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pH Measurement of Organic Solvents

Need Precise pH of Organic Solvents? Discover the Tools and Methods to Measure Confidently

Home Expertise Library pH Measurement of Organic Solvents

Need Precise pH of Organic Solvents? Discover the Tools and Methods to Measure Confidently

If you work with chemicals, pharmaceuticals, food and beverages, or paints, you're likely familiar with the complexities of measuring pH of organic solvents.

Non-aqueous solvents alter the activity of hydrogen ions and affect the pH electrode, causing shifts in the liquid junction potential. The result is drift in response time and readings that are inconsistent and difficult to reproduce. The KCl electrolyte in standard electrodes may not mix well with the sample, and if it crystallizes, it can clog the junction and disrupt electrolyte flow, adding further instability to the readings. Selecting the right sensor is therefore essential for reliable pH measurement of organic solvents.

This white paper examines these challenges in detail. It provides guidance on achieving accurate results for both water-miscible and non-water-miscible organic solvents. It also explores best practices that help keep your results reliable and extend sensor life.

Click the link below to access the PDF version of the white paper.

Laboratorist in Lab

Why Non-Aqueous Solvents Need a Different Measurement Approach

Non-aqueous samples behave differently from aqueous ones, and that difference starts at the molecular level. Water dissociates according to a known equilibrium constant (Kw = 10⁻¹⁴), which forms the basis of the standard pH scale. This is one of the primary reasons why the question of whether you can measure pH of organic solvent systems requires a different technical approach than conventional aqueous pH measurement.

Closeup of the InLab™

This distinction becomes clearer when solvents are grouped by their relationship with water. Water-miscible solvents blend with water to form a single, homogeneous solution. Water-immiscible solvents, on the other hand, separate into distinct layers when combined with water. Each behaves differently during measurement, and each calls for its own handling approach, which is why this study addresses them separately.

To address these challenges, specialized sensors designed for non-aqueous applications are recommended. Sensors such as the InLab™ Science Pro-ISM and InLab™ Science Pro-DES by METTLER TOLEDO are designed to support reliable pH measurements in organic solvents by helping maintain stable performance across a wide range of water-miscible and water-immiscible samples.

The following sections examine measurement considerations and experimental results obtained with the InLab™ Science Pro-ISM sensor.

Left figure: Closeup of the InLab™ Science Pro-ISM sensor.

How the pH of Organic Solvents Was Measured

Application InLab Science Pro-DES 1 Coloured samples Emulsion Chemicals and Cosmetics

Treating Water-Miscible and Water-Immiscible Solvents Differently

To study pH behavior in these samples, measurements were divided into two sections: water-miscible and water-immiscible organic solvents. Water-miscible solvents form homogeneous solutions with water, so their pH is measured as a mixture of aqueous and organic content. In the case of immiscible solvents, the organic layer separates from the aqueous phase, with the density of the solvent determining the order of the layers. This is why the two require a different measurement approach.

Various experiments were carried out in each section to study the trend of pH values in organic solvents. Each behavior is discussed individually, focusing on the factors that affect the precision of pH measurement in organic solvents in these samples. The advantages offered by the METTLER TOLEDO specialist sensor in measuring the pH of these samples are demonstrated in the sections that follow.

Outline of the Topics Covered in This Study

Outline of the topics covered in this study
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Chemicals and Reagents Used in the Experiment

Organic solvents, namely Methanol, Ethanol, IPA, DMSO and Acetonitrile were used as a representative to study water-miscible solvents. Chloroform, Toluene, Hexane were used for the study of water-immiscible solvents. 

Typical pharma and industrial samples of non-aqueous nature were used for the pH measurement studies. All chemicals used were of Analytical grade from MERCK (purity above 99.5%). 

METTLER TOLEDO technical buffers were used for the calibration and experimental studies of the pH electrode. Electrolyte filling solution 3 mol/L KCl (P/N: 51343180), 1 mol/L LiCl in ethanol (P/N: 51350088) were used as electrolytes for the pH electrode. 

