Lab pH Instruments

pH is a scale used to specify the acidity or alkalinity of aqueous solutions. The pH value correlates with the concentration (to be more precise: with activity) of hydrogen ions. Solutions with a pH less than 7 are acidic (high concentration of hydrogen ions) and solutions with a pH greater than 7 are basic (low concentration of hydrogen ions).

pH is measured to:

• Produce products with defined properties
  
• Produce products at reduced costs
  
• Ensure the quality of products, in order to avoid damage to people, materials and the environment
  
• Fulfill regulatory requirements
• Protect equipment
  
• To gain knowledge for research and development

Lab pH instruments are used in various industries such as:

• Pharma and biotech
• Dairy
• Soils and sewage treatment
• Cosmetics
• Water filtration
  
• Food and beverages

Additionally, pH instruments are needed for applications outside of the laboratory. This includes locations near or at industrial production and in the field (for measuring water, sewage, soil ,etc.).

The tools necessary for pH measurements are relatively uncomplicated and provide reliable measurements when used correctly. A typical lab pH instrument consists of the following:

• pH meter: A potentiometer that measures the voltage difference between a glass electrode and a reference electrode and calculates the pH value.
• Sensors: A reference and a pH electrode for completing the circuit. Nowadays they can be combined, and are called combined pH electrodes.

Other tools required are:

• Calibration solutions: Before measuring the pH of a sample, two or more reference solutions of known pH values must be used for the calibration of a pH electrode.
• Sample: The sample is the solution to be measured, which needs to be an aqueous solution or contain enough water for the pH measurement to be possible.

Yes, pH and conductivity are related, but not linearly or in an absolute manner.
A pH sensor responds solely to H⁺ in a solution, whereas in conductivity the sensors measure the activity of all charged ions (anions and cations) present in a solution. The higher the concentration of ions, the higher the conductivity.

Furthermore, the mobility of an ion has an enhancing impact on conductivity. Among the common ions in a solution, the most mobile cation is the Hydrogen ion [H⁺] with a value of 350 units, and the most mobile anion is the Hydroxyl ion [OH^-], 199 units. Other common ions have values ranging between 40 and 80 units. This means strongly acidic (or strongly basic) solutions will have high conductivities. Since pH is a measure of the concentration of Hydrogen ions, the following rules apply:

• In acidic solutions (< pH 7): the lower the pH (i.e. the higher the H⁺ concentration) the higher the conductivity.
  
• In alkaline solutions (> pH 7): the conductivity increases with increase of the pH (increase of OH^- ion).
  
• Neutral pH (pH 7) is due to equal concentration of H⁺ and OH^- ions. But it does not mean the solution does not contain any other ions that would contribute to the conductivity of the solution.
  
  

Let's consider an example: pH of deionized water is theoretically 7.0 and conductivity is 0.055 µS/cm. If you add NaCl salt to it, the resulting NaCl solution will still be of neutral pH, but the conductivity of the solution could greatly increase depending on the amount of NaCl added.

In summary: pH and conductivity of a sample must be determined separately for each of the samples and cannot be theoretically correlated.

pH measurements depend on a sample’s temperature. The below points are important to keep in mind:

a. Temperature influence on the slope of a electrode:
The pH electrode provides a potential (mV) between the measuring and the reference half-cell. The lab pH instrument calculates the pH value from this potential using the temperature dependent factor -2.3 * R * T / F where R is the universal gas constant, T the temperature in Kelvin and F the Faraday constant. At 298 K (25 °C), the factor is -59.16 mV/pH. This is called the theoretical slope of the electrode at the reference temperature (25 °C). At different temperatures the slope values can be calculated accordingly. E.g.: -56.18 mV/pH at 10 °C, -58.17 mV/pH at 20 °C, -60.15 mV/pH at 30 °C and so on. This influence of the temperature on the pH measurement is corrected by automatic (ATC) or manual temperature compensation (MTC). Hence, it is important to know the temperature of a sample or to use a temperature probe. A wrongly set temperature results in an error of 0.12 pH units per 5 °C difference.

b. Temperature influences the pH value of a sample:
The pH value of a sample changes with the temperature. This is a chemical effect and therefore individual for each type of sample. This influence cannot be compensated; only the real pH value at the actual temperature is displayed. Hence, it is important to compare only pH values measured at the same temperature.

Exception: the temperature dependence of pH of many commercial buffer solutions is stored in the instrument. As result the electrode can be calibrated at different temperatures because the measured potentials are automatically referred to 25 °C or 20 °C. To benefit from this feature, it is important to select the correct buffer group and to measure the temperature during calibration.

The conductivity measurement is strongly temperature-dependent (about 2% variation per °C). Results can only be compared if the temperature of all samples is identical or if the value refers to a certain reference temperature.

In most cases, the linear temperature compensation is used. The operator has to select 20 °C or 25 °C as the reference temperature. The difference between the measured and the reference temperature is then multiplied by a compensation factor called α (unit; %/°C), which in turn compensates the conductivity.

In order to do this correctly, the linear compensation coefficient α must be determined for each sample. Even though the temperature dependence is considered linear, in reality this “linear” coefficient itself depends on the ion concentration and temperature of a sample. The factory setting for α is 2.00 %/°C. In all Five and Seven meters α can be adjusted from 0.00 %/°C - which means no temperature compensation at all - to 10 %/°C.

The METTLER TOLEDO pH Competence and Support Center (pH CSC) comprises of a team of experts in direct electrochemical analysis. Due to the team's close contact with customers, technical support, product management and product development, quick advice and effective solutions can be provided, making this service rather unique in the world of pH analysis.

The offered technical and applicative support encompasses the following measuring parameters and the related METTLER TOLEDO pH Lab equipment:

• pH
• Redox (ORP)
• Ion concentration (ISE)
• Conductivity
• Dissolved oxygen (DO)