In chemistry, hardness of water translates to the amount of minerals, mainly calcium and magnesium ions dissolved in water.

The hardness of water is one of the most essential parameters in many parts of the food industries, particularly in water treatment facilities.

Basically, the hardness level determines how well soap will react with water, or soap consuming property. Hard water requires more soap to produce foam or lather. Therefore, cleaning using hard water may require more soap.

Examples of soft water is tap water, or drinking water (equal or below 60 ppm). Interestingly, water from the mountains are also very low in minerals. One good example of hard water, on the other hand, is seawater, and for obvious reasons. Seawater is rich in dissolved salts and has a typical hardness of at least 6,600 ppm (very hard).

Let’s discuss further.

The chemistry behind soap with water

Soap is made through a chemical reaction between alkali and fats— has long fatty acids of sodium salts. The calcium ions (Ca2+) and magnesium ions (Mg2+) dissolved in water react with soap. Please note that other heavy metal ions may also be present such as Mn2+, Fe2+ and Al2+. The sodium salts in the soap are converted into the corresponding calcium and magnesium salts. These salts dissolve very easily in soft water. But in hard water, they are instead precipitated as insoluble white solid called soap scum.

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Key differences between soft and hard water

The differences between soft and hard water are determined by the amount of dissolved minerals, primarily calcium and magnesium. The more minerals the water contain, the less soap lather is produced. See below table.

2 types of water hardness

Temporary hardness

Temporary hardness refers to the hardness of water caused by carbonates of calcium, and other metals. This type of hardness can be removed by boiling. The elevated temperature converts calcium bicarbonates into calcium carbonate, water, and carbon dioxide. Magnesium bicarbonate is converted into magnesium hydroxide and carbon dioxide. While carbon oxide escapes, the resulting salts calcium carbonate and magnesium carbonates are water-insoluble and only precipitates to the bottom of the container. See below reaction.

The remaining Ca2+ and Mg2+ ions can be eliminated by filtration, turning water into soft water.

Temporary hardness is very evident in equipment associated with heat like boilers and kettles. The solid deposits are usually white, but the colors may vary, depending on the metals present. Iron is responsible for the reddish-brown color. One example of this is your brown-stained sink. Are you sourcing your water underground? Maybe it is iron-rich.

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Aside from boiling and filtration, permanent hardness can also be eliminated by Clark method, also called lime softening. The Clark method works by adding lime or Calcium hydroxide (Ca(OH)₂₎ to precipitate the calcium and magnesium ions. Use lime with caution though. Because Ca(OH)₂ is a source of calcium ions itself, so it must be used very carefully.

Permanent hardness refers to the hardness of water due to chlorides and sulphates of calcium and magnesium, and other metals. It is so called because boiling is ineffective to remove it. Instead, a chemical method may be applied such as the following:

• Calgon’s method

With Calgons method, the water is treated with sodium hexametaphosphate (NaPO₃)_6. This displaces sodium ions and in the process, eliminates the calcium and magnesium ions, making the water soft.

• Ion-exchange method

Like the name suggests, Ion-exchange work by exchanging the calcium and magnesium ions in the water for sodium ions. Due to its ability to remove other substances, Ion-exchange is used for denitrification, dealkalization, and disinfection as well.

• Treatment with washing soda

Washing soda is the common name for sodium carbonate (Na₂CO₃). We actually use washing soda in the form of detergent to clean dishes or when doing the laundry. Its carbonate ions react well with calcium and magnesium ions dissolved in the water. The resulting precipitates of CaCO₃ and MgCO₃ are removed by filtration to make the water soft.

• Synthetic resins method

The synthetic resins method uses synthetic ion-exchange resins (RNa⁺) that when treated with sodium chloride (NaCl) exchanges sodium ions with Ca2+ and Mg2+ ions. With Ca2+ and Mg2+ ions removed, the water becomes soft.

In the food industry

The hardness level of water is a must-checked part of food manufacturing. This is especially true for processes that require heating water. Take a bottle washer in soft drinks processing that requires washing temperatures of around 158 °F (70°C), for example. A bottle washer has numerous pipes and nozzles. In order to keep it from running efficiently (no clogging), soft water is necessary all throughout to prevent scale buildup. To achieve this, the hardness is kept low using a certain water softening method—one is Clark’s method. The hardness is religiously monitored and recorded.

