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Determination of pH value of aqueous solutions and emulsions
What is pH?
pH is the main indicator of aqueous solutions. Water as a weak electrolyte dissociates to an insignificant degree into H+ and OH- ions, which are in equilibrium:
H20 ⇄ H+ + OH-
The product of the concentrations of hydrogen ions and hydroxide ions in water at a certain temperature is constant (called the ionic product of water). The constancy of the product [Н+] [OH-] means that in any aqueous solution, neither the concentration of hydrogen ions, nor the concentration of hydroxide ions can be equal to zero. In other words, any aqueous solution of an acid, alkali or salt contains both H+ and OH- ions. Thus, the acidity and alkalinity of a solution can be expressed through the concentration of either H+ or OH- ions. In practice, the first method is usually being used.
Concentration of hydrogen ions for neutral solution [H+] = 10-7 mol/l. To avoid the inconvenience associated with the use of numbers with negative exponents, the concentration of hydrogen ions is usually expressed in terms of the pH.
pH is the decimal logarithm of the concentration of hydrogen ions taken with the opposite sign:
pH = -lg [H+]
With the help of pH, solutions are characterized as follows:
- neutral pH = 7
- acidic pH < 7
- alkaline pH > 7
Here are the approximate pH values of the most common solutions:
- pH of gastric juice 1.7 (highly acidic environment);
- pH of beer wort 5.0 (slightly acidic environment);
- pH of rainwater. 6.0 (slightly acidic environment);
- pH of the treated water 7.0 (neutral medium);
- blood pH 7.4 (slightly alkaline environment);
- pH of hand soap 9.0 (alkaline).
The H values given are approximate and depend on many factors.
pH is not a static value and tends to change (for example, the ph of water from a well changes throughout the year depending on the season). In order to measure the pH value in a particular solution, pH meters are used.
How to measure pH?
By the nature of their use, pH meters are divided into laboratory and portable. A great advantage of laboratory instruments is the measurement accuracy. For example,HACH LANGE electrodes have an accuracy of 0.02 ph. A plus for increasing accuracy will be the fact that, in laboratory conditions, measurements are carried out at certain temperature and humidity. The temperature value greatly affects the pH value, this must be taken into account. The most accurate results are obtained at temperatures close to 20 ℃.
The advantage of portable pH meters is their compactness and the ability to measure the pH value in reservoirs or at different production sampling points. Modern pH electrodes are equipped with temperature sensors and are capable of recalculating values, which significantly increases their accuracy.
How can I improve the accuracy of my pH measurement?
An important point is the choice of an electrode for a pH meter: the determining factor is the solution in which the pH measurement is planned and the operating conditions. So, for example, the HACH product range includes electrodes for:
- aggressive media (alkaline or acidic solutions)
- dirty solutions (waste water)
- for semi-solid products (meat / cheeses)
- for drinking water
- samples which are high in protein (beer)
- for viscous samples (gel products and pastes)
Calibration Frequency: Calibrating the pH meter and checking daily against standard buffers is a must and should not be neglected. Typically you should perform a 3-point calibration: 4.01, 7.00, 10.01, but variations are possible. For example, calibration with a buffer solution with ph = 2.00, 6.88, 9.22. After calibration, it is important to check the pH meter with buffer solutions, and as close as possible to the values of the test samples. For example, if you frequently measure the pH of wort, you should check the pH meter against a buffer solution with a pH of 5.00.
Pay attention to how long the electrode "holds" the calibration. If several days pass after calibration and the electrode shows the correct value during daily check, then everything is ok. If the electrode has to be frequently recalibrated or the calibration does not work from the first time, you should think about replacing the electrode and check the storage conditions of the electrode between measurements.
The manufacturer's recommended electrode service life is 6 months. It happens that the service life is significantly reduced when using the electrode in aggressive environments or due to improper operation. Conversely, with proper maintenance, the electrode can work correctly for more than a year.
AquaAnalytics® is the official representative of Hach in Russia and the CIS countries. We offer test equipment, training and software to monitor and maintain pH in a wide variety of process applications. If you have any additional questions, please contact us in a convenient way.
Representative office in Russia: +7 (495) 201-53-02 info@aquaanalytics.ru
Representative office in Uzbekistan: +998 (90) 174-57-84 info@aquaanalytics-tekhnika.ru
