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Tricky pH measurement - (Oct/26/2007 )

One of my little tasks is making up small volumes ( less than 1 mL) of retrorsine solution using .5 M NaOH and .5 M HCl to pH, then diiluting with water to the desired volume.

To sum up the procedure, the retrorsine is initially dissolved in NaOH, then pHed to 7.0 with the HCl, then an additional volume of NaOH is added to bring the pH to 7.05, then diluted in water to the final volume. The trouble comes in when I add the additional NaOH. The specified volume works perfectly for the lab we received he procedure from, and worked well too when I made up a very large volume and could use the pH meter rather than pH strips. With the pH strips I get a reading of over 8 at that point.

I'm wondering if there's some reason the pH strips might be especially unreliable in this case, and if so how I might be able to work around that. Making up large volumes isn't an option, unfortunately. We only use a little every two weeks and the other lab recommended we not store it for any real length of time.

Thanks!

-D

-dianawinters-

pH strips are neither accurate enough nor do they have the resolution to reliably read a .05 difference in pH (or even a 0.5 difference).

if you need to make a low volume you can use a combined pH electrode or, if the volume has to be even lower, you can use a combined microelectrode (see this website: microelectrodes, inc.) to monitor the adjustment of the pH of your solution.

-mdfenko-

QUOTE (dianawinters @ Oct 26 2007, 03:11 PM)
One of my little tasks is making up small volumes ( less than 1 mL) of retrorsine solution using .5 M NaOH and .5 M HCl to pH, then diiluting with water to the desired volume.

To sum up the procedure, the retrorsine is initially dissolved in NaOH, then pHed to 7.0 with the HCl, then an additional volume of NaOH is added to bring the pH to 7.05, then diluted in water to the final volume. The trouble comes in when I add the additional NaOH. The specified volume works perfectly for the lab we received he procedure from, and worked well too when I made up a very large volume and could use the pH meter rather than pH strips. With the pH strips I get a reading of over 8 at that point.

I'm wondering if there's some reason the pH strips might be especially unreliable in this case, and if so how I might be able to work around that. Making up large volumes isn't an option, unfortunately. We only use a little every two weeks and the other lab recommended we not store it for any real length of time.

Thanks!

-D


If the pH strips are new and dry, then they should work to within .2 or .3 pH. Perhaps the Molar concentration of the NaOh is not accurately known and your pipette is putting in too much NaOh.

-dave2-

QUOTE (dave2 @ Nov 4 2007, 11:42 AM)
QUOTE (dianawinters @ Oct 26 2007, 03:11 PM)
One of my little tasks is making up small volumes ( less than 1 mL) of retrorsine solution using .5 M NaOH and .5 M HCl to pH, then diiluting with water to the desired volume.

To sum up the procedure, the retrorsine is initially dissolved in NaOH, then pHed to 7.0 with the HCl, then an additional volume of NaOH is added to bring the pH to 7.05, then diluted in water to the final volume. The trouble comes in when I add the additional NaOH. The specified volume works perfectly for the lab we received he procedure from, and worked well too when I made up a very large volume and could use the pH meter rather than pH strips. With the pH strips I get a reading of over 8 at that point.

I'm wondering if there's some reason the pH strips might be especially unreliable in this case, and if so how I might be able to work around that. Making up large volumes isn't an option, unfortunately. We only use a little every two weeks and the other lab recommended we not store it for any real length of time.

Thanks!

-D


If the pH strips are new and dry, then they should work to within .2 or .3 pH. Perhaps the Molar concentration of the NaOh is not accurately known and your pipette is putting in too much NaOh.


I thought of that myself - that there was some confusion about the NaOH concentration - but that wouldn't explain why I get the results I expect when using the more sensitive pH meter, and don't with the strips. wacko.gif Argh.

-dianawinters-

QUOTE (dave2 @ Nov 4 2007, 10:42 AM)
If the pH strips are new and dry, then they should work to within .2 or .3 pH.


Thank you Dave2 for the information.

I didn't know it.

I am using pH strips that are 6 years old.

-Minnie Mouse-

QUOTE (dianawinters @ Nov 4 2007, 11:08 AM)
QUOTE (dave2 @ Nov 4 2007, 11:42 AM)
QUOTE (dianawinters @ Oct 26 2007, 03:11 PM)
One of my little tasks is making up small volumes ( less than 1 mL) of retrorsine solution using .5 M NaOH and .5 M HCl to pH, then diiluting with water to the desired volume.

To sum up the procedure, the retrorsine is initially dissolved in NaOH, then pHed to 7.0 with the HCl, then an additional volume of NaOH is added to bring the pH to 7.05, then diluted in water to the final volume. The trouble comes in when I add the additional NaOH. The specified volume works perfectly for the lab we received he procedure from, and worked well too when I made up a very large volume and could use the pH meter rather than pH strips. With the pH strips I get a reading of over 8 at that point.

I'm wondering if there's some reason the pH strips might be especially unreliable in this case, and if so how I might be able to work around that. Making up large volumes isn't an option, unfortunately. We only use a little every two weeks and the other lab recommended we not store it for any real length of time.

Thanks!

-D


If the pH strips are new and dry, then they should work to within .2 or .3 pH. Perhaps the Molar concentration of the NaOh is not accurately known and your pipette is putting in too much NaOh.


I thought of that myself - that there was some confusion about the NaOH concentration - but that wouldn't explain why I get the results I expect when using the more sensitive pH meter, and don't with the strips. wacko.gif Argh.


Perhaps your probe is an old gel that hasn't been calibrated to the meter in a while. I use the most accurate probe I can find - - I use a ROSS. And I use a $300 plus meter.

-dave2-