How To Measure Water Hardness

Whether the total hardness of water meets the standard is an essential reference data for tap water. It mainly describes the content of calcium and magnesium ions. There are different types of total hardness of water according to different standards. There are also other conversion units in different countries.

Water hardness is a vital monitoring index of water quality. We can identify whether it can be used in industrial production and daily life by monitoring it. For example:

• High hardness water can precipitate soap and significantly reduce the effectiveness of detergent.
• Too high hardness of the water in the textile industry makes textile fabrics rough and difficult to dye;
• It is easy to block the pipeline, resulting in a boiler explosion;
• Bad taste, which even affects gastrointestinal after drinking;
• It can cause the abortion of pregnant animals.

Therefore, we should determine the water hardness for better use. Currently, there are many chemical and instrumental analyses to measure water’s hardness.

What does the total hardness of water mean?

The total hardness of water refers to the total amount of Ca2 +, Mg2 + in water, which includes temporary and permanent hardness. When the part of Ca2 +, Mg2 + in the form of carbonate in water can be removed by heating, which is called temporary hardness; While the part in the form of sulfate, nitrate, and chloride is called permanent hardness due to it cannot be removed by heating.

Conversion unit

Conversion unit of various water quality hardness:

1) German: 1 degree is equivalent to 10mg Cao in 1L water;

2) British: 1 degree is equivalent to 10mg CaCO3 in 0.7L water;

3) French: 1 degree is equivalent to 10mg CaCO3 in 1L water;

4) American: 1 degree is equivalent to 1mg CaCO3 in 1L water.

Type of water hardness

calcium hardness and magnesium hardness.

Calcium hardness: the content of Ca2 + in water is called calcium hardness.

Magnesium hardness: the content of Mg2 + in water is called magnesium hardness.

Carbonate hardness and non-carbonate hardness.

Carbonate hardness is formed by bicarbonate of calcium and magnesium [Ca (HCO3) 2, Mg (HCO3) 2], and a small amount of carbonate hardness. Carbonate hardness is decomposed into sediment after heating and removed from the water, so it is also called temporary hardness.

Noncarbonate hardness is formed by sulfate, chloride, nitrate, and other calcium and magnesium salts such as CaSO4, MgSO4, CaCl2, MgCl2, Ca (NO3) 2, Mg (NO3) 2, etc. This kind of hardness cannot be removed by heating decomposition, which is called permanent hardness.

Total hardness refers to the sum of carbonate hardness and non-carbonate hardness.

Negative hardness: When water’s total hardness is less than the total alkalinity, the difference between them is called negative hardness.

Why measure water hardness

Water hardness detection and analysis is an essential work of water quality analysis, affecting public production and life safety. Water hardness is the main factor that forms pot scale and affects product quality. Therefore, determining the total hardness of water provides a basis for determining water quality and water treatment.

However, most of the determination methods of the total hardness are laboratory determination, less on-site online analysis. There are many methods to measure the total hardness.

How to measure water hardness?

Instrumental analysis

Spectrophotometry is used for qualitative and quantitative element analysis based on the Lambert-Beer law. The light absorption intensity value determines the concentration value of element ions.

Pros:

High sensitivity, simple and rapid operation

Cons:

It needs an appropriate chromogenic agent

ICP-AES method

ICP-AES method, namely inductively coupled plasma emission spectrometry, can analyze aqueous solution or organic solution and dissolved solid elements and detect about 72 pieces (including P, B, Si, as, and other non-metallic elements) in the sample at the same time.

Pros:

continuous single element operation

continuous multi-element operation.

Chromatographic analysis

Ion chromatography (IC) is a kind of liquid chromatography, which is a liquid chromatography method for analyzing ions. Determining hardness in water by ion chromatography can effectively avoid the interference of organic matter and do not consider the influence of magnesium ion. It can still be determined directly when the magnesium content is too low.

Pros:

less dosage, simplicity, rapidity, and accuracy.

Electrochemical analysis

Automatic potentiometric titration determines the endpoint automatically according to the titration curve. The error between the stoichiometric point and the endpoint is very small, and the accuracy is high, which avoids the mistake of chemical analysis titration.

