ASTM D664 for acid number of petroleum products
ASTM D664 for acid number of petroleum products by potentiometric titration
APPENDIX
(Nonmandatory Information)
X1. CHECK FOR ELECTRODE PERFORMANCE
X1.1 The kinetic electrode test measures the kinetic response of the electrode. Electrodes can calibrate with acceptable slope and intercept values yet still not have a response good enough for titration. The speed of response and subsequent stability is important for a titration electrode. A manual check is described in this Appendix that can be carried out with a pH meter or titrator set to read millivolts continuously.
X1.2 The essence of this check is to challenge the electrode coming from rest in a water solution with buffers and measure the potential after 30 and 60 s. A fast electrode reaches a stable point in less than 30 s and changes little from 30 to 60 s. Use buffers pH 4, pH 7, and pH 11 for this check, as needed.
X1.3 Procedure
X1.3.1 Set the titrator or pH meter to read millivolts continuously. Have provision for stirring the buffer solution at the same speed used for the titrations.
X1.3.2 Allow the electrode to stabilize for 1 min in distilled or equivalent deionized water.
X1.3.3 Remove the electrodes from the water, and place them in the pH 4 buffer. Start a stopwatch at about the moment when the buffer touches the electrode.
X1.3.4 After 30 s, note the potential. After an additional 30 s, note the potential again. The difference between the two potentials is termed the drift.
X1.3.5 Repeat the procedure for pH 7 buffer and pH 11 buffer.
X1.4 Calculate the drift for each of the three buffers. The electrode response may be judged as follows:
drift < 1 excellent
1 < drift < 2 good
2 < drift < 3 acceptable
3 < drift < 4 questionable
4 < drift unacceptable
X1.5 The difference between the 60-s potentials for pH 4 buffer and pH 7 buffer should be greater than 162 mV, or 54 mV / pH number. Electrodes with a slope less than 54 mV / pH number are not reliable for titration.
SUMMARY OF CHANGES
Subcommittee D02.06 has identified the location of selected changes to this standard since the last issue (D664–06a<e1>) that may impact the use of this standard. (Approved July 1, 2007.)
(1) Added Note 3 stating a few laboratories observed discrepancies in results between aqueous and nonaqueous buffers.
Subcommittee D02.06 has identified the location of selected changes to this standard since the last issue (D664–06) that may impact the use of this standard. (Approved Nov. 1, 2006.)
(1) Clarified requirement for purity of water in 7.2.
Subcommittee D02.06 has identified the location of selected changes to this standard since the last issue (D664–04<e1>) that may impact the use of this standard. (Approved Jan. 1, 2006.)
(1) Added Appendix X1 to provide a possible procedure for checking the electrode performance.
