Sorensen’s pH Scale, pH Determination (Electrometric and Calorimetric) : Pharmaceutical Guidelines -->

Editable Pharmaceutical Documents in MS-Word Format


Sorensen’s pH Scale, pH Determination (Electrometric and Calorimetric)

Ions are produced when electrolytes dissociate in water. As acids react with water, hydrogen ions or hydronium ions are produced.

pH and Sorensen’s pH

Ions are produced when electrolytes dissociate in water. As acids react with water, hydrogen ions or hydronium ions are produced. According to thermodynamics, pH can be calculated by calculating the negative logarithm of hydrogen ions' activity.


pH = -log aH+ --------------------(1)

aH+ = activity of hydronium ion

In terms of activity, hydronium ion concentration, divided by rational activity coefficient, is equal to hydronium ion activity.

aH+ = c x γ ± ----------------(2)

Putting equation (2) in place of equation (1) gives

pH = -log (c x γ ±) ------------------(3)

When salts are not added, activities are equal to concentration, since ionic strength is small. When neutral salts are added, the hydronium ions' activity is altered and activity coefficient should therefore be used, i.e. equation (3)

For Sorenson, pH is the logarithm of hydrogen ion concentration times the reciprocal.


you can write pH as

pH = log 1 -------- (4)

Putting [H3O+] Equation (4) in another way:

pH = log 1- log[H3O+] ----------------(5)

Log 1 being zero, the equation (5) becomes:

pH = -log[H30+] ------------------(6)

The pH value is therefore defined as the negative logarithm of the hydrogen ion concentration. Sorensen created the term pH, which stands for hydrogen ion potential. Negative logarithms are expressed by P. Molecules/liter and moles/liter are the units of concentration for H30+.

In pH, why is H capitalized and p is small?

In Danish, pH is "potenz," which means "power." Sorensen, the Danish Chemist, compiled the scale. H is the symbol for Hydrogen. Hence, the capital letter H is used.

Sorensen pH scale

A system of grading pH levels and different concentrations of H30+ ions have been devised and named after Sorenson, who was the original developer. Having a pH value of zero indicates a hydrogen ion concentration of 1(or 100). pH levels of this level are indicative of highly acidic solutions. This indicates strong alkaline properties. pH 14 means hydrogen ion concentration is between 10-14, i.e., the solution has a high pH. As [H3O+] is equal to [OH], the central point pH in the scale is 7.0, meaning there is a concentration of 10-7 hydrogen ions.

It represents neutrality as 7. Generally, a pH value under 7 indicates an acidic region, and a pH value above 7.0 indicates an alkaline area. By measuring the pH levels of colloidal and non-aqueous solutions, the activity scale has little correlation.

Applications of pH

  • Enhancing purity
  • Enhancing stability
  • Increasing solubility
  • Maximizing biological activity
  • Improving the body's comfort
  • Storing products

pH determination


Scope and application

  • In this method, the pH of wastes containing aqueous phases and those containing multiphase wastes that have 20% or more aqueous phases is determined.
  • Neither the corrosion nor the corrosivity of concentrated acidic or basic solutions, or those that are mixed with inert substances, can be measured. This is due to the requirements of measuring pH.
It can be measured electrically by using either an electrode that uses a glass electrode associated with a reference potential or by using an electrode/reference potential combination. An array of standard solutions with known pH values serves as calibration standards for the measuring device.

Glass electrodes are not susceptible to solution interference in terms of color, turbidity, colloidal matter, oxidants, reductants, or moderate salinity (<0.1 molar solution). By using electrodes that experience low sodium errors at pH levels above 10, sodium errors can be decreased or eliminated.

The electrode response can be affected if it is coated with oily or particulate matter. In most cases, these coatings can be removed by gently wiping them away or by washing them with detergent and then rinsing them with distilled water. For any remaining film, further treatment with hydrochloric acid (1:10) may be necessary.

Temperature influences pH measurement by electrometric means in two ways. Firstly, they are due to the change in output of electrodes as a function of time and temperature. Instruments that compensate for temperature differences or calibrate the electrode-instrument system to match the samples' temperature would minimize this interference. Another consequence of changing temperature changes in pH occurs when the sample is changed. Temperature effects on the pH are entirely dependent upon the sample. As a result, both pH and temperature should be reported when performing an analysis.

Apparatus and materials needed

  • pH meter - The pH meter is available in a laboratory model or field model. Commercially available instruments come with a wide range of specifications, as well as optional equipment.
  • Glass electrode
  • Reference electrode - Alternatively, a silver-silver chloride or another constant potential electrode can be used as a reference electrode.
  • This stirrer has a Teflon-coated rod and magnetic stirring mechanism.
  • For automatic temperature compensation, a thermometer with an integrated temperature sensor is used.
Note - It is convenient to use combination electrodes that can measure and reference a signal, and are usually filled with gel-like solid materials that are easy to maintain.


All tests must be conducted with reagent-grade chemicals. As a general rule, the Committee on Analytical Reagents of the American Chemical Society, where the specifications exist, are to be followed in the case of all reagents, except in certain rare cases. If another grade of reagent is available, it must first be determined that it is of high purity sufficient to be used without reducing the accuracy of the determination.

