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FREE WEBINAR  - Intro to ASTM D974 by Automatic Titration for Low Range Acid Number. Thursday, April 29 @ 2pm.

Sunlight on limestone rocks under water in Lake Michigan

What Does Water Hardness Mean Anyways?

Water hardness refers to the amount of dissolved minerals in water and largely results from calcium and magnesium. Based on the concentration of these dissolved minerals, water may be classified as “soft” or as varying degrees of “hard” (Figure 1), where such minerals are inherent to the source water due to geographical location and makeup of the land (Figure 2).

Figure 1. Concentration ranges of water hardness in milligrams per liter. [1]
Classification CaCO3 (mg/L)
Soft ≤ 60
Moderately Hard 61 - 120
Hard 121 - 180
Very Hard > 180


Figure 2. Map of mean water hardness in the United States.[2]


How Do I Know If I Have Hard Water?

If hardness levels are high enough to deem water “hard,” you may feel the effects when using soap. The high levels of calcium and other dissolved minerals present in hard water react with soap, producing fewer suds, reducing the overall effectiveness of the cleaning agent, and eventually forming deposits known as “soap scum.” If you’ve ever done a load of dishes and noticed the presence of spots and/or films after washing and drying, you understand the aesthetic consequences hard water can have on the cleaning process. When hard water is heated, however, these dissolved minerals are more prone to precipitate out. The most common offender is calcium carbonate (CaCO3), which has the potential to cause costly mechanical problems for both industrial and domestic water users. These deposits are often termed “scale;” scale can raise the costs of heating water and reduce the efficiency and life of electric water heaters and other equipment. CaCO3 can also build up in pipes, reducing pressure and causing clogs. As such, the measurement of water hardness is commonly tested in water treatment and water-based industrial applications, such as drinking water, wastewater, and boiling and cooling towers.

Calcium and Magnesium Are Key Players

When testing for water hardness, calcium and magnesium are the two constituents used to determine the total hardness of water since both are the major contributors. Results for hardness are expressed as the milligrams of CaCO3 equivalent to the total amount of calcium and magnesium present in one liter of water, or mg/L CaCO3.

How Do I Test Water For Hardness?

Titration is the gold standard when measuring water hardness, where calcium and magnesium form complexes with the titrant, EDTA. When performing this titration with an automatic potentiometric titrator, there are two different means of monitoring this reaction: (1) with a calcium ion-selective electrode (ISE), or (2) a photometric electrode. The use of one versus the other is based on overall measurement objective and user preference, with both methods having pros and cons (Figure 3). We will detail the differences between the two procedures below in order to assist you in determining which method is more appropriate for your application.

Figure 3. Pros and cons of hardness determination with an ISE and photometric electrode.[3]
Pros Calcium ISE Photometric Electrode
Determine calcium and magnesium in one titration (one sample). Minimal electrode maintenance required.
Minimal changes to manual titration procedure.



Significantly more electrode maintenance required. Requires two separate titrations (two samples) to determine calcium and magnesium hardness.
Higher startup and recurring costs (ISE, modules, and required chemicals).

Calcium ISE Method

HI4104_2016-720x720-4820e198-9002-4c1b-a69c-c17a873df3bbIn the presence of TRIS buffer, the calcium ISE can be used to detect both calcium and magnesium in one titration, where each ion is differentiated and displayed as their own specific equivalence point (Figure 4). Upon completion of the titration, results are automatically calculated for total hardness, calcium hardness, and magnesium hardness. This allows for one sample to be used for all three analytes, benefiting users who have a high throughput of samples. However, this method requires slightly higher startup and recurring costs for necessary materials and consumables; a calcium ISE averages between $800 to $1,000. Furthermore, the ISE requires more time for electrode preparation and maintenance, rendering a steeper learning curve for those unfamiliar with ion-selective electrodes.

Figure 4. Two equivalence point (EQPT) titration via Calcium ISE method, where EQPT 1 (7.528 mL) refers to calcium hardness and EQPT 2 (1.682 mL) refers to magnesium hardness.


Interested in the Calcium ISE? Click Here

Photometric Electrode Method

HI90060x-Wave-GroupAs dictated by Standard Methods for the Examination of Water and Wastewater[3], total hardness and calcium hardness can be determined with a color indicator, where magnesium hardness is calculated by deduction as seen in (1):

Magnesium hardness = Total hardness (Titration #1) – Calcium hardness (Titration #2) (1)

In the presence of an indicating dye, the testing solution will change color, signaling the end of the titration. Indicating dyes are used when manually titrating for hardness and can also be used in automatic titration with use of a photometric electrode. Where manual titrations are often affected by human error (due to subjective determination of the color end point), automatic titrations omit this subjectivity by use of an electrode to detect the color change.

With the release of Hanna Instruments’ application-based photometric electrodes, this alternative detection of color indicators is now more accessible and affordable than ever. By creating four different photometric electrodes at varying wavelengths, Hanna Instruments has significantly reduced the cost of photometric determination, permitting purchase of a singular wavelength at roughly $400 as opposed to an unnecessary multiwavelength-based electrode, costing upwards of $3,000. Photometric electrodes require minimal maintenance and significantly lower recurring costs where, for customers currently testing via manual titration, current procedures and chemicals can be seamlessly transferred to automation. However, to determine all hardness constituents with this methodology, two separate titrations (and therefore two separate samples) are required. For some, this may prove to be problematic for high throughput processing or when there are limitations with low sample volume.

Take a Look at the 525nm Photometric Electrode!

Figure 5. Hardness determination procedure by ISE and photometric method.
Automatic Titrator
525nm Photometric Electrode OR Calcium ISE
Titration #1 Titration #2   Titration

Total Hardness
pH 10 - 12
Color Indicator #1

Calcium Hardness
pH > 12
Color Indicator #2
  Total Hardness, Calcium Hardness, and Magnesium Hardness
pH 10 - 12
NO Color Indicator Requires
Magnesium Hardness = Total Hardness (Titr. #1) - Calcium Hardness (Titr. #2)  


A Quick Recap

To summarize, in researching options for in-house automation of water hardness, it is important to consider the means and objectives of doing so:

  1. Are you currently performing in house manual titrations and prefer to limit changes to the procedure?
  2. Do you have a high throughput (>25) of samples per day?
  3. How much time will be allotted to daily tests?
  4. Do a large majority of samples require total hardness, and calcium and magnesium hardness?
  5. What is the technical background of those operating the tests?

By answering these questions, you will be better equipped to determine which methodology is most appropriate for your application and organization.


Got Questions?

For more information regarding how Hanna Instruments can help you with your hardness titrations, contact us, as sales@hannainst.com or 1-800-426-6287.


[1] USGS (https://www.usgs.gov/special-topic/water-science-school/science/hardness-water?qt-science_center_objects=0#qt-science_center_objects)
[2] USGS (https://www.usgs.gov/media/images/map-water-hardness-united-states)
[3] Standard Methods for the Examination of Water and Wastewater Methods 2340 C. and 3500-Ca D., EDTA Titrimetric Method