Question

I have a basic question on a reagent (solution) preparation. Here are the steps that I take to prepare the reagent:

Weigh the chemical (solid form, ammonium bicarbonate) and dissolve with DW to make 100 mM solution. The weight (or volume needed to make 100 mM) can vary depending on the # of the samples to be analyzed

Calibrate the pH meter and adjust the pH of the above solution to a certain pH

I would like to make my life easier by not using the pH meter (gets tedious having to wait for the pH calibration and whatnot). I do know that theoretically, if I make the weight of the chemical the same each time I prepare the solution, I can add the same amount of base to set the pH. But is it correct that the volume of base added changes depending on the amount of chemical weighed?

I don't remember my chemistry very well, but from my memory, pH is concentration dependent so I was wondering if adding the same volume of base to a solution of desired concentration (but may have different volumes as I may weigh the chemical differently) is reproducible.

So basically, I want to simply weigh the chemical, dissolve with required volume of DW to make 100 mM concentration, and then add the same volume of base without having to use my pH meter, which will save a lot of time in the morning.

Answer 1

To adjust the pH of a 100 mM ammonium bicarbonate solution without a pH meter could be possible by adding an exact volume of base, assuming the solid weighed is consistent. However, due to factors like temperature and impurities, periodic validation with a pH meter is recommended for accuracy.

Step-by-step explanation:

The accuracy of pH adjustment in a solution is dependent on the specific stoichiometry of the acid-base neutralization reaction. When preparing a 100 mM ammonium bicarbonate solution and adjusting its pH, the amount of base required for pH adjustment can indeed vary depending on the initial moles of the chemical weighed.

pH does not depend on the volume of the solution, as long as the solution is not so dilute that the autoionization of water becomes significant.

Therefore, if you consistently weigh the exact same amount of ammonium bicarbonate each time, you could potentially add the same volume of a titrant to reach the desired pH, provided that the concentration of the base is consistent and the reaction goes to completion.

However, it is important to remember that factors such as temperature and impurities can influence the pH, making it advisable to occasionally verify the pH with a pH meter to ensure reproducibility.