InLab Science Pro-ISM pH Sensor for Measurement of Non-Aqueous Samples

Outline of the topics covered in this study
Set of electrolyte filling solution of 1mol/L LiCl in ethanol

Instrumentation and Measurement Setup Followed

SevenExcellence S400 meter was used for all the pH measurements. The data collection and analysis were processed using LabX®. 

The pH of all the samples in this study was measured using a specialist pH electrode InLab Science Pro-ISM (P/N: 51344072). The "Pro" designation indicates an integrated temperature sensor, while ISM™ (Intelligent Sensor Management) technology enables the storage and transfer of calibration history and sensor data directly to the instrument.

The sensor features an original ARGENTHAL™/KCl reference system and a double electrolyte chamber design. This configuration allows the outer bridge electrolyte to be changed from a KCl/water system to a LiCl/ethanol system, improving compatibility with non-aqueous sample matrices. The ability to adapt to the electrolyte system helps address common challenges associated with measuring pH of organic solvents. Additional details are provided in the Specialist Sensor section.

All experiments were conducted under laboratory conditions at an ambient temperature of 25 ± 2 °C.

Right figure: Set of electrolyte filling solution of 1mol/L LiCl in ethanol

Assessing the Performance in Water-Miscible Organic Solvents

Understanding the pH Value Shift in Solvent System

Understanding the pH Value Shift in Solvent System

The conventional pH scale of 0 to 14 is based on the dissociation behavior of water. In case of samples dis solved in organic solvent, there is a change in the dissociation equilibrium (depending on the ion product of the solvent involved) that results in completely different concentration ranges for H+ ions compared to the ionic product of water. Hence, the pH value recorded in water as solvent is different compared to the pH value in organic solvents. 

A simple experiment, taking the conventional '3 mol/L KCl in water' as reference electrolyte, is performed below to observe the shift in pH values when the solvent is changed from water to ethanol. The experiment was carried out using pH buffers. Buffers are composed of a mixture of a weak acid/base and its corresponding conjugate base/acid salt and have the ability to maintain their pH despite addition of a certain quantity of acid or base, depending on their buffer capacity. 

Therefore, if water is added to buffer solutions, no major shift in pH values is expected. The effect of adding ethanol, a miscible organic solvent, to the same buffers was studied. Fixed proportions of this solvent were added to the buffers and the corresponding changes in pH values were noted and compared with the same buffer modified with addition of water. In this case, addition of ethanol leads to increase in the pH values of these buffers. The graph plot indicates the variation in the pH values due to presence of this organic phase.

The deviation from the original pH value is clearly visible from the graph. While the equivalent proportion of water does not increase the pH value above 0.1 pH, the presence of the organic phase increases the pH value up to 2.1 pH units. This is because the dissociation of the hydrogen ions from the buffer in water is different from the dissociation in ethanol. The dissociation constant of water (Kw) is 10−14 at 25°C whereas for ethanol, it is different. 

This graph illustrates pH ranges for some common solvents. For a mixed aqueous/ water-miscible solvent systems, the effects are intermediate and highly non-linear and are better explained in the literature. The pH is relative in such mixed systems and serves as an indicator of the pH value. Only purely aqueous samples would have absolute pH values.

Outline of the topics covered in this study

Left figure: Variation in pH value of buffer (pH 4.01 and pH 7.00) in addition of water/ethanol.

Right figure: pH value in different solvents.

How to Measure pH in Organic (Water-Miscible) Solvents

Water-miscible organic solvents form a single phase with the aqueous system, but the presence of inorganic salts can lead to turbidity and precipitation in the mixture (especially if the concentration of the organic solvent is more than 80%). 

A direct effect of this is noticed when a drop of 3 mol/L KCl solution, which is a conventional reference electrolyte, is added to the organic solvents. Solvents like ethanol, isopropanol, acetonitrile and DMSO, if present above 80%, precipitate out the KCl salt, because KCl is not as soluble in these solvents as it is in water. 

