Buffer preparation

[syx]

When I make a phosphate buffer solution with pH 6.2 – 8.2, for example: 0.025M phosphate buffer pH 7.0, I prepare the buffer solution using this method:

-Transfer 3.4023 g of KH2PO4 to a 1-liter volumetric flask, add water to dissolve, adjust to pH 7.0 with 10% KOH solution, dilute with water to volume, and mix.

In this method phosphate concentration is remaining constant: 0.025M and K+ is in higher concentration than phosphate. I think, sometime, we need this K+ to compete with analyte to interact with ionized free silanols.

[Chris Pohl]

While I agree that development of a proper analytical system that is relatively insensitive to pH is the best solution to this issue, the use of pellets is not a good surrogate for concentrated solutions of sodium hydroxide when preparing buffer solutions with sodium hydroxide. If you've ever noticed the difference in appearance of pellets from a freshly open container and an old container that's been around for years, you'll see that there is a progressively increasing film of sodium carbonate on the outside surface of the pellets. Uncontaminated pellets are translucent in appearance whereas highly contaminated pellets ultimately become opaque with a thick film of carbonate on the exterior surface. The use of pellets is inadvisable considering the fact that the percent by weight of sodium carbonate can vary significantly depending upon the size of the pellets and the number of times the container has been opened. A concentrated solution of sodium hydroxide on the other hand has a reproducible contamination level of sodium carbonate dictated by the solubility limit of carbonate in concentrated sodium hydroxide solutions. This makes concentrated solutions of sodium hydroxide a much better option when choosing to use sodium hydroxide as a component in buffer preparation.

[HW Mueller]

Thought this had run its cause.... but:
Syx,
if you did your adjustment with the KOH via monitoring with a pH meter (not by weighing of a standard sol.) than this is what some of us tried to label as questionable.
Chris,
I must have garbled my statements again. So here an attempt to clarify.
a. using hydroxides, here, is limited to cases where other techniques don´t go.
b. you all convinced me that weighing standard hydroxide solutions should be the best method if hydroxides are required (with the exception that there might be some interference, in rare cases, from material emanating from the plastic bottle).
c. guessing the amounts of pellets, as described above, allows one to handle these so fast that CO2/H2O absorption is quite low so the pellets stay translucent for a long time. Water and CO2 picked up, even if it were a lot, is of no consequence as the pellets are not weighed. Once more: I just threw enough pellets into the acetic acid solution to just get at a higher pH than wanted, then adjusted with the other portion of acetic. The dilution by water on the pellets is obviously negligible, any added CO2 would come to the same equilibrium as that from air. So this method is as good as any which uses mixtures of acid and base solutions of the same buffer.

[syx]

Sorry, out of topic but still have correlation.
One type of our pH-meter needs to be calibrated with 2 buffers solutions. Is it still a valid calibration compared with calibration using 3 buffers solutions?

[HW Mueller]

Nobody prevents you to check the validity of your pH meter with 3 or 10 or 100 or.... standards (via weighed primary standard, for instance). You have to ask yourself: Do I want to get an answer or do I want to satisfy a regulation?