pH/ORP Buffer Calibration Solutions

Primary reference buffer 

These buffers are not commercial buffers and mainly used in metrological institutes. These buffers show the lowest uncertainty in pH values, ±0.003. 

Standard Buffer (secondary reference buffer)

Standard buffer solutions are used as standards for accurate measurements especially in laboratories and production of technical buffers. They are traceable to the primary standards. The constituents of these buffers are defined by international standards such as DIN19266, IEC 726 and NIST. The uncertainty is 0.002 and 0.004 pH units (at 25°C), depending on the buffer.

Technical buffer

They are commercial buffers and used mainly for calibration of industrial pH measurements. The buffer values of technical buffers are traceable to the standard buffer. The DIN19267 defines standards for these solutions. The uncertainty is 0.02 a pH units (at 25°C), depending on the buffer.

Examples of preferred buffers by Yokogawa are shown in the table below. Buffer solutions prepared from these substances conform to the recommendations of the DIN Standards Committee and the National Institute of Standards and Technology (NIST). The substances were chosen for their particular suitability as calibration standards for precision pH meters.

Temperature dependence

The temperature dependence of the pH of a buffer solution is generally specified in terms of measured pH values at certain discrete temperatures. Many buffer tables are preprogrammed in Yokogawa Analyzers. So if during calibration the temperature compensator is immersed in the buffer liquid, an automatic adjustment for temperature variations will be done. Any stated pH value is only meaningful if the measuring temperature is also specified. 

Be Aware

Buffers with a pH above 7 are particularly sensitive to atmospheric CO2. Buffer showing any sign of turbidity must be discarded immediately. For accuracy it is recommended that a buffer should not be used for more than a month after opening. Buffers should be stored in tightly sealed, preferably air-tight bottles made of polyethene or borosilicate glass. Buffers should not be returned to the bottles once removed.

 

ORP Standard Solutions

When verification or calibration of an ORP sensor is required, there are two types of Standard Solutions that are commonly used. The first are premade solutions designed to provide a specific stable mV value, typically one that falls within the process ORP range. The second type of solutions, and probably the most common, are those that are made using the standard pH 4 and pH 7 buffers with quinhydrone crystals mixed in until saturation is reached. Either of these pH buffer solutions can be used for calibration of an ORP measuring system and are very practical if pH loops are also being maintained. Preparation and use of both types of solutions are discussed below: 

Quinhydrone¹⁾ Solution

To create an ORP solution using a pH buffer (either 4.0 or 7.0) stir in a small amount, approximately < 0.5 gm, of quinhydrone into 200 mls of solution. Quinhydrone is not very soluble, so only a small amount will dissolve in the buffer changing the solution to an amber color. If all of the quinhydrone dissolves, then continue to add small amounts and stir again. Saturation is achieved when a small amount of quinhydrone remains undissolved after mixing. 

Whether it is a 4.0 or a 7.0 buffer you are using,  the table below shows the mV reading you should obtain depending on which reference electrode is being used. As an example, a quinhydrone/pH 4.0 solution should give a 253 mV (± 30 mV) at 25°C for a reference electrode that has 3M KCl internal fill.

Note 1: The quinhydrone powder poses a moderate health risk, causing irritation of the lungs with prolonged exposure. The premade calibration solutions are fairly innocuous unless ingested in large amounts. Both types should be handled carefully following good laboratory practices.
Note 2: SCE = Saturated Calomel Electrode
Note 3: SHE = Standard Hydrogen Electrode

Pre-made Stabilized ORP Solutions

Reference electrodes with different internal fill solutions will have different mV outputs when they are put in the same Standard Solution. This is because the Standard Solution was prepared with one specific reference fill solution in mind. Table below lists in the left-most column, some of the most commonly used reference electrode fill solutions. Across the top of the table are the possible reference fill solutions that Standard Solution was prepared against. To use the chart below, you have to know what (1) reference solution is used in the reference electrode and (2) what reference solution the known pre-made solution is being compared to. For example, if you have a pre-made 250 mV solution that is referenced to SHE (Standard Hydrogen Electrode) and the reference electrode in your measuring loop uses a 1 M KCl fill solution, then on the transmitter you would NOT read 250 mV, but instead you would read only 19 mV at 25° C. This is the 250 mV value on the solution minus the 231 mV value shown as the difference between the SHE and the 1M KCl references. This would be 19 mV.

Note 1: SCE = Saturated Calomel Electrode
Note 2: SHE = Standard Hydrogen Electrode