In such a situation, the KCl electrolyte outflowing from the sleeve junction of the electrode precipitates and clogs the sensor junction. Hence, pH measurement in organic solvents using 3 mol/L KCl as the electrolyte is not advisable. 1 mol/L LiCl in ethanol solution can be used as the reference electrolyte in such cases. Indeed, LiCl has a much better solubility in ethanol than KCl (5.84 mol · kg−1 of solvent versus 0.0064 mol · kg−1 at 25 °C for KCl), while remaining perfectly soluble in water. 

Additionally, the concentration is kept low at 1 mol/L to further improve solubility. The solvent medium for this electrolyte being ethanol helps miscibility in water-miscible organic solvents. The InLab Science Pro-ISM electrode enables the outer bridge electrolyte to 1 mol/L LiCl in ethanol for ensuring reliable pH measurement of non-aqueous samples in water miscible organic solvent. 

Unlike KCl/water reference electrolyte, LiCl/ethanol fill solution is miscible in organic solvents in all proportions. This provides a stable junction potential during pH measurement, resulting in decreased response time and improved precision when compared with a conventional pH electrode. 

The absolute pH values measured using KCl/water system would be different from LiCl/ethanol system due to the change in the overall reference system.

Typical Examples for pH of Samples in Organic (Water-Miscible) Solvents

As a guiding principle, the analyst can check the solubility of 3 mol/L KCl/water electrolyte in the non-aqueous sample solution. To do this, take a few ml of the non-aqueous sample and add a few drops of the KCl electrolyte in it. If no precipitation or turbidity arises, the KCl reference electrolyte can be used for the pH measurement. 

However, if turbidity is seen, switching the electrolyte to 1 mol/L LiCl in ethanol is recommended. The sample nature can directly influence the response time of the pH electrode, and the pH value keeps drifting in some cases. Allowing ample time to stabilize the pH reading is a best practice to get consistent results in aqueous samples. 

That said, in non-aqueous samples such as alcohol-based samples or ethanol fuel and fuel blends, ASTM D6423 recommends that the pH measurement is done at a timed endpoint of 30 seconds, upon immersion of the pH electrode in the sample.  

pH measurement of two common pharmaceutical samples was carried out using both conventional 3M KCl water and 1M LiCl-ethanol reference electrolyte. The samples contained relatively low amounts of organic solvent and were fully miscible with both electrolyte systems. As a result, measurements could be successfully performed using either reference electrolyte. All measurements were carried out in triplicate to assess repeatability and measurement precision.

Sample (~Alcohol %)3M KCl-waterStd Dev (n=3)

pHStd Dev (n=3)Avg. Response TimepHStd Dev (n=3)Avg. Response Time
Elixir (6%)3.5700.0313 s3.7010.0188 s
Listerine (26%)4.3500.01717 s4.2150.0087 s

Above figure: pH measurement of samples dissolved in miscible organic solvents.


From the above data, one can observe that the use of 1M LiCl-ethanol electrolyte helped in improving the precision of the measurement and the response time. We note change in the absolute pH values recorded for the samples using different electrolytes and are accounted to the change of reference system in either case. For a sample with increased percent of organic (water miscible) solvent, the use of 1M LiCl-ethanol electrolyte becomes necessary. 

Performance in Water-Immiscible Organic Solvents

Water-immiscible organic solvents such as hexane, chloroform, and toluene present unique challenges for pH measurement. Unlike aqueous solutions, these solvents exhibit very different acid-base behavior and the lack of ions, make direct pH determination with conventional glass electrodes difficult.

When a water-immiscible organic solvent is mixed with water, two separate phases form and reach equilibrium. Under these conditions, dissolved species can establish the same hydrogen ion activity in both layers. As a result, the extracted aqueous phase can be used to estimate the acidity of the organic sample. In principle, the pH measured in the water layer serves as an indirect reflection of the acidity of the organic phase.

However, this approach has limitations. Some non-polar substances remain dissolved exclusively in the organic phase and cannot establish an equilibrium with the aqueous layer because of their low water solubility. This leads to the development of a potential at the interface of the layers. The measurement by this technique gives us a relative pH that remains constant for a fixed concentration of the dissolved species and correlates with its increase or decrease in the organic solvent under similar conditions.

