H changes due to absorption of basic(or acidic)materials in the storage environment, and bacterial growth. Commercially pre- pared buffers should be stored in their original containers. The storage of individually prepared buffers is discussed below. The importance of adequate labeling, including preparation date. composition,pH, the preparer's name, and ideally a notebook number or other reference to the exact procedure used for the preparation, cannot be overemphasized. Absorption of bases The most common base absorbed by acidic buffers is ammonia. Most acidic buffers should be stored in glass vessels. The common indicator of buffer being neutralized by base is failure to achieve he target pH. In acidic buffers the pH would end up too high Absorption of acids Basic buffers can readily absorb CO2 from the atmosphere forming bicarbonate, resulting in neutralization of the base. This is very common with strong bases(NaoH, KOH), but often the effect will be negligible unless the system is sensitive to the pres ence of bicarbonate(as are some ion chromatography systems)or he base is very old. If high concentrations of acids(e.g, acetic neutralized by these as well. a similar common problem is improper storage of a basic solution in glass. Since silicic materi als are acidic and will be attacked and dissolved by bases, long term storage of basic buffers in glass can lead to etching of the glass and neutralization of the base Microbial contamination Buffers in the near-neutral pH range can often readily sup port microbial growth. This is particularly true for phosphate containing buffers. Common indicators of bacterial contamination are cloudiness of the solution and contamination of assays or plat Strategies for avoiding microbial contamination include steril zing buffers, manipulating them using sterile technique, refriger- ated storage, and maintaining stock solutions of sufficiently high ionic concentration. A concentration of 0.5 M works well for phos- phate buffers. For analytical chemistry procedures, phosphate buffers in target concentration ranges(typically 0. 1-0.5M) should be refrigerated and kept no more than one week. Other buffers could often be stored longer, but usually not more than two weeks.pH changes due to absorption of basic (or acidic) materials in the storage environment, and bacterial growth. Commercially pre￾pared buffers should be stored in their original containers. The storage of individually prepared buffers is discussed below. The importance of adequate labeling, including preparation date, composition, pH, the preparer’s name, and ideally a notebook number or other reference to the exact procedure used for the preparation, cannot be overemphasized. Absorption of Bases The most common base absorbed by acidic buffers is ammonia. Most acidic buffers should be stored in glass vessels. The common indicator of buffer being neutralized by base is failure to achieve the target pH. In acidic buffers the pH would end up too high. Absorption of Acids Basic buffers can readily absorb CO2 from the atmosphere, forming bicarbonate, resulting in neutralization of the base. This is very common with strong bases (NaOH, KOH), but often the effect will be negligible unless the system is sensitive to the pres￾ence of bicarbonate (as are some ion chromatography systems) or the base is very old. If high concentrations of acids (e.g., acetic acid) are present in the local environment, basic buffers can be neutralized by these as well. A similar common problem is improper storage of a basic solution in glass. Since silicic materi￾als are acidic and will be attacked and dissolved by bases, long￾term storage of basic buffers in glass can lead to etching of the glass and neutralization of the base. Microbial Contamination Buffers in the near-neutral pH range can often readily sup￾port microbial growth. This is particularly true for phosphate￾containing buffers. Common indicators of bacterial contamination are cloudiness of the solution and contamination of assays or plates. Strategies for avoiding microbial contamination include steril￾izing buffers, manipulating them using sterile technique, refriger￾ated storage, and maintaining stock solutions of sufficiently high ionic concentration.A concentration of 0.5M works well for phos￾phate buffers. For analytical chemistry procedures, phosphate buffers in target concentration ranges (typically 0.1–0.5M) should be refrigerated and kept no more than one week. Other buffers could often be stored longer, but usually not more than two weeks. 38 Pfannkoch