Gas Evolving

Primary emphasis is to prevent or retard growth of bacteria which produce gas. Under natural conditions a species of bacteria maintains itself by continuous multiplication, a balance being established between the rates of death and multiplication. This balance can be upset by changes in moisture content, availability of oxygen and of nutrients, presence of inhibitory agents, competition with other species, and other factors encountered in the environment. In the marine environment a direct correlation has been observed between survival at increased hydrostatic pressure, temperature, and the salinity of the medium (Litchfield, 1976). The methods that can cause the death of bacteria and can potentially be employed are the following (Zobell, 1946 Clifton, 1957)  

Injection of a chemical into the sample such as formaldehyde. 

X-raying the sample at high intensity prior to testing. 

Testing using an inert gas, such as nitrogen to restrict available oxygen. 

The injection of a chemical into the sample to prevent the growth of microorganisms has been used previously by Allison (1947) in a study of permeability with mixed results. One difficulty with this technique is from possible interference with the original pore water chemistry. 

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One of the most readily observed reactions in chemistry is the familiar production of bubbles of a colourless gas when certain metals (for example, iron, zinc) react with dilute acids. Cavendish investigated these reactions rather more than 200 years ago, and found the gas evolved to be the same in each case the gas, later named hydrogen, was much lighter than air and when burned in air produced water. 

Fig 23(A) shows an assembly for boiling a liquid under reflux whilst adding another liquid at a rate which can be clearly seen cf. preparation of acetophenone, p. 253). The outlet A allows expansion of the vapour content, and can be fitted with a calcium chloride or soda-lime tube. The outlet A can also be used for collecting a gas evolved during the reaction cf, preparation of acetylene,... 

Carbon monoxide test. Warm together carefully 0 5 ml. of formic acid (or 0 5 g. of a formate) and i ml. of cone. 112804. Identify the carbon monoxide by igniting the gas evolved and observing the pale blue flame travel down the test-tube. Note that dilute solutions of formic acid will not give this test. HCOOH — HjO — CO. 

Methyl iodide, ethyl bromide and ethyl iodide also evolve small amounts of ethylene when treated as above. If this is suspected, a small quantity of the substance should be heated with alcoholic NaOH solution in a small flask, fitted with a knee delivery-tube. Pass the gas evolved through a very dilute solution of KMn04 which has been made alkaline with aqueous NagCOj solution. If ethylene has been formed, a brown precipitate of MnOj will be produced (a transient green colour may appear). 

Coal tar is the condensation product obtained by cooling to approximately ambient temperature, the gas evolved in the destmctive distillation of coal. It is a black viscous Hquid denser than water and composed primarily of a complex mixture of condensed ring aromatic hydrocarbons. It may contain phenoHc compounds, aromatic nitrogen bases and their alkyl derivatives, and paraffinic and olefinic hydrocarbons. Coal-tar pitch is the residue from the distillation of coal tar. It is a black soHd having a softening point of 30—180°C (86—359°F). 

Hydrogen iodide (anhydrous) [10034-85-2] M 127.9, b -35.5°. After removal of free iodine from aqueous HI, the solution is frozen, then covered with P2O5 and allowed to melt under vacuum. The gas evolved is dried by passage through P2O5 on glass wool. It can be freed from iodine contamination by repeated fractional distillation at low temperatures. Fumes in moist air. HARMFUL VAPOURS. 

In some cases the amount of gas evolved or consumed according to Eqs. (2-17) to (2-19) can be asked for. Then Eq. (2-6) is applicable. To obtain the volume, the following applies ... 

The isocyanate group may be terminal on a polyester chain or may be part of the unchanged di-isocyanate. The density of the product, which depends on the amount of gas evolved, can be reduced by increasing the isocyanate content of the reaction mixture and by correspondingly increasing the amount of water to react with the excess isocyanate (that is excess over that required for chain extension and cross-linking). 

Formulations should be based on stoichiometric considerations. Based on a knowledge of the hydroxyl value of the polyol the amount of isocyanate necessary to cause chain growth should be calculated. The gas evolved will depend on the water content and additional isocyanate must be incorporated corresponding to the water present. When the isocyanate used equals the theoretical amount the system is said to have a TDI index of 100. In practice a slight excess of isocyanate is used (TDI index 105-110) to ensure complete... 

It is apparent (Fig. 1.21) that at potentials removed from the equilibrium potential see equation 1.30) the rate of charge transfer of (a) silver cations from the metal to the solution (anodic reaction), (b) silver aquo cations from the solution to the metal (cathodic reaction) and (c) electrons through the metallic circuit from anode to cathode, are equal, so that any one may be used to evaluate the rates of the others. The rate is most conveniently determined from the rate of transfer of electrons in the metallic circuit (the current 1) by means of an ammeter, and if / is maintained constant it can eilso be used to eveduate the extent. A more precise method of determining the quantity of charge transferred is the coulometer, in which the extent of a single well-defined reaction is determined accurately, e.g. by the quantity of metal electrodeposited, by the volume of gas evolved, etc. The reaction Ag (aq.) -t- e = Ag is utilised in the silver coulometer, and provides one of the most accurate methods of determining the extent of charge transfer. 

The performance of graphite in seawater, where chlorine is the principal gas evolved, is considerably better than in fresh water where oxygen is produced. Graphite is immune to chlorine and has a long history in the chemical industry in this and similar applications . 

Scale removal is also assisted by the dissolution of the underlying metal by normal acid corrosion processes, which undermines the scale, and by the physical effect of hydrogen gas evolved in this latter reaction. Some authors attribute major effects to the latter. 

The presence of nitrate as acelerator