How to Measure pH in Water-Immiscible Organic Solvents

The following procedure is recommended to measure pH of samples dissolved in water-immiscible organic solvents:

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Take sample/ water-immiscible organic solvents and deionized water of controlled pH in a separating funnel and mix vigorously. The ratio of sample to water can be 1:1 or as per specified method. 

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Allow the mixture to stand until the aqueous and organic phases have completely separated.


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Collect the aqueous phase and measure its pH using standard aqueous measurement practices.

For this type of measurement, specialized sensors such as the InLab™ Science Pro-ISM and InLab™ Science Pro-DES can be used with a 3 mol/L KCl/water outer bridge electrolyte when measuring the extracted aqueous phase.

Why Direct Measurement Is Generally Not Recommended

Direct pH measurement in the organic phase is generally discouraged for two key reasons:

Risk of sensor fouling and dehydration: Glass pH membranes require hydration to function correctly. Prolonged exposure to water-immiscible organic solvents can dehydrate the membrane surface, leading to unstable or inaccurate readings.

Reduced measurement quality: Direct measurements often exhibit longer response times and greater measurement variability compared with measurements performed in the extracted aqueous phase.

When Direct Measurement Cannot Be Avoided

If direct measurement in a water-immiscible organic solvent is required, specialized sensors such as the InLab™ Science Pro-ISM and InLab™ Science Pro-DES can be configured with 1 mol/L LiCl in ethanol as the outer bridge electrolyte. Because ethanol is compatible with organic solvent systems, this electrolyte minimizes solvent incompatibility issues that may occur with conventional 3 mol/L KCl/water electrolytes.

After measurement, rinse the sensor thoroughly with an appropriate water-miscible organic solvent, followed by deionized water, to help remove sample residues and maintain sensor performance.

pH measurements in pure water-immiscible organic solvents should always be performed with caution. Improper handling or prolonged exposure to such media can promote sensor fouling, compromise measurement quality, and potentially reduce sensor lifetime.

Typical Examples for pH of Samples in Water-Immiscible Solvents

Samples dissolved in such organic solvents are very often used in industrial applications. pH measurement of these samples is required in paints, plastics, coating and life science industries. 

One such example is the purification and extraction of nucleic acids–DNA and RNA in life science industry, using a phenol-chloroform mixture. Herein, the mixture is extracted with an equal amount of aqueous phase. The acidic pH of the mixture ensures that only RNA is precipitated into the aqueous phase whereas DNA remains at the interface with the organic phase.

Nucleic Acid Extraction Using Phenol-Chloroform Mixture

Outline of the topics covered in this study

Above figure: pH driven extraction technique for nucleic acid.


For samples that are dissolved in immiscible organic solvents, an extraction into an aqueous phase and subsequent measurement of their relative pH values is recommended. The concentration extracted in the aqueous phase depends on the relative solubility of the sample in this phase and on the partition coefficient in the respective solvent phases. 

Hence, the absolute pH values, obtained by dissolving the same concentration in water, could be different from the extracted aqueous phase. However, the water extraction technique gives reproducible pH values that are representative of the sample-organic solvent system without risking the electrode life. 

Some experiments were carried out on few non-aqueous samples, wherein, the pH of samples when dissolved directly in water were compared to pH values when they were dissolved in water-immiscible organic solvents. The pH of the samples dissolved in water was measured directly in the beaker. For samples dissolved in organic solvents, the extracting technique, consisting of vigorously mixing deionized water in a separating funnel, was used. 

On resting, the water layer was extracted, and the pH was measured normally, using InLab Science Pro ISM pH sensor with 3 mol/L KCl/water as outer-bridge electrolyte.

pH Measurement of Non-Aqueous Samples Dissolved in Different Solvents

SampleWater (pH)Water Layer Extracted from


Chloroform (pH)Toluene (pH)Hexane (pH)
Acrylic acid (10% v/v)2.10 ± 0.012.25 ± 0.022.16 ± 0.012.11 ± 0.01
Phenol (6% w/v)5.53 ± 0.035.46 ± 0.025.73 ± 0.016.02 ± 0.01
Aniline (10% v/v)8.36 ± 0.017.22 ± 0.017.32 ± 0.017.88 ± 0.01
Diisopropylamine (10% v/v)12.51 ± 0.0211.48 ± 0.0111.91 ± 0.0112.04 ± 0.01

The samples were analyzed in triplicates. Across different organic solvents, variance in pH values is observed, depending on the dissociation and partitioning in respective solvents. Nevertheless, the relative pH values adequately reflected the pH in organic solvents and were found to be reproducible.

