4 mm pin plug connections.
Applications: Universal use in the chemical, pharmaceutical and food industries.
3
Schott Instruments
Specifications
0 to 100°C 0 to 14 pH
Temperature sensor: NTC 30 k ohm
Length/Dia.: 120 mm/12 mm
Electrolyte: Fluid, KCl 3 mol/l
Diaphragm: Platinum
Membrane resistance at 25°C: 300 M ohm
Refill hole seal: Slide valve
________________________________________

The meter is giving a error indicating that the probe is faulty. How do I get the liquid (3M KCl per liter)out of the probe. There is a filling hole but I cannot empty the probe.

I presume that I connect the four mm plug (temperature?) to the temperature hole on the back of the pH meter?

What is the Reference Electrode socket for?

Will an 'ordinary' probe (BNC to DIN converter required) with no temperature connecter coming from it work with this meter, if I note the temperature of the sample and compensate accordingly (manually)?

There seems to be a bubble at the bottom of the probe where KCl solution will not fill up. Is this normal?
The probe has lots of KCl solution in it.

Thanks for your time,

Dann2

densest - 17-11-2009 at 16:20

First try it without connecting the 4mm plug. The temperature compensation should be sufficient for most uses. Check the manual for your pH meter for the correct connection and setup for a probe with reference electrode. How did you test the system? Are you using standard calibration solutions? Did you let the probe dry out? Do you know what sensor resistance your pH meter is designed for?

Ozone - 18-11-2009 at 14:49

I don't know about yours, specifically, but mine are filled with a saturated AgCl solution. KCl might cause a few problems...

[edit]
Apparently, some solutions contain KCl, as well as AgCl because the solubility of AgCl is greater in water containing a large amount of Cl-. Fill and storage solutions of 4M KCl saturated with AgCl are known.

Interestingly, none of the Ag/AgCl probes in your pdf indicated AgCl in the filling solution?
[edit]

If your solution is, in fact AgCl, and all else looks OK, it might be fouled or biased. This can usually be fixed by immersion in 1:1 HCl (conc):iPrOH (or water, if you prefer) overnight.

Cheers,

O3



[Edited on 19-11-2009 by Ozone]

dann2 - 19-11-2009 at 13:54

Hello,

Thanks for replys. The filling solution is 3 Moles per liter KCl solution. Says it in manual.
The meter is a SCHOTT CG 842
Picture of one here


I will test the meter by putting a milli Volt power supply to it (the DIN plug) and see what it says on the display. I should get something.
I can always purchase cheap probe if I think the meter is OK and then perhaps splash out on something more expensive.

Input impedence of meter is >= 10^12 Ohms.

I purhcased the meter and probe so I don't know what it's history is. It just gives a E3 error (check the probe it says in manual) when in pH mode. The probe has a temperature sensor integrated. The temperature displayed stays at 25 Degrees C. There is also a CalError sign on display.(when in pH mode).

When you switch over to use the probe to read mV the display moves/floats from -200mV -80mV with the probe not connected. With the probe connected the display reads 1400mV and moves up from that value to OFL (overflow).

The probe has a boot on it and has not been let dry out (my me) but I have no idea of history.

Also wondering if I were to make my own refilling solution, what purity of KCl do I need. I have lot's of fertilizer grade. If I recrystalize a few times?

Cheers.



Dann2


[Edited on 19-11-2009 by dann2]

densest - 20-11-2009 at 14:08

Short the input to the meter with a paper clip - it should register 0 mV (or very close). 1400 mV from a probe is way way off, so one of the two is broken.

dann2 - 22-11-2009 at 08:44

Hello,
Could not get zero Volts from meter by shorting input so I took the cover off and (low and behold) there was a printed circuit cable out of its socket. Don't know if this was the fault of a fall or perhaps someone had been in there before me. The meter started to read mV OK but still showed a pH probe error. I then went through a reset procedure of the meter and it sprang into correct operation. (pH anyways).
The temperature measurement on the probe does not seem to be working correctly though. It reads approx. 8 degrees C too low at (15C). The temperature probe is in the pH probe. Still need to calibrate but all seems to be OK and I am not doing precision work anyways.

Does setting the meter to Milli Volts (Redox potential) and 'reading mV' with the pH probe mean anything at all. I presume you need a Redox probe for to do that?
I cannot get the meter (I presume pH meters do not have the facility and why would you want it anyways) to display the actual milli Volts (0.059mV per pH point I believe from reading Wiki) coming directly from the pH probe.

Thanks, Dann2

densest - 22-11-2009 at 15:53

Quote: Originally posted by dann2  
Hello,
Could not get zero Volts from meter by shorting input so I took the cover off and (low and behold) there was a printed circuit cable out of its socket. Don't know if this was the fault of a fall or perhaps someone had been in there before me. The meter started to read mV OK but still showed a pH probe error. I then went through a reset procedure of the meter and it sprang into correct operation. (pH anyways).
The temperature measurement on the probe does not seem to be working correctly though. It reads approx. 8 degrees C too low at (15C). The temperature probe is in the pH probe. Still need to calibrate but all seems to be OK and I am not doing precision work anyways.

