Verification of a Buffer

Most buffers prepared with COg-free distilled water and with the salts accurately weighed will be precise for a considerable time. Over a period of time, however, many buffers are subject to mold growth, they polymerize, or are affected by pick-up of carbon dioxide. In the proper container, normally glass or polyethylene, sealed from contamination, a standard NBS-type buffer should last at least 6 months. Most manufacturers of buffer solutions include a fungicide, such as butaben, in the buffer to inhibit mold growth. 

The only practical method of verifying the accuracy of a buffer is to compare it to another known buffer solution. NBS-certified salts may be used and the NBS procedure followed to produce a primary buffer whose pH value is closest to the pH value of the buffer in question. The comparison of buffers should be made at the same temperature, using the same electrodes and employing good technique. 

There are many different parameters which can affect the pH measurement, but only two are, normally, sources of significant error. These two parameters are the electrodes and the technique employed. This chapter discusses the components of proper technique and summarizes the various factors of accuracy. The discussion is directed at such questions as what response time can be expected, should the sample be stirred or static, and how does temperature effect the pH reading  

The objective of rinsing the electrodes between measurements is to prevent contamination by carry-over on the electrodes. It has become common practice to use distilled water from a squirt bottle to rinse the electrodes and then to blot them with a tissue. This practice, however, has led to the improper technique of wiping rather than blotting the electrodes. Wiping often results in slower responding electrodes, thus requiring considerable time before a stable pH reading is obtained. 

The point of this discussion is to blot and not to wipe the electrodes. In other words, do not provide a pathway for charge transfer. Since the rinse is done with distilled water which has very little buffering capacity, any excess water on the electrodes is not going to have a significant effect on the next solution. 

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