The hardness level must be maintained equal or below 60 ppm as CaCO₃. Scum or scale deposit may form if the equipment has been in contact with hard water for an extended period of time.

A few unfavorable results of scale buildup in food processing equipment include:

• Clogging of equipment parts

• Reduced efficiency

• Reduced equipment lifespan

• Increased consumption of cleaning agent

• Processes are more costly

In the laboratory, using hard water is a definite no-no. This is because using it will only add more calcium and magnesium ions, and give you false readings. Use distilled or deionized water instead.

At homes

The last time you washed your hands, did you feel some residues left on your hands? How about the last time you did the dishes? If you did, you may be using hard water at home. No worries though, as it not dangerous to your health. But like in any industry, hard water is somewhat a concern for several reasons.

Here are some.

• Difficulty in foaming of soap

• Scale deposits on glass wares, sinks, faucets, showers with heater and tubs

• Increased fuel used during cooking (elevated water boiling point)

• The need for fabric conditioner when doing laundry

• More energy usage of appliances (clogged nozzles and pipes)

Water hardness analysis

When we determine the hardness level of water, we usually talk about the total hardness, the sum temporary and permanent hardness.

Total hardness of water

The total hardness of water is the sum of temporary and permanent hardness. It is determined by chemical titration. The result is commonly reported in parts per million (ppm), or milligrams per liter (mg/l) as calcium carbonate. Here is how it is done.

What you need:

• Ethylenediaminetetraacetic acid (EDTA) – a chelating agent that reacts with Ca and Mg ions
• Ammonia buffer solution- to keep pH level at 10
• Eriochrome Black T (EBT) indicator
• Deionized water
• 50.0 ml burette
• Sample water

1. To perform this, fill the burette with EDTA. Make sure you have already rinsed it with deionized water and EDTA.
2. Using a sample-rinsed 10.0 ml pipette, pipette 10.0 ml of your sample into a conical flask.
3. Add 5.0 ml of ammonia buffer solution to keep pH at 10.
4. Add a bit of EBT indicator. 2 Drops is sufficient. This will turn the sample wine-red in color.
5. Titrate your sample against EDTA solution until the color changes from wine red to blue, your end point. At this point, all the Ca and Mg ions have already been complexed by EDTA.
6. Titrate your sample thrice to get 3 results for precision.
7. Compute the total water hardness using the formula provided below.

Temporary and permanent hardness

So above only finds the total hardness water, right? How do you find the temporary hardness? The permanent hardness?

It is simple.

Boil the water. This will eliminate the temporary hardness of the water. Now titrate the boiled water. As expected, you will get a smaller value— this is the water’s permanent hardness.

Here is an example.

Let’s say before boiling, the water’s total hardness was 50 ppm CaCO₃.

Then after boiling, instead of 50 ppm CaCO₃, the value is now down to 38 ppm CaCO₃. Since you removed the temporary hardness by boiling, this 38 ppm CaCO₃ is now your permanent hardness. To get the value of the temporary hardness, simply substract 38 from 50. That will give you a temporary hardness of 12 ppm CaCO₃.

Classification of water hardness

Generally, any water that registers 60 ppm or below is soft water.

Check the below table for the classification.

Health concerns

There is this misconception that consuming hard water is harmful to human health. This is definitely not true. Hard water is generally safe to consume.

Well, first of all, hard water is as it is because of the essential minerals— mainly calcium and magnesium— it contains. And as we all know, our body requires minerals in order for our body to grow, develop and keep it healthy. Calcium promotes strong bones, among other important functions. Magnesium helps regulate the muscle, blood sugar level, and nerve function. And consuming hard water helps you fulfill the dietary intake of these minerals. And besides, the World Health Organization says that there are no strong evidences to prove that hard water causes adverse effects to human health. Read this PDF file from the (WHO) regarding the health effects of hard water.

Water hardness is an interesting subject to cover. Did we miss anything important? Or perhaps you found something new. Feel free to comment down below.

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