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What is naphtha?.
Naphtha is a refined light distillate fraction, usually boiling below 250C, but often with a fairly wide boiling range. Gasoline and kerosine are the most well-known, but there are a whole range of special-purpose hydrocarbon fractions that can be described as naphtha. The petroleum refining industry calls the 0-100C fraction from the distillation of crude oil "light virgin naphtha" and the 100-200C fraction " heavy virgin naphtha". The product stream from the fluid catalytic cracker is often split into three fractions, <105C = "light FCC naphtha", 105-160C = "intermediate FCC naphtha" and 160-200C "heavy FCC naphtha".
What are white spirits?.
White spirits are petroleum fractions that boil between 150-220C. They can have aromatics contents between 0-100%, and Shell lists eight grades with aromatics contents below 50%, and six grades with aromatics contents above 50%. The two common "white spirits" are defined by British Standard 245, which states Type A should have aromatics content of less that 25% v/v and Type B should have an aromatics content of 25-50% v/v. The most common " white spirit" is type A, and it typically has an aromatics content of 20%, boils between 150-200C, and has an aniline point of 58C, and is sometimes known as Low Aromatic White Spirits. The next most common is Mineral Turpentine (aka High Aromatic White Spirits ), which typically has an aromatics content of 50%, boils between 150-200C and has an aniline point of 25C. For safety reasons, most White Spirits have Flash Points above ambient, and usually above 35C. Note that "white gas" is not white spirits, but is a volatile gasoline fraction that has a flash point below 0C, which is also known by several other names. Do not confuse the two when purchasing fuel for camping stoves and lamps, ensure you purchase the correct fuel.
Is vinegar just acetic acid?.
Most countries have food regulations that permit the use of acetic acid as clearly-labelled "synthetic white vinegar". Most vinegars are actually malt vinegars ( fermented ), and synthetic acetic acid is not allowed to be sold as Malt Vinegar. Most natural, unfortified, malt vinegars are appropriately labelled. The classification can get rather messy when bulk suppliers dilute malt vinegar concentrates with acetic acid, which itself could either be synthetic, or from another fermentation process. Regulations usually require any addition of acetic acid to be clearly marked on the label, and the product is not normally legally sold as pure "malt vinegar". The amount of acetic acid in "natural" malt, cider, or wine vinegars usually ranges from 4% - 6%, but some examples can have up to approximately 20%. Vinegar is produced by the exothermic aerobic bacterial oxidation of ethanol to acetic acid via acetaldehyde.
What are the different grades of laboratory water?.
There are several techniques used in chemical laboratories to obtain the required purity of water. There are several grading systems for water, but the most well-known is the ASTM system, although certain applications (HPLC) often require purer water than ASTM Type I, consequently additional treatments such as ultrafiltration and UV oxidation may also be used to reduce concentrations of uncontrolled impurities, such as organics.
ASTM Type I II III Specific Conductance (max. uMhos/cm.) <0.06 <1.0 <1.0 Specific Resistance (min. Mohms/cm.) >16.67 >1.0 >1.0 Total Matter ( max. mg/l ) <0.1 <0.1 <1.0 Silicate ( max. mg/l ) N/D N/D 0.01 KMnO4 Reduction ( min. mins ) >60.0 >60.0 >10.0
Type A B C Colony Count (Colony forming units/ml) 0 Bacteria <10 <100 pH NA NA 6.2-7.5
The techniques to purify natural waters - which may be almost saturated with some contaminants - are frequently used in combination to obtain high purity laboratory water. Some purification techniques use less energy than distilling the water, and may be used in combination where large volumes of "pure" water are required. The design of purified water systems, and the materials used for construction, are selected according to the important contaminants of the water. For some applications, 316L stainless steel may be required, whereas other applications may require polyvinylidene difluoride and polytetrafluoroethylene materials. Systems are carefully designed to minimise the volume of water remaining static and in "dead ends" - where microbes could grow.
The first treatment is usually a coarse physical filtration using a depth filter that can remove undissolved large particles and other insoluble material in the feed water.
