Methods Manual for Salt Lake Studies/Hardness

Hardness overview

"Hardness" is a measure of calcium and magnesium in brine or water. The following titration, based on APHA methods 2340C, 3500-Ca D and 3500-Mg E (Eaton et al, 1995) provides a measure of Ca and Mg separately, and also a combined measure of "hardness" .

What is Hardness?

The Metals Handbook defines hardness as "Resistance of metal to plastic deformation, usually by indentation. However, the term may also refer to stiffness or temper, or to resistance to scratching, abrasion, or cutting. It is the property of a metal, which gives it the ability to resist being permanently, deformed (bent, broken, or have its shape changed), when a load is applied. The greater the hardness of the metal, the greater resistance it has to deformation.

In mineralogy the property of matter commonly described as the resistance of a substance to being scratched by another substance. In metallurgy hardness is defined as the ability of a material to resist plastic deformation.

The dictionary of Metallurgy defines the indentation hardness as the resistance of a material to indentation. This is the usual type of hardness test, in which a pointed or rounded indenter is pressed into a surface under a substantially static load.

Hardness Measurement

Hardness measurement can be defined as macro-, micro- or nano- scale according to the forces applied and displacements obtained

Measurement of the macro-hardness of materials is a quick and simple method of obtaining mechanical property data for the bulk material from a small sample. It is also widely used for the quality control of surface treatments processes. However, when concerned with coatings and surface properties of importance to friction and wear processes for instance, the macro-indentation depth would be too large relative to the surface-scale features.

Where materials have a fine microstructure, are multi-phase, non-homogeneous or prone to cracking, macro-hardness measurements will be highly variable and will not identify individual surface features. It is here that micro-hardness measurements are appropriate.

Microhardness is the hardness of a material as determined by forcing an indenter such as a Vickers or Knoop indenter into the surface of the material under 15 to 1000 gf load; usually, the indentations are so small that they must be measured with a microscope. Capable of determining hardness of different microconstituents within a structure, or measuring steep hardness gradients such as those encountered in casehardening. Conversions from microhardness values to tensile strength and other hardness scales (e.g. Rockwell) are available for many metals and alloys

Micro-indenters works by pressing a tip into a sample and continuously measuring: applied load, penetration depth and cycle time.

Nano-indentation tests measure hardness by indenting using very small, on the order of 1 nano-Newton, indentation forces and measuring the depth of the indention that was made. These tests are based on new technology that allows precise measurement and control of the indenting forces and precise measurement of the indentation depths. By measuring the depth of the indentation, progressive levels of forcing are measurable on the same piece. This allows the tester to determine the maximum indentation load that is possible before the hardness is compromised and the film is no longer within the testing ranges. This also allows a check to be completed to determine if the hardness remains constant even after an indentation has been made.

There are various mechanisms and methods that have been designed to complete nano-indentation hardness tests. One method of force application is using a coil and magnet assembly on a loading column to drive the indenter downward. This method uses a capacitance displacement gauge. Such gages detect displacements of 0.2 to 0.3 NM (nanometer) at the time of force application. The loading column is suspended by springs, which damps external motion and allows the load to be released slightly to recover the elastic portion of deformation before measuring the indentation depth. This type of nano-indentation machine can be seen in Figure 1.

Hardness Measurement Methods

There are three types of tests used with accuracy by the metals industry; they are the Brinell hardness test, the Rockwell hardness test, and the Vickers hardness test. Since the definitions of metallurgic ultimate strength and hardness are rather similar, it can generally be assumed that a strong metal is also a hard metal. The way the three of these hardness tests measure a metal's hardness is to determine the metal's resistance to the penetration of a non-deformable ball or cone. The tests determine the depth which such a ball or cone will sink into the metal, under a given load, within a specific period of time. The followings are the most common hardness test methods used in today`s technology:

Rockwell hardness test
Brinell hardness
Vickers
Knoop hardness
Shore
Barcol hardness

Equipment and reagents required

beaker for sample analysis,
pipettes for measuring sample & reagent volumes,
burette tube or Dr Schilling self-zeroing burette for titre,
sample,
distilled water,
normal HCl,
normal KOH,
calcium indicator,
EDTA titre,
solochrome black indicator

Sample to be tested

Between 10 mL and 100mL of brine, depending on salinity. Use 10mL for brines approaching the saturation point for salt, ranging up to 100 mL for brackish waters. Test a blank (deionised water) alongside the sample.

Determining sample calcium content

Put sample in beaker and make up to 100mL with deionised water
Add 5 mL of Normal (N) HCl
Add 20 mL of Normal (N) KOH
Add 10 mL of Calcium indicator
Fill the Burette tube to zero with EDTA
Stir the sample for approximately 1 minute
Add EDTA to sample until the vortex changes colour from orange to yellow, then add EDTA dropwise until the entire beaker changes colour. To prevent over titration, wait for initial colour change to take effect before adding more EDTA.
Record EDTA used from burette tube in the lab notebook.

Ca Calculation

Correct EDTA use for any blank value obtained. Calculate Calcium concentration for a 10 mL sample by multiplying mL of .02 N EDTA by 40.08 and report as mg/L Ca. For larger or smaller samples than 10mL, correct for the sample size.

Determining sample magnesium content

Using the same sample that you used to determine the calcium,
Add 25 mL Normal HCl to sample to decolorize the sample
Add 5 mL Ammonia buffer to sample
Add approximately 10 drops of Solochrome Black indicator to sample
Stir for 1 minute
Fill Burette tube to zero level with EDTA
Add EDTA to sample until the vortex changes colour from purple to blue then add EDTA dropwise until the entire beaker changes colour. To prevent over titration, wait for initial colour change to take effect before adding more EDTA.
Record EDTA used from burette tube in the lab notebook.

Mg Calculation

Correct EDTA use for any blank value obtained. Calculate Magnesium concentration for a 10 mL sample by multiplying mL of .02 N EDTA by 24.3 and report as mg/L Mg. For larger or smaller samples than 10mL, correct for the sample size.

Total hardness calculation

If you need the results as total hardness, rather than as separate Ca and Mg, the following calculation will provide hardness as CaCO₃ mg/L

Hardness mg equivalent CaCO₃ mg/L = 2.497[Ca mg/L] + 4.118[Mg mg/L]^[1]

Record results in the laboratory daybook

Things to be aware of with this method

Contaminated reagents or deionised water can invalidate results. Test against Calcium and Magnesium standards monthly and test a blank in every analytical run. Record the results of the blank and use in calculation.

Old or out-of-date reagents may produce inaccurate results. Always check the age of reagents. Exhausted ammonia buffer produces a wine colour that is diagnostic.

Acids and bases can cause burns when splashed on skin or clothing. Take care not to splash chemicals. Use safety glasses. Any splashes should be immediately rinsed with water.

References

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