The answer to the analytical problems associated with gypsum titration is to replace the manual colorometric method with the modern 809 Titrando potentiometric autotitrator from Metrohm UK
When formulating modern day construction plaster, strict control and monitoring of additives to gypsum such as lime are vital.
The total lime content on plaster formulations is historically carried out using a manual titration method with phenolphthalein colour indicator.
This manual method is open to the many errors that are associated with manual colorometric titration determinations.
These sources of error include inaccuracies associated with differing colour endpoints between analysts, overtitrating due to burette precision and the most common problem with gypsum titrations, the colour and the turbidity of the gypsum solution almost eliminates any visible colour change.
Titrando alone is not the answer: a robust glass electrode which can withstand the difficult matrix of the plaster solution is essential for accurate and precise reproducible results.
The best electrode for the job is the Unitrode sensor (6.0259.100).
Pure gypsum is a white rock but sometimes impurities colour it grey, brown or pink It is a naturally occurring crystal of calcium sulphate (CaSO4 2H2O).
For every molecule of gypsum, there are two molecules of water therefore if a piece of gypsum is ground to powder and heated, it will lose about three quarters of its water.
If this powder is then mixed with water, the paste or slurry will re-crystallise and set rock hard.
The chemically-combined water, previously removed, has re-combined and the material has reverted to the original composition of the rock.
If gypsum is heated to about 150C, then only some of the water is lost and the hemi-hydrate is formed (CaSO4 1/2H2O).
The hemi-hydrated gypsum plaster we know as plaster of Paris and the name derives from the fact that the city of Paris was built over ground which contained gypsum and this has been mined and quarried, particularly in the district of Montmartre.
Plaster of Paris, casting plaster, dental plaster, and Helix are all forms of hemi-hydrated gypsum.
They all set rapidly (within 10-20 minutes) by recrystallising when mixed with water.
Modern bagged plasters are also made from hemi-hydrated gypsum, but contain retarders (such as lime) which slow down the speed of the set, and also include fillers and other additives.
If gypsum is heated to higher temperatures, then all of the water can be driven off.
However, fully hydrated gypsum will not readily recombine with water and can only be used as a plaster when an accelerator is added to it.
Lime and gypsum are obviously different materials with different properties.
Lime sets slowly by absorbing carbon dioxide from the air, whereas gypsum plaster sets rapidly by crystallising (even fully hydrated gypsum plaster sets within about a day).
Also, as a lime plaster dries it shrinks slightly, while a gypsum plaster expands slightly as it sets.
Historically gypsum has generally tended to be used as an additive to lime.
The speed at which a mix of lime and gypsum set (which is slightly slower than gypsum on its own) means that by varying the combination of gypsum and lime plasters with different setting speeds can be formulated.
Gypsum is found in every continent of the world.
France has already been mentioned, but gypsum is also mined and quarried throughout Europe.
India and Pakistan have gypsum and so have Africa, Australia, China, Japan and South America.
In 1964 Jamaica issued a stamp picturing its gypsum industry.
There are gypsum mines across North America, and Utah possesses large deserts of powdery gypsum rock.
Gypsum is now also obtained as a by-product of reducing the emissions levels of coal-fired power stations.
Flue-gas desulpherisation (FGD) systems are a great step forward in keeping the air you breathe clean and provide a sustainable and ecologically sound source of pure gypsum.
Gypsum in Britain derived from around 160 to 200 million years ago when the climate was hot and arid.
During this time the huge inland seas dried up and left large layers of gypsum.
Gypsum is an evaporate mineral and comes from such drying up of large bodies of water, or from the evaporation of sea water in mud flats at the edge of the sea.
During the following millions of years, when the British Isles were being formed, the gypsum deposits were folded and bent while the land surfaces rose and fell, and the earth's crust moved.
Very large earth movements led to the rocks being faulted with the gypsum being displayed and distorted.
This explains why it is possible to quarry gypsum in one place while nearby it is necessary to mine at depths of over 180m to win it.
If the gypsum was displayed deeply, then the water associated with the gypsum would be driven off by pressure and temperature leaving anhydrite (also an important ingredient in the manufacture of modern cement).
When the ice age came the land surface was eroded or ground down to bring the anhydrite nearer the surface where it encountered water and rehydrated to gypsum.
These areas now form the gypsum deposits we mine, or quarry today around the UK.
Below are some figures from the gypsum industry in Britain and Ireland from a few years ago.
(Source of data: GB and Ireland producers, Gypsum Products Development Association).
Total value of the gypsum sales in Britain and Ireland is around £600 million.
Total production in Great Britain of gypsum (natural and synthetic) in year 2000 was reported as being 3 million metric tons.
Ireland 0.5 million metric tonnes.
UK annual production of plaster is around 700 million kg (more than 60 million bags) and over 200 million square metres of plasterboard.
UK per capita consumption in 2001 was forecast at 12kg of plaster and 3.5m2 of plasterboard.
In the UK direct employment in the gypsum industry is around 2500 with over 20,000 indirectly employed (merchants, delivery, installation etc).
Strict control of products for this huge business in the UK is essential.
Metrohm UK provides the instrumentation to enable precise and accurate titration of gypsum plaster products.
Total lime titrations are determined by titrating HCl acid into gypsum plaster mixed into de-ionised water.
Using the Metrohm 809 Titrando (2.809.0010) potentiometric autotitrator, errors associated with manual titrations which were mentioned at the start of this article are eliminated.
When undertaking a manual addition titration, the pink phenolphthalein colour indication is used to indicate when the titration endpoint is reached.
This is very difficult to see in a solution of gypsum, particularly if the gypsum is of a pink type.
Using the 809 autotitrator with a glass electrode sensor, this problem is eliminated.
However, the sample matrix of gypsum is a very difficult for many electrode types.
The particulates in the sample matrix will block the ceramic pin diaphragm of standard electrodes and the alkali nature of the formulation will reduce the response of the electrode causing what is termed an alkali error.
Therefore, a robust high specification electrode is essential.
The Metrohm Unitrode sensor (6.0259.100) has been designed for such samples.
The Unitrode has a fixed ground-joint diaphragm; the large surface area of this diaphragm ensures the sensor is insensitive to blockage by particulates or contamination.
In addition, the glass membrane of the Unitrode electrode is suitable for alkali conditions such as those found in solutions of plaster.
This is because they are made from a specialised U-glass membrane which has been developed to have a very low alkali error.
