The sections of Method 1311 (TCLP) that deals with the preparation of Leachate fluids 1 and 2 states that if the fluids do not fall between a certain pH range, they must be remade.
Why can't a few drops of the individual reagent (i.e. sodium hydroxide solution or acetic acid solution) be used to adjust the pH of the extraction fluid, instead of having to discard the bulk solution and start again?
Since Method 1311 is a Method Defined Parameter (MDP) method, it is exempt from the flexibility clause that is in effect for most of the other SW‐846 methods. As such, it must be followed prescriptively, especially in a regulatory situation. Getting the exact pH with buffered solutions that are used in TCLP depends on using the right amount and concentration of the reagents. The range of pH allowed is fairly easy to accommodate using regular laboratory glassware and equipment to measure volumes and prepare reagents. Some laboratories measure everything out by weight (accounting for the density) to get more accuracy, which is also allowed.
Technically, the nature of the fluid is changed by pH adjustments. As more acid or base is added to get the pH within range, the buffering capacity of the fluid is changed. Glacial acetic acid is pure, so diluting it with the right amount of water to prepare fluid #2 is straightforward. Fluid #1 requires the use of 1N NaOH, which can be prepared improperly. If prepared from NaOH pellets, which are hygroscopic, significant amounts of water can be absorbed if the pellets are old or exposed to the atmosphere for any length of time. Consequently, you may not have 1N NaOH, and using the prescribed volumes in the method will give you a pH that is too low. If the NaOH concentration is verified after making it from scratch or 1N NaOH is purchased, the prescribed amount of NaOH and glacial acetic acid per liter will give the right pH, within the allowed window. We have also seen situations where the fluids were prepared correctly, but the pH probe was calibrated improperly. Adjusting the pH to the supposed "proper" pH in that case, would have led to improper fluid.