For smaller volumes, distillation is the pretreatment method of choice. Distilled water is water that has been boiled in a still and the vapour condensed to obtained distilled water. While many impurities are removed ( especially dissolved and undissolved inorganics that make water "hard", most organisms, etc. ), some impurities do remain ( volatile and some non-volatile organics, dissolved gases, and trace quantities of fine particulates ). Distilled water has lost many of the ionic species that provided a pH buffer effect so, as it dissolves some CO2 from the air during condensation and storage, the pH moves to around 5.5 ( usually from close to the neutral pH of 7.0 ). Distilled water has the vast majority of impurities removed, but often those residual compounds still make it unsuitable for demanding applications, so there are alternative methods of purifying water to remove specific undesirable species.
The next common treatment is ion-exchange, which involves using a bed of resin that exchanges with unwanted dissolved species, such as those that cause "hardness" ( calcium, magnesium ) in water. Two resins are used, one that exchanges anions ( usually a strong anion exchanger such as Amberlite IRA-400 - a quaternary ammonium compound on polystyrene ), and one that exchanges cations ( usually a strong cation exchanger such as Amberlite IR-120 - a sulfonic acid compound on polystyrene ). These resins can also be combined in "mixed bed" resins, such as Amberlite MB-1A, which is a mixture of IRA-400 [OH- form] and IR-120 [H+ form]. The porosity of the polystyrene-based resin is dependant on the amount of cross-linking, which is, in turn, dependant on the proportion of divinyl benzene used in the process. Unfortunately, selectivity of a highly porous resin is inferior to that of a less porous, more cross-linked, resin, so a balance between the rate of exchange and the selectivity is sought. Agarose, cellulose, or dextran can be used in place of the polystyrene base. Sophisticated systems can have many beds in sequence, using both stronger and weaker ion exchange resins.
The exchange potential for ions depends on a number of factors, including molecular size, valency and concentration. In dilute solutions, exchange potentials increase with increasing valency, but in concentrated solutions the effect of valency is reversed, favouring the absorption of univalent ions rather than polyvalent ions. This explains why calcium and magnesium can be strongly absorbed from feedwater in softening processes, but then are easily removed from the ion exchange resin when concentrated sodium chloride is used as regenerant. In dilute solutions, the order of common anion exchange potentials on strong anion exchangers is sulfate > chromate > citrate > nitrate > phosphate > iodide > chloride. In dilute solutions, the order of common cation exchange potentials on strong cation exchangers is Fe3+ > Al2+ > Ba2+ > Pb2+ > Ca2+ > Cu2+ > Zn2+ = Mg2+ > NH4+ = K+ > Na+ > H+ > Hg2+.
There are two forms of ion exchange for water purification. To "deionise" feed water, the resins are in the OH- ( anion exchanger ) and H+ ( cation exchanger ) forms. If sodium chloride was present in the feed water, the sodium ion would displace the hydrogen ion from the cation resin, while the chloride would displace the hydroxyl ion from the anion resin. The displaced ions can combine to form water. Separate beds of resins can be regenerated using 1 Normal acid ( HCl or H2SO4 ) for strongly-acid cation resins, or 1 Normal sodium hydroxide for strongly-basic anion resins. The amount of regenerant is approximately 150 - 500% of the theoretical exchange capacity of the bed.
If the intention is to merely "soften" the feed water to reduce deposits, the beds can be in the Cl- ( anion exchanger ) and Na+ ( cation exchanger ) forms. These are replaced by the dilute polyvalent species in the water that rapidly form undesirable insoluble deposits as process water evaporates, like calcium, magnesium and sulfate. The beds can be regenerated by passing highly concentrated salt ( sodium chloride ) solutions through them until all the polyvalent ions on the resins have been replaced. This technique produces "soft" process water that used in industry.
When a dilute feedwater solution containing salt passes through a cation exchange resin bed in the hydrogen form, the reaction that occurs is:- Na+ + Cl + R.SO3H <=> H+ + Cl- + R.SO3Na Obviously, the acidity of the water strongly increases as it moves down the bed, which inhibits the exchange process. If a mixed bed is used, the products soon encounter the anion exchange resin and are also removed:- H+ + Cl- + R.NH2 <=> R.NH3 + Cl- H+ + Cl- + R.NH3OH <=> R.NH3 + Cl- + H2O Mixed bed resins are usually more efficient than equivalent single beds.