The Advantage of Choosing a Specialist Sensor for Non-Aqueous pH Measurements

Specialized sensors are essential for achieving reliable pH measurements in non-aqueous samples. Sensors such as the InLab™ Science Pro-ISM and InLab™ Science Pro-DES are designed to address the unique challenges associated with measuring pH in organic solvents, including electrolyte compatibility, junction stability, and sensor durability. 

The sensor features a chemically resistant glass shaft and an A41 glass pH-sensitive membrane, providing excellent chemical resistance and thermal stability at temperatures up to 130 °C. A movable sleeve junction promotes consistent electrolyte outflow into the sample and offers a practical advantage over conventional ceramic junctions. The sleeve can also be cleaned easily if precipitation forms around the junction during measurement. 

Right Figure: Application of the InLab Science Pro-DES sensor in complex media such as paints.

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A key advantage of this sensor design is its double electrolyte system, which allows the outer bridge electrolyte to be adapted to the application. For conventional aqueous measurements, 3 mol/L KCl is typically used. For many non-aqueous applications, the outer bridge electrolyte can be changed to 1 mol/L LiCl in ethanol, improving compatibility with organic solvents and helping to enhance measurement stability, response time, and precision.

Kindly note that the inner electrolyte is 3 mol/L KCl solution for this electrode and should not be changed, since the main reference system should remain Ag | AgCl | 3M KCl. 

A slope of 95-105% ensures reliable measurements. By contrast, offsets are typically shifted to 40 mV or more when switching to LiCl/Ethanol electrolyte. As long as the slope is in the expected range, this is no reason for concern, and the sensor can be used.

Calibration and Maintenance Considerations

Calibration values obtained with a LiCl/ethanol bridge electrolyte may differ from those obtained with a conventional KCl/water electrolyte because the overall reference system changes. This is expected behavior, provided calibration results remain within the recommended operating range. 

If they do not, refer to our pH Sensor Troubleshooting White Paper, to receive tips and hints on how to maintain your electrode.

Residual KCl crystals within the sensor can interfere with measurements, potentially causing elevated offset values, slower response, and reduced measurement precision. Careful rinsing during electrolyte changes helps prevent this issue.

Ethanol in the outer bridge electrolyte can dehydrate the pH sensitive membrane; hence always keep the pH sensitive membrane immersed in InLab storage solution for rehydration when not in use. During storage, the sleeve junction should not be immersed in the storage medium, as contact between KCl-containing storage solution and ethanol from the outer reference chamber may lead to precipitation at the junction. 

For long-term storing in the wetting cap or in storage solution, the outer electrolyte system should be changed back to the original 3 mol/L KCl electrolyte. This helps prevent KCl diffusion and crystal formation within the sensor, reducing the risk of junction blockage, offset errors, and measurement instability over time.

Reference Junction Clogging Caused by Improper Refilling of Organic Solvent Electrolyte

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Figure above: Picture A shows a regular InLab Science Pro-ISM sensor. Picture B and C are the magnified image of the circled section. There is no clogging in B whereas C shows clogging and precipitation due to white KCl crystals. Improper refilling of outer reference electrolyte with 1 mol/L LiCl-ethanol leads to such problems

Best Practices for Reliable Results

To make the most of your pH measurement in organic solvents, follow these essential tips and best practices.

  • Ensure the use of fresh buffers for electrode calibration.
  • Make sure that the SafeLock (electrolyte filling hole) is open during pH calibration and measurement. For specialized sensors such as the InLab™ Science Pro-ISM and InLab™ Science Pro-DES, this guidance applies to both the inner and outer electrolyte chambers where applicable.

