Translator for HPLC HINTS and TIPS for Chromatographers

Saturday, December 26, 2015

Common pKa Values for ACIDS & BASES used in HPLC and LC/MS Method Development

pKa (25°C)                              ACID
0.3                                           Trifluoroacetic acid
2.15                                          Phosphoric acid (pK#1)
3.13                                          Citric acid (pK#1)
3.75                                          Formic acid
4.76                                          Acetic acid
4.76                                          Citric acid (pK#2)
4.86                                          Propionic acid
6.35                                          Carbonic acid (pK#1)
6.40                                          Citric acid (pK#3)
7.20                                          Phosphoric acid (pK#2)
8.06                                          Tris
9.23                                          Boric acid
9.25                                          Ammonia
9.78                                          Glycine (pK#2)
10.33                                        Carbonic acid (pK#2)
10.72                                        Triethylamine
11.27                                        Pyrrolidine
12.33                                        Phosphoric acid (pK#3)

Notes: (1) This is a general list of commonly used acids & bases for chromatography applications and not meant to be a comprehensive list of all values. (2) TFA is an overused and very strong acid for many chromatography applications. It also has strong ion pairing properties and can result in high UV noise, vacuum degasser and/or MS contamination. If you must use it, try and use the lowest concentration which results in the desired pH. Example: 0.1 % TFA ~ pH 2.0, 0.02% TFA ~ pH 2.7. (3) Formic acid is a popular alternative to TFA for many applications, esp LC/MS.

Reference: CRC Handbook of Chemistry & Physics.