If the water feeding the resin beds has already been distilled ( very common in laboratories - the resin beds then last much, much longer, and the distillation has also removed other impurities ), then the water is called "distilled and deionised". Laboratory water that has had most of the ionic impurities removed will have a high electrical resistance, and is often known as "18.3 megohm" water because the electrical resistance is >18,300,000 ohm/cm, but note that non-ionic impurities may still be present.
An alternative process that has increasingly replaced ion-exchange is reverse-osmosis, which uses osmotic pressure across special membranes to remove most of the impurities. It is called reverse-osmosis because the feed side is pressurised to drive the purified water through the membrane in the opposite direction than would occur if both sides were the same pressure. The two common membrane materials are cellulose acetate or polysulfone coated with polyamine, and typical rejection characteristics are:- Monovalent Divalent Pyrogens, Bacteria Ions Ions Organics > 200 MW Cellulose Acetate >88% >94% >99% Polyamine >90% >95% >99%
The huge advantage of RO is that membranes can easily be maintained ( occasional chemical sterilisations ), are largely self-cleaning, and can produce large amounts of water with no chemical regeneration and minimal energy requirements - just the pressure ( 200 psi ) required to push the water along the membrane surfaces and improve the osmotic yield. RO is commonly used as a pretreatment stage when very pure water is required, and for situations where large volumes of reasonably pure water are required.
Organic species and free chlorine are usually removed from water by passing the water through a bed of activated carbon where they form a low energy chemical link with the carbon. These filters are often installed upstream of the ion-exchange and reverse osmosis stages to protect them from chlorine and organics in the feed water. Polyamine RO membranes require feedwater containing <0.1ppm free chlorine, however cellulose acetate membranes can tolerate up to 1.5ppm free chlorine.
The final stage of producing "pure" laboratory water usually involves passing the deionised water through a 0.22um filter, which is sufficiently small to remove the vast majority of organisms ( the smallest known bacterium is around 0.3um ), thus sterilising the water.
Recently, ultrafiltration has become popular as a means of reducing pyrogens ( they are usually lipopolysaccharides from the degradation of gram negative bacteria ). They are measured by either injecting a sample into test rabbits and measuring body temperature increase or by the more sensitive Limulus Amebocyte Lysate (LAL) test. The internal membrane of an ultrafiltration system has a pore size of <0.005um. This will remove most particles, colloidal silica, and high MW organics such as pyrogens, down to about 10,000MW. These are usually for cell-culture and DNA research, and are located at the point of use, however the ultrafiltration unit has to be regularly sanitized to prevent microbial growth.
Ultraviolet irradiation can be used as a bactericide (254nm) or to destroy organics by photo-oxidation (185nm). The water is exposed to UV for periods up to 30 minutes, and the UV interacts with dissolved oxygen to produce ozone. The ozone promotes hydroxyl radical formation, which result in the destruction of organic material. Usually a high intensity, quartz mercury vapour lamp is used, and is followed by an ion exchange and organic scavenger cartridge to collect decomposition products. The product water is very low in total organic carbon.
Dissolved gases can be removed by passing the water through a vacuum degassing module that utilises an inert, gas-permeable membrane surrounded by a vacuum to remove dissolved gases from the water.
The purest laboratory water is usually obtained after passing through a system that can include reverse osmosis or distillation of the feed water, followed by activated carbon to remove chlorine and organics. The water is passed through ion exchange resins to remove inorganic ions, through a UV oxidation stage, followed by a combined ion exchange and organic scavenger cartridge, and finally through a 0.22um filter. An additional stage of vacuum degassing to remove dissolved gases may be added for some applications - such as for semiconductor production.
These pure water systems are regarded as " point-of-use ", because it is extremely difficult to prevent the reintroduction of contamination during storage and distribution. The water is commonly known as " 18.3 Megohm " water, because it has a specific resistance greater than 18.3 Megohm-cm at 25C. It also contains < 5 ppb of total organic carbon, < 10 ppb of total dissolved solids, and < 1 colony forming unit / mL of micro-organisms.
Details of laboratory and industrial water-purification processes are available in the catalogues of equipment suppliers such as Barnstead [16] and Millipore [17].