Congratulations on fixing it!

There is no standard temperature sensor for pH probes. If you can find the manual for the meter it might tell you what resistance value it expects for the sensor. Many manufacturers use a specified type of 10000 (10K) ohm thermistor (temperature dependent resistor). The catalog page you posted said the probe uses 30K ohms. Since the resistance value decreases with increasing temperature, that would cause the meter to read low. Crudely, one could try various resistors in parallel (I would start around 100,000 (100k) ohms) between the sensor terminal and the ground (connector shell) until it reads correctly near room temperature. A smaller resistor will make the meter read a higher temperature. It would be increasingly incorrect at temperatures further from the one correct reading, but might be usable.
Quote: Originally posted by dann2  
Does setting the meter to Milli Volts (Redox potential) and 'reading mV' with the pH probe mean anything at all. I presume you need a Redox probe for to do that?
I cannot get the meter (I presume pH meters do not have the facility and why would you want it anyways) to display the actual milli Volts (0.059mV per pH point I believe from reading Wiki) coming directly from the pH probe.


As far as I know, a setting of mV should display real millivolts. If you could find a 1 megohm (anything between 10K and 10meg would do) resistor and a small old dry cell, you could put the resistor in series with the dry cell and connect that to the meter input. The resistor is to protect the meter input.

It should read somewhere between 800-1400 mV depending on how thoroughly drained the dry cell is. A fresh dry cell can read as high as 1650 mV which might be out of range for the meter. Remember, near ph 7 the probe will read 0 volts, not 0.42 mV.

A redox probe is a standardized pair of electrodes which should read directly in mV.

watson.fawkes - 22-11-2009 at 16:08

Quote: Originally posted by dann2  
The temperature measurement on the probe does not seem to be working correctly though. It reads approx. 8 degrees C too low at (15C).
Offset errors on temperature probes are generic, enough that controllers that take arbitrary sensors typically have a configuration parameter for setting the offset manually. My guess would be that there's something analogous in your system. It might be in hardware or software, depending on the age of the equipment.

dann2 - 22-11-2009 at 17:57

Hello,

When my meter is reading a pH of 7.94 (pH option selected) it reads -52.0mV when you go over to the mV option.
At a pH of 10.44 (pH option selected) it reads -190mV when you go over to the mV option.
At a pH of 0.82 (pH option selected) it reads 342.8mV when you go over to the mV option.

When I connect a dry cell with a Voltage divider using one fixed resistor and a pot the meter reads mV OK and in agreement with a multimeter. Seems strange.
Would there be an amplifier in the probe?

Regarding the non functioning temperature. You can manually put in a measured temperature from keys on front of meter.
Also the meter may be set for the wrong temperature sensor. Will have to examine it further. I have a manual for the meter so it's easy enough to check.
Dann2



[Edited on 23-11-2009 by dann2]

densest - 22-11-2009 at 21:40

The voltage from the probe is about right - 59 mV per pH unit. The article you read may have been sloppy and used mV and V together without carefully specifying where the 10^3 was supposed to be.

watson.fawkes - 23-11-2009 at 06:56

Quote: Originally posted by dann2  
Regarding the non functioning temperature. You can manually put in a measured temperature from keys on front of meter.
There's your offset correction. This device does it by having you enter an absolute temperature. It then internally computes an offset and stores it.

Swede - 2-12-2009 at 06:59

I am not a huge fan of pH meters with ATC. They tend to require non-standard pH probes using DIN or similar rather than BNC. Most ATC use some sort of PT-100 or similar resistive element, and it is not hard to make a set of standard resistances to plug into the ATC port if the meter does not accept manual temperature inputs, using 1% resistors.

Better is simply measuring the temp and entering it, or using a corrective table. Or living with the modest error if you can.

Dann2, I recently bought a refillable lab electrode that also uses KCl 3M as a refill. I wrote to the manufacturer, and here is his reply - maybe it will help with your probe.
Quote:

To exchange/refill electrolyte solution remove the small retaining bolt from the side of the probe and locate the access hole into the center of the probe with a small paperclip. If you purchased your unit from this United States distribution center you would have received several needle nose pipettes. with the needle nose pipette insert into the located access hole and remove the old electrolite solution. Once the probe is fully drained refill using electrolyte solution MA9011 which is rated at 3.5m- KCL -- hook up the probe and turn on meter to charge electrolite inside the probe ( 10 to 15 min. ) then recalibrate --- ALWAYS KEEP YOUR PROBE HYDRATED AND CLEAN


So some probes have a flap or port that requires a super-fine tipped pipette to withdraw the old solution.

As the probe response gets slower, a fairly aggressive cleaning solution made of acidified KCl (maybe 1M KCl and 1% to 3% HCl) seems to restore a more rapid response and allows easier calibration. The more I work with these units, the more I understand the need for frequent calibration. Even stored wet with a proper storage solution, they age and lose calibration, and a pH probe is the ONE ITEM I simply will not buy on eBay. Even a $500 unused probe that is 5 years old is going to be very problematic. Buying (or maintaining a good refillable glass lab probe) is part of the cost of doing business.

Good luck with your unit.