For situations requiring extraordinary accuracy, the National Institute of Standards and Technology (NIST) offers the primary standard buffer salt. To prepare references from these salts, different handling procedures and precautions are required. For instance, dilution water that has a low conductivity, drying ovens, and carbon dioxide-free purges should be utilized. Replacement of these solutions is recommended at least once a month.

In addition to NIST salts and commercial buffers, secondary standards can be prepared using commercial solutions. A comparison of these commercially available solutions with NIST standards validates their effectiveness and recommends using them routinely.


Having so many types of pH meters and accessories to choose from makes it impossible to include detailed operating procedures in this method. Analysts must be proficient in the operation of all systems as well as knowledgeable about all instrument functions. Maintaining electrodes should be done with special care.

Each instrument or electrode system should have been calibrated at a minimum of two points three or more pH units apart and at a pH level at least two pH units below the samples' expected pH. If the acidic wastes have pH levels above 10.0, then the pH meter calibration should include a buffer of pH 2 and a buffer of pH 12). If acidic wastes have pH levels above 11.5, then the pH meter calibration should be 25 ± 1 EC. Instrument designs may include dials or slope adjustments (to "balance" or "standardize" the sample).

Within 0.05 pH units for successive portions of both buffer solutions, the readings should be within the buffer solution value.

Fill the clean glass beaker with the sample or buffer solution so that it covers the electrode sensing elements and is large enough to allow the magnetic stirring bar to move freely. It is possible to submerge the electrodes directly in the sample stream to a sufficient depth and move them in a manner that ensures sufficient sample movement across the electrode-sensing element so that errors in the measurements are minimized (<0.1 pH).

Temperature differences between the sample and buffer solution exceeding two degrees Celsius must be corrected when measuring pH. When instruments are fitted with automatic or manual temperature compensation, the compensation is adjusted electronically.

After cleaning the electrodes thoroughly, gently wipe them before attempting to measure pH. Gently stir the sample stream or sample beaker at a constant rate to keep the electrodes submerged and provide homogeneity. Take note of the pH and temperature of the sample. Analyze successive aliquots of the sample until the differences from each other do not exceed <0.1 pH units. The volume should be changed two or three times.



pH colorimetric reagent for use with the Arena or Gallery automated Thermo Scientific analyzers for determining pH inhomogeneous liquid samples.


An aqueous pH indicator dye was used in the colorimetric test. A 575 nm filter is used with a side wavelength of 700 nm with the method carried out at 37 °C.


A 575 nm filter is used with a side wavelength of 700 nm with the method carried out at 37 °C. As a result of the color change, an absorbance change can be measured spectrophotometrically.


Preservatives may include sodium azide (<0.1%). It should not be swallowed. Do not contact skin or mucous membranes with the solution. Precautions should be taken when handling laboratory reagents.


Ready-to-use reagent.
Note: Check that the vials do not contain any bubbles when you insert them into the analyzer.

Storage and stability

As long as the expiration date is printed on the label, unopened vials of reagents are stable at 2...8 °C. Expose the reagents to light or do not freeze them. The on-board stability of reagents should be determined by consulting your analyzer's application notes.


Sample type
It is intended mostly for wort and beer samples. Sample types other than beer and wort require user validation.

A sample application for Arenas/Galleries
There is a separate application note for details related to the method. The pH test does not support automatically diluted samples or calibrators.

Quality control
Quality controls should be performed at least once every day, every time a new bottle of reagent is used, and after each calibration. A minimum of two separate controls should be used. It is important to adjust control intervals and limits according to the laboratory's needs. As long as the quality control sample(s) falls within the laboratory's test limits, it should be accepted.

The colorimetric analysis is a way of determining the concentration of chemical elements and chemical compounds in a solution by using color reagents. Using it with or without an enzymatic enzyme stage is possible for organic compounds as well as for inorganic compounds. The method is widely used in laboratory environments as well as for industrial purposes, e.g., the selection of water samples for treatment as part of industrial water treatment.
Get subject wise pdf documentsView Here

Ankur Choudhary is India's first professional pharmaceutical blogger, author and founder of Pharmaceutical Guidelines, a widely-read pharmaceutical blog since 2008. Sign-up for the free email updates for your daily dose of pharmaceutical tips.
.moc.enilediugamrahp@ofni :liamENeed Help: Ask Question

No comments: Read Comment Policy ▼

Post a Comment

Please don't spam. Comments having links would not be published.

Popular Categories

QA SOPs QC SOPs Micro SOPs HVAC Production SOPs Stores SOPs Checklists Maintenance SOPs HPLC Sterile GLP Validation Protocols Water System GDP Regulatory Maintenance Calibration Warning Letters Education B.Pharmacy
Download COVID-19 SOP for Home

Follow Pharmaguideline



Editable Pharmaceutical Documents in MS-Word Format. Ready to use SOPs, Protocols, Master Plans, Manuals and more...




Pharmaceutical Updates

✔ Worldwide Regulatory Updates
✔ Pharmaceutical News Updates
✔ Interview Questions and Answers
✔ All Guidelines in One Place


Recent Posts