pH measurements are typically made in solutions which contain relatively large amounts of acid or base, or which contain substantial amounts of dissolved salts. Under these conditions, conventional pH electrodes can make measurements quickly and precisely.

There has been a growing interest in making pH measurements in "pure water": waters in which the total amount of acid or base is very small, and in which there is a low level of dissolved salts. The terms "pure water" and "low ionic strength" can be used interchangeably. Sample which may fall into this catagory include:

Measurement in these pure water samples is more difficult. Although electrodes respond quickly in buffers, in pure water the electrode response is often unsatisfactory - slow, noisy, drifty, non-reproducible, and inaccurate.

II. The Problem

These commonly observed problems can be attributed to the low conductivity of the sample, differences between low ionic strength solutions and normal ionic strength buffers, changes in the liquid junction potential, and absorption of carbon dioxide.

Since pure water solutions are poor conductors, the solution will tend to act as an "antenna" and response can be noisy. Standardization of an electrode in a high ionic strength buffer will increase the time required for stabilization when measuring a low ionic strength sample. In addition, the possibility of sample contamination will be increased. For most precise measurements, buffers and samples should be of similar ionic strength. When any two solutions come in contact, diffusion occurs until equilibrium is reached. Since ions have different mobility and diffuse at different rates, a charge imbalance occurs at the point of contact. A junction potential occurs when the reference electrode filling solution meets the sample. This charge imbalance will be large if there is a large difference in composition between the filling solution and the sample. It is important that the junction potential be constant during measurement. If the two solutions are quite different, normal fluctuations in the boundary layer will produce noise. Constant, reproducible junction potentials are achieved by measuring in samples and standards with ionic strengths similar to the filling solution.

Since pure waters contain little dissolved material, their buffering capacity is small. Absorption of carbon dioxide from the atmosphere will cause a slow change in pH, observed as a "drifting" pH reading and a different pH from the original sample. Samples that are not previously saturated with carbon dioxide must be handled with care.

III. Conventional approaches

The most widely accepted solutions to these problems involved the use of low resistance glass pH electrodes or the use of a reference electrode with a fast, continuous leak rate. When placed in a pure water sample, these electrodes show improved time response and stability, due to dissolution of the low resistance glass into the low ionic strength solutions and the non-quantitative addition of a salt solution from the reference into the sample, respectively. Both techniques raise the conductivity, but, at the same time, may change the sample pH at the electrode surface. Response is improved, but an error is added to the measurement, which depends upon the amount of dissolved material. Increased sample conductivity is desirable, whereas variable alteration of pH is not.

IV. The Pure Water ^TM Solution

Orion has developed an easy-to-use method that minimizes the problems encountered when measuring pH in pure waters. The method uses a high quality glass pH electrode, and a kit consisting of a pure water ionic strength adjustor (Pure Water pHisa^TM) and a special set of Pure Water L.I.S. buffers containing the same background of Pure Water pHisa. For the best results, the ROSS electrode Orion 81-02 is recommended.

Adding Pure Water pHisa to samples increases the ionic strength, thus reducing noise and improving response time. The shift in pH causedby the addition of Pure Water pHisa is minimal, between 0.005 and 0.01 pH units. Since the same amount of Pure Water pHisa is added tothe buffers and samples any effect on the pH is neglible.

V. Why the Pure Water Solution is Better

Calibration is performed using Pure Water L.I.S. buffers with pHisa already added. Measuring with samples and buffers of the same ionic strength improves accuracy, precision, and electrode stability. Contamination due to carryover from higher ionic strength buffers is also minimized.

Errors in pH measurement due to liquid junction potential variations are minimized by using buffers and samples at similar ionic strengths. Addition of Pure Water pHisa to both the buffers and samples achieves this condition.

Junction potentials will vary depending on the style of junction and choice of electrode filling solution. A high quality pH electrode will provide better response when compared to the universal standard, the hydrogen electrode. To optimize the measurement, Orion recommends using the glass ROSS pH electrode, Orion 81-02.

The Pure Water method uses:

• Pure Water L.I.S. calibration buffers
• Pure Water pHisa ionic strength adjustor
• a ROSS pH electrode

These special dilute buffers and adjustor are provided in the Pure Water pH Test Kit, Orion No. 700001, or may be purchased separately. The Orion 81-02 ROSS pH electrode must be ordered as a separate item. See Ordering Information for more details.

VI. Ordering Information

VII. Instructions

Measuring Hints

• Before measuring samples, perform a two buffer calibration. Restandardize with one buffer periodically during the day.
• Measure the temperature of samples and buffers. If the temperatures are different, temperature compensation is recommended. Refer to the meter instruction manual for calibration procedures using temperature compensation.
• Use fresh buffers for each calibration.
• Add Pure Water pHisa to each sample, 1 mL Pure Water pHisa to 100 mL of sample. Other sample volumes may be used, as long as the ratio of Pure Water pHisa to sample remains 1:100.
• Stir all buffers and samples. Place a piece of insulating material (e.g. Styrofoam or cardboard) between the magnetic stirrer and the beaker to prevent drift due to heat transfer.
• Before placing electrodes into any solution, rinse with an additional aliquot of the solution. Do not rinse into the solution being measured. Do not wipe the electrode since contamination or polarization may occur.

Procedure

1. Condition the electrode as described in the electrode instruction manual. Refer to the meter instruction manual for meter calibration details.
2. Choose two of the Pure Water L.I.S. pH Buffers which bracket the expected sample pH.
3. Pour 100 mL of the first Pure Water L.I.S. pH Buffer into a 150 mL beaker. 
4. Rinse the electrode with the buffer used in step 3, and place electrode into the beaker.
5. When a stable reading is obtained, calibrate the meter to display the pH value of the buffer at the measured temperature. A table of pH values at various temperatures is supplied on the bottle.
6. Pour 100 mL of the next Pure Water L.I.S. pH Buffer into another 150 mL beaker.
7. Rinse the electrode with the buffer used in step 6, and place electrode into this second beaker.
8. When a stable reading is obtained, set the meter to display the pH value of the second buffer at the measured temperature.
9. Pour 100 mL of sample into a 150 mL beaker. With the syringe provided, add 1 mL of pHisa to this sample solution.
10. Rinse the electrode with sample, and place into sample beaker. Wait for a stable reading and record the sample pH.