Automatic potentiometric titration does not need an indicator to titrate colored samples, turbid samples, and samples without appropriate indicators.

Pros:

• fast and simple
• high accurate
• reliable results
• good reproducibility.
• It is suitable for measuring the content of total hardness in water.

Electrode method

An ion electrode is an indicator electrode with a preference for a specific ion—this kind of electrode with a special electrode film has a selective response to specific ions. 

The relationship between the point position of the electrode film and the content of ions to be measured conforms to the Nernst formula.

Pros:

Good selectivity, short equilibrium time, simple equipment, and convenient operation.

Chemical analysis

EDTA complexometric titration is widely used in chemical analysis to determine water’s hardness. Under certain conditions, with chrome black T as an indicator, and ammonia ammonium chloride as a buffer solution, EDTA produces a stable complex with calcium and magnesium ions to determine the total amount of calcium and magnesium in water.

However, this method is prone to errors such as controlling the number of indicators, the indicating endpoint and metering point, and the operator’s judgment of the endpoint color.

When analyzing samples, if the total alkalinity of a water sample is very high, the blue color will soon return to purplish-red after titration to the endpoint, which is caused by suspended particles of calcium and magnesium salts that affects the determination results.

We can be acidified and boiled with hydrochloric acid to remove the alkalinity in the water sample. After cooling, neutralize with sodium hydroxide solution, add buffer solution and indicator agent for titration, and the endpoint will be sharper.

Since the indicator chrome black T is easy to be oxidized, the titration should be completed as soon as possible after adding chrome black T, but it is best to drop it every 2 ~ 3 s at the terminal point and shake it thoroughly;

In addition, we can add an appropriate amount of equivalent EDTA magnesium salt to the buffer solution to make the endpoint obvious; During titration, the temperature of the water sample should be 20 ~ 30 ℃.

Determination principle of water hardness by EDTA complexometric titration

When measuring water hardness, EDTA complexometric titration is a standard analytical method stipulated in the world. It is suitable for drinking, boiler, cooling, groundwater, and surface water without severe pollution.

The hardness of water refers to the concentration of calcium and magnesium in water. Calcium ion (Ca2 +) and magnesium ion (Mg2 +) can form a stable complex with EDTA. As long as we know the total amount of EDTA that completely complexes calcium and magnesium ions, we can calculate the concentration of calcium and magnesium ions and get the hardness of the water. Considering that EDTA is affected by acid, the pH value of the solution is controlled to 10.

Therefore, titrate with EDTA standard solution in PH10 ammonia buffer solution with chrome black T as an indicator. Since the complex of chrome black T and Mg2 + is more stable than Ca2 +, if there is no or very little Mg2 + in the water sample, the color change of the endpoint is not sensitive enough. We can now add a small amount of mgna2y solution or acid chrome blue K as the indicator.

Steps

1) Preparation of EDTA solution with C (EDTA) = 0.01mol/l

Weigh 1.99g of disodium ethylenediamine tetraacetic acid and dissolve it in 300ml water by heating appropriately. After cooling, transfer to reagent bottle, dilute with water to 500ml and shake well.

2) Calibration of C (EDTA) = 0.01mol/l EDTA solution

Weigh 0.25g (accurate to 0.0001g) of the reference calcium carbonate into a 200ml beaker, wet it with a small amount of water, cover the watch glass, and add 4mol / L HCl drop by drop until the calcium carbonate is wholly dissolved (Avoid adding excess acid). Add 100ml water and boil over low heat for 10min to remove CO2. Cool it to room temperature, rinse the surface dish with a small amount of water, transfer it quantitatively to a 250ml volumetric flask, dilute it with water to the scale, and shake it well.

Pipette 25ml of the above Ca2 + standard solution into a 250ml conical flask. Add 20ml distilled water, 4ml buffer solution, and 3 drops of black T indicator solution (or 50-100mg indicator), titrate with the prepared EDTA solution immediately and shake it thoroughly. When the color of the solution changes from purple-red to pure blue, the determination of six samples, the absolute deviation of calcium and magnesium content shall be less than 0.04mol/l.