pH

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What is pH?

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.

Why Monitor the pH of Water? 

pH measurement is a key step in many water testing and purification processes:

  • A change in the pH level of water can alter the behavior of chemicals in the water.
  • pH affects product quality and consumer safety. Changes in pH can alter flavor, color, shelf-life, product stability and acidity.
  • Inadequate pH of tap water can cause corrosion in the distribution system and can allow harmful heavy metals to leach out.
  • Managing industrial water pH environments helps prevent corrosion and damage to equipment.
  • In natural environments, pH can affect plants and animals.

Hach ® offers the testing equipment, resources, training and software to successfully monitor and maintain pH in a wide array of process applications.


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Featured Products for Monitoring pH

HQ Series Portable Meters

Our new portable platform will allow you to collect intuitive, accurate measurements, manage data, and easily review results, while supplying an IP67 robustness rating.

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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.

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Test Strips

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

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pH Buffers

Hach calibration pH buffers are available in a variety of color-coded or colorless solutions, in different packaging types and sizes from 15 mL up to 20 L.

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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. Normal pH range for tap water is 6.5-8.5 throughout both treatment process and distribution. Drinking water pH level must be monitored to minimize corrosion, contamination and adverse effects on taste, odor, color and appearance.

A drinking water treatment plant monitors pH in several stages of 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 - NPDES.

This equalization basin monitors pH to ensure pH levels comply with regulation. pH is essential to metal precipitation and in microbial digestion.

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.

A water boiler. pH is an important water parameter to measure in boilers because it can indicate corrosion, which damages equipment.

Methods for Monitoring pH


Hach pH sensing electrodes. Electrodes are used with a pH meter for electrochemical measurement to determine where the tested fluid falls on the pH scale.

Potentiometric Method

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

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

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


Hach Wide Range pH Indicator Solution, 100 mL MDB

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.

Advantages

This method is simple, inexpensive and portable.

Disadvantages

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.

Liquid Dyes

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

pH Indicator Solution

Paper Strips/Test Strips

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


Hach DR300 Colorimetric meter for pH measurement

Photometric Method

Like the visual method used in test strips and color wheels, 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 photometer (colorimeter or spectrophotometer) to produce a more precise pH reading.

Advantages

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

Disadvantages

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

Colorimeters Used to Test pH

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 grams of the soil sample with 25 grams 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 HQ Series ® meter and 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, as well as online sensors that can be placed in the water stream.

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. There are a variety of processes and laboratory electrochemical pH sensors, including those built on combination and differential technologies (pHD sensor). The most common and accurate pH measurement 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.