A Word About Buffers

pH Buffer solutions are an indispensable tool for maintaining an accurate pH measurement. A pH measurement loop requires regular calibration of the electrodes to compensate for their aging and deterioration, therefore it is essential to have a reliable standard with which to do this calibration. This is the task of pH buffers. They are the reference points to which the measurement electrodes are calibrated. Buffer solutions are used as a means of keeping pH at a nearly constant value in a wide variety of chemical applications. Consequently, if the buffers are not accurate themselves, the calibration serves no useful purpose.

The fundamental concept of buffers is that it is a solution designed to maintain its rated value even with the intrusion of contaminants. Buffer solutions are one which resists large changes in pH following the addition of either an acid (H+) or a base (OH-). This is because the weak acid and its conjugate base can react with both the acid and base. A buffer solution is an aqueous solution consisting of a mixture of a weak acid and its conjugate base or a weak base and its conjugate acid, mixed in a 1:1 ratio (equilibrium). These are called N.I.S.T. Buff ers. National Institute of Standards and Technology (NIST), formerly known as the National Bureau of Standards (NBS); is a federal technology agency that develops and promotes measurement, standards, andtechnology. The level of contaminants a Buffer can withstand before becoming inaccurate has a great deal to do with how close to equilibrium the solution is in the first place. The pH of such a solution is calculated by the following formula:

pH = pKa+log(salt)/(Acid)

If the Salt/Acid ratio is 1, then we have:

pH = pKa, because log 1 = 0

N.I.S.T. lists the pKa values for buffers at 25ºC:

4.008- Potassium Hydrogen Phtalate
6.865- Potassium Dihydrogen Phosphate/ Disodium Phosphate
9.180- Sodium Teraborate Decahydrate (Borax)

Therefore, the pH values for the most stable buffer solutions are 4.01, 6.865 and 9.18 respectively after rounding at 25°C. These are the equilibrium values at which the buffer can accept the maximum level of contaminants before becoming inaccurate. This is commonly referred to as the "buffer capacity." The constituents of these liquids are defined by international standards like DIN19266, IEC 726 and NIST.

What Buffer values do you use... 4.00, 7.00, and 10.00? If you use 4.00, 7.00, and 10.00 you are not using N.I.S.T. Buffers, you are using what are known as Technical or "Traceable to N.I.S.T." Buffers. You can tell a N.I.S.T. Buffer from a Traceable Buffer because no N.I.S.T. Buffer at reference temperature (25ºC) is a whole number and all traceable buffers are whole numbers (4.00, 7.00, and 10.00).

A Traceable or Technical Buffer is made by taking the N.I.S.T. formulations for true buffered solutions and adjusting them away from the 1:1 ratio to get the whole number, 4.00, 7.00, or 10.00. This was done 15-20 year ago because with the past old Analog Transmitter it was simpler and straightforward to read a whole number during calibration. Which would you be able to see better on the transmitter below in Figure 1, a pH of 6.86 or a pH of 7.00? The answer is simple 7.00 with out a doubt.

Examples of the N.I.S.T. buffers used by YOKOGAWA are shown in Table 1. Buffer solutions prepared from these substances conform to the recommendations of the DIN Standards Committee and the National Instit ute of Standards and Technology (NIST). The substances were chosen for their particular suitability as calibration standards for precision pH meters. Examples of the Traceable buffers are shown in Table 2. All Yokogawa pH Transmitters are preprogrammed with N.I.S.T. Buffer tables for calibration. Refer to Technical Note TNA0905 "PH450 Analyzer Programming Customer Buffer Tables", before calibrating a Yokogawa EXA PH series transmitter with Traceable or Technical Buffers; you will need to program th e buffer table to reflect the buffer table of the buffer you are using. Refer to Technical Note TNA0919 "PH450 Analyzer Free Programmable Buffer Tables" for help.

Table 1

EXA Auto CAL NBS Buffer Table
Temperature		pH Value
°C	°F	pH7	pH4	pH9
0	32	6.98	4.00	9.46
5	41	6.95	4.00	9.40
10	50	6.92	4.00	9.33
15	59	6.90	4.00	9.28
20	68	6.88	4.00	9.23
25	77	6.86	4.01	9.18
30	86	6.85	4.02	9.14
35	95	6.84	4.02	9.10
40	104	6.84	4.04	9.07
45	113	6.83	4.05	9.04
50	122	6.83	4.06	9.01

Table 2

Typical NBS Traceable Buffer Table
Temperature		pH Value
°C	°F	pH7	pH4	pH9
0	32	7.13	4.00	10.34
5	41	7.10	4.00	10.26
10	50	7.07	4.00	10.19
15	59	7.05	4.00	10.12
20	68	7.02	4.00	10.06
25	77	7.00	4.00	10.00
30	86	6.99	4.02	9.94
35	95	6.98	4.02	9.90
40	104	6.97	4.04	9.85
45	113	6.97	4.05	9.82
50	122	6.97	4.06	9.78

Temperature Dependence

The temperature dependence of the pH of a buffer solution is generally specified in terms of measured pH values at certain di screte temperatures. Since the Yokogawa EXA PH series instruments are pre-programmed with N.I.S.T. Buffer tables, you can see from table 1 above that as the temperature ch anges the pH of the buffer solution also changes. So if during calibration the temperature compensator needs to be immersed in the buffer liquid, an automatic adjustment for temperature variations will be done in the transmitter. Any pH value listed is only meaningful if the measuring temperature is also specified. High caustic Buffers (9.18 and 10.00) are the most temperature sensitive buffer that can be used. pH 4 Buffer is th e most temperature stable buffer that can be used.

Be Aware

• Solutions with a pH above 7 (base or caustic)are particularly sensitive to the ingress of atmospheric C02. When C02 from the air dissolved in the caustic buffer the conductivity increases and the pH value decreases. Why? Because the Carbon Dioxide from the air dissolves and forms Carbonic Acid... this causes the pH value to go down. The longer the caustic buffer sits open to air or the older the Buffer is, the greater the ingress of the C02, the buffer values will be low. Therefore it is good practice to only use the 9.18 pH buffer for checking the upper range of the transmitter, and use 6.86 and 4.01 pH Buffers to perform calibration.
• Solutions showing any sign of contamination must be discarded immediately and fresh solutions used.
• No solution should be used for more than a month after preparation, as it may no longer be accurate.
• Solutions should be stored in tightly sealed, preferably air-tight bottles made of polythene or borosilicate glass.
• Solutions used in calibration should NOT be returned to the original bottle, as they will contaminate the buffer.