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What is pH and What Does It Measure?

pH is a measure of the hydrogen ion activity in a solution. Pure water that contains an equal balance of positive hydrogen ions (H+) and negative hydroxide ions (OH-) has a neutral pH.

  • Solutions with a higher concentration of hydrogen ions (H+) than pure water are acidic and have a pH less than 7.
  • Solutions with a higher concentration of hydroxide ions (OH-) than water are basic (alkaline) and have a pH greater than 7.

Featured Products for Monitoring pH

HQ Series/HQD Meters/Intellical Probes

The rugged and durable HQD® portable meters and laboratory meters are designed to give drinking water and wastewater professionals the greatest confidence and flexibility.

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Intellical Probes

Red Rod pH probes offer exceptional performance and response time across a wide variety of sample types in the laboratory. Rugged stainless steel outdoor probes are extremely practical for on-site analysis.

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pH & ORP Sensors

Hach offers a variety of analog and digital pH sensors and ORP sensors to meet your application needs for inline pH or ORP monitoring of water or other fluids.

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Color Disc Test Kits

Color discs use a visual comparison method (color matching) to determine concentration and are accurate, portable, and require no batteries. Simply add reagent, turn the wheel until the sample matches a reference color and concentration is displayed in a window.

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Test Strips

High-quality Hach chemistry on an easy-to-use test strip. Simply dip the strip in sample and compare with colors printed on the bottle to find a reading. Test strips are accurate, portable, and low-cost.

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pH Buffers

Hach calibration pH buffers are available in a variety of color-coded or colourless solutions, in different packaging types and sizes from 15 mL up to 20 L. pH buffers are also available in convenient single-use pouches (Singlet) and unit-dose powder pillows for on-site and field analyses ensuring freshness, eliminating the possibility of contamination.

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Which Processes Require pH Monitoring?


Drinking Water Purification

  • pH of drinking water is measured at multiple stages in a drinking water plant.

  • Optimum tap water pH level is 6.5-8.5.

  • Drinking water pH level must be monitored to minimize corrosion, contamination and adverse effects on taste/odor/color/appearance.

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Drinking Water Purification

Wastewater Treatment

  • pH adjustment is necessary for the precipitation of heavy metals in wastewater plants.

  • In biological treatment processes, pH can affect the microorganism metabolizing waste. If the pH is too high or low in aeration basins, the microorganisms lose the ability to convert waste to energy and raw materials. In nitrification basins, if the pH drops too low the nitrifying bacteria begin to inhibit the nitrifying process.

  • Anerobic digesters must maintain a balance between several microbial populations. If the pH increases outside of acceptable limits, methane production stops and the digester system will fail.

  • To comply with regulation pH must be measured in effluent. (Facilities must abide by the National Pollutant Discharge Elimination System.)

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Wastewater Treatment

Boiler Water Applications

  • Proper pH balance is critical to overall boiler chemistry at nearly every stage and to maintaining high purity water.

  • pH is an important factor in monitoring the corrosive properties of water which can damage equipment.

Boiler Water Applications

Methods for Monitoring pH


pH Method Potentiometric Method

Potentiometric Method

This method uses a pH sensing system (including a pH meter, a pH sensing electrode & a pH reference electrode) to take an electrochemical measurement to precisely determine where the tested fluid falls on the pH scale.

Potentiometric pH Equipment

Our online technology course provides more information on the importance of monitoring and measuring pH.

pH Buffer Solutions to Calibrate Equipment

Advantages: This is the most accurate method for pH measurement. Portable, lab or process/online options are available for this method. This method can be EPA approved for monitoring pH.

Disadvantages of this Method: This method requires proper handling, care and maintenance of the pH probe and involves higher instrument costs.


pH Method Photometric Method

Photometric Method:

Like the visual method, an indicator solution is used to create a color change in the solution being tested. However, with this method the color change is not assessed by the human eye, it is measured by a colorimeter or spectrophotometer to produce a more precise pH reading.

Colorimeters Used to Test pH

Advantages: This method removes the subjectivity of the human eye, it is portable, it does not require complex pH meter calibration.

Disadvantages of this Method: Solutions being tested with this method must be colorless. No online/process options exist for this method, there are costs associated with instruments. This method is not an EPA-approved method for monitoring pH.


pH Method Visual Method

Visual Method

An indicator solution changes color when added to the solution being tested. That color change is then visually compared against a color chart to determine pH.

Liquid Dyes

Liquid dyes are added directly to the solution to produce a color change that indicates pH.

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Paper Strips/Test Strips

Dye-infused paper strips are dipped into a solution and then change color to indicate pH. This method is not affected by color or turbidity in a sample.

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Advantages: This method is simple, inexpensive and portable.

Disadvantages of this Method: Solutions being tested with this method must be free of color and have no turbidity. Solutions being tested with this method must not have a secondary reaction with the indicator solution. No online or process options exist for this method. This method does not produce exact measurements.

Frequently Asked Questions

What is the best way to measure pH in soil samples?

When testing the pH of soil-based samples it is advisable to use a pH probe with a strong glass tip and high electrolyte outflow. Mix 5 g of the soil sample with 25 g of deionized water while stirring carefully. Let the mixture stand without stirring for 10 minutes to separate the particles from the liquid. Insert the pH probe in such a way that the glass bulb is totally covered by particles, but the diaphragm is not. Wait for a stable reading.


I need to measure pH and report the results to a regulatory agency. What do I need to do this?

A pH meter and electrode such as an HQD meter/Intellical probe must be used if your results need to be reported to an agency such as the USEPA. You can review the Hach USEPA pH electrode method 8156 for additional information.

pH meters will give the best accuracy and sensitivity, measure across the full pH range, and can be used with most water samples, compared to colorimetric or test strip measurements. However, they require regular calibration with pH buffer solutions. Hach has several different kits available, complete with calibration buffers, for simplified measurement solutions.


How do I measure pH in water?

pH is a critical water quality indicator. There are several common techniques available for measuring pH: titrimetric, colorimetric, litmus paper, and electrochemical. The most common and accurate pH technique is by electrochemical combination electrode. A combination pH electrode responds to the hydrogen ion (H+) concentration (activity) by developing an electrical potential across the electrode bulb/water interface. This potential difference is measured with a voltmeter. Electrodes are designed for optimal performance in various matrices. Hach HQd meters and a versatile assortment of IntelliCAL probes bring simplicity and consistency to pH measurements in any matrix. See this document for more information.