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A prerequisite for an accurate pH calibration is the availability of a suitable liquid buffer solution.

pH buffer solutions are mixtures of weak acids and the salt of these acids combined with a strong base, or mixtures of weak bases and the salt of these bases combined with a strong acid. Buffer solutions are notable by the fact that they resist change to their pH value regardless of additions of small quantities of acids or bases. Their hydrogen ion activity is stable over a wide range of dilution or concentration.

The National Institute of Standards and Technology (NIST) recommends nine different buffer solutions for the exact calibration of pH measuring systems. These NIST buffer solutions also serve as reference points for the pH scale, as it is impossible to prove the activity of single hydrogen ions by measurement.

All buffer solutions which are produced according to the NIST formulas are called "Standard Buffer Solutions". As the activity of the hydrogen ions is temperature dependent, so is the pH value of any buffer solution. The temperature dependency of the NIST standard buffer solutions is given in the table below.

pH Values of Standard Buffer Solutions according to NIST

Untitled Document
NBS A B H C D E F I G
Temp. C Potassium
Tetra-oxalate
Potassium
Hydrogen Tartrate
Potassium
Dihydrogen Citrate
Potassium
Hydrogen Phthalate
Phosphate Phosphate Borax Sodium Carbonate/
Sodium Hydrogen
Carbonate
Calcium
Hydroxide
0 3.863 4.010 6.984 7.534 9.464 10.137 13.423
5 1.668 3.840 4.004 6.951 7.500 9.395 10.245 13.207
10 1.670 3.820 4.000 6.923 7.472 9.332 10.179 13.003
15 1.672 3.802 3.999 6.900 7.448 9.276 10.118 12.810
20 1.675 3.788 4.001 6.881 7.429 9.225 10.062 12.627
25 1.679 3.557 3.776 4.006 6.865 7.413 9.180 10.012 12.454
30 1.683 3.552 3.766 4.012 6.853 7.400 9.139 9.966 12.289
35 1.688 3.549 3.759 4.021 6.844 7.389 9.102 9.925 12.133
38 1.691 3.548 3.755 4.027 6.840 7.384 9.081 9.903 12.043
40 1.694 3.547 3.753 4.031 6.838 7.380 9.068 9.889 11.984
45 1.700 3.547 3.750 4.043 6.834 7.373 9.038 9.856 11.841
50 1.707 3.549 3.749 4.057 6.833 7.367 9.011 9.828 11.705
55 1.715 3.554 3.750 4.071 6.834 8.985 11.574
60 1.723 3.560 3.753 4.087 6.836 8.962 11.449
70 1.743 3.580 3.763 4.126 6.845 8.921
80 1.766 3.609 3.780 4.164 6.859 8.885
90 1.792 3.650 3.802 4.205 6.877 8.850
95 1.806 3.674 3.815 4.227 6.886 8.833

Standard buffer solutions have an accuracy better than +/- 0.005 pH units between 0°C and 60°C. Between 60°C and 95°C their accuracies are not worse than +/- 0.008 pH units. NIST buffer solutions are exclusively used in laboratory and critical measurement applications. For industrial use, where the demand on absolute accuracy is normally not as high as in the analytical laboratory, manufacturers offer so-called “Technical Buffer Solutions”.

Technical buffer solutions are more stable than standard buffer solutions and easier to manufacture. Their accuracies are given as +/- 0.02 pH units, but may differ from manufacturer to manufacturer. The temperature dependency of technical buffer solutions is normally printed on their container by the manufacturer. Conventional technical buffer solutions have a limited shelf-life, most of them only one year in sealed bottles. Once opened, their shelf-life is reduced to a few months. Especially alkaline buffer solutions which are affected by CO2 contamination from the atmosphere.

DuraCal pH buffer solutions from Hamilton are different. These patented technical buffer solutions provide a pH stability never achieved before; their high accuracies are guaranteed for 5 years after date of manufacture. The pH 9.21 and pH 10.01 buffer solutions are stable even in open air.

Each bottle of Hamilton buffer solutions is certified, including the actual pH value and expiry date. The certification is traceable to primary standards from NIST and PTB. An overview of buffer stability and accuracy is found in a separate article.

Stability Comparison of Hamilton Alkaline Buffer to Conventional Alkaline Buffer

Both buffer solutions have been exposed to blowing air:

Used buffer solutions should always be discarded and never be returned to their original storage bottle. For this reason Hamilton have developed a unique storage bottle which includes a calibration compartment with a non-return valve at the bottom, preventing the used buffer solution to be returned into the storage bottle. This Calpack bottle is practical: no additional calibration container is required, and it is economical: only the required amount of buffer solution is used.

It must always be remembered: one cannot achieve a better measuring accuracy than the accuracy of the buffer solution one uses for calibration.

Values and Accuracies of Hamilton Buffer Solutions

Buffer SolutionpH ValueAccuracyStability in Months
Hamilton DuraCal1.09+/- 0.02 pH60
Hamilton DuraCal1.68+/- 0.02 pH60
Hamilton DuraCal2.00+/- 0.02 pH60
Hamilton DuraCal3.06+/- 0.02 pH60
Hamilton DuraCal4.01+/- 0.02 pH60
Hamilton DuraCal5.00+/- 0.02 pH60
Hamilton DuraCal6.00+/- 0.02 pH60
Hamilton DuraCal7.00+/- 0.02 pH60
Hamilton DuraCal8.00+/- 0.02 pH60
Hamilton DuraCal9.21+/- 0.02 pH60
Hamilton DuraCal10.01+/- 0.02 pH60
Hamilton11.00+/- 0.05 pH24
Hamilton12.00+/- 0.05 pH24

Prior Article - pH Probe Accessories

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