  • As a general guideline, additional care should be taken when measuring organic solvent systems containing more than 80% organic solvent. When measuring water-immiscible organic solvents, first rinse the sensor with a water-miscible solvent such as ethanol or isopropyl alcohol, and then rinse with water. This helps remove solvent residues more effectively and protects the sensor from contamination.
  • Thoroughly rinse the sensor after pH measurements in non-aqueous samples to prevent residue buildup and cross-contamination between measurements.

  • After exposure to organic solvents, allow the sensor to rehydrate by soaking it in an appropriate aqueous storage solution, such as InLab™ Storage Solution. Rehydration helps maintain the performance of the pH-sensitive glass membrane and supports reliable measurements.

  • Between measurements, or whenever the electrode is not in use, keep only the electrode tip (the pH-sensitive membrane) immersed in the storage solution. This helps recondition the glass membrane by enriching it with ions. Never store the sensor in ion-poor pure water.

  • Routine maintenance is essential for maximizing sensor performance and service life. Regularly inspect the sensor and replenish electrolyte solutions as required to ensure optimal performance.

To learn more, watch the InLab Science Pro-DES video: Sensor Handling for Optimal Sensor Performance

A Simplified Workflow for Selecting the Right Approach to pH Measurement in Organic Solvents

A simplified workflow for selecting the right approach to pH measurement in organic solvents

Key Takeaways

Obtaining reliable pH measurements in organic solvents requires the right combination of measurement technique, reference electrolyte, and sensor design. Since water-miscible and water-immiscible solvents behave differently, the measurement approach should be selected based on the sample composition.

Specialized sensors such as the InLab™ Science Pro-ISM and InLab™ Science Pro-DES are designed to support these demanding applications. When used with the appropriate electrolyte system and measurement workflow, they help achieve reliable and reproducible results across a wide range of organic solvent samples.

The recommended measurement practices outlined in this guide support accurate and reproducible readings for calculating pH of organic solvents for research, quality control, and industrial applications.

Why is measuring pH in organic solvents challenging?

The traditional pH scale of 0 to 14 does not apply when measuring pH of organic solvents due to variations in how samples dissociate in different solvents. Organic solvents are often ion-deficient, leading to unstable measurements that require longer stabilization times. The aqueous outer electrolyte (3M KCl) is not suitable for use in an organic medium and must be replaced with an ethanolic solution of LiCl to obtain stable readings. Such samples require additional understanding to perform the pH measurement and sensor selection is key to obtain results with improved accuracy.

What is the InLab Science Pro-ISM pH sensor, and why is it suitable for measuring pH in organic solvents?

The InLab™ Science Pro-ISM pH sensor by METTLER TOLEDO is specifically designed for non-aqueous applications. The movable sleeve junction ensures easy and adequate outflow of reference electrolyte into the sample and is easy to clean in case of any blockage. This sensor has the added advantage of using two electrolytes, of which the outer bridge electrolyte can be changed from a conventional 3 mol/L KCl solution to 1 mol/L LiCl in an ethanol solution or any other electrolyte as needed. These factors contribute to obtaining stable readings and giving reliable pH results.

To learn more about the advantages of using this sensor, read this white paper.

Where is the pH of organic solvents measured and why is it important?

Several industries, including paints and pigments, inks, oils, petroleum and petrochemicals, pharmaceuticals, and biofuels, require routine pH measurements of samples that are dispensed or dissolved in organic solvents. Accurate pH measurement is crucial for maintaining product quality and compliance with industry standards.

How can I measure the pH of organic solvents?

pH measurements in organic solvents can be categorized into two sections: Organic solvents are classified as water-miscible and water-immiscible solvents. Therefore, pH measurements in non-aqueous samples can be categorized into two sections: 1. The pH measurement of water-miscible solvents, which is conducted by immersing the electrode in the solvent or its mixture (with water) 2. The pH measurement of water-immiscible organic solvents, which is conducted by performing a solvent extraction of the sample in the aqueous phase with deionized water, followed by the pH measurement

For further details, download our white paper on pH measurement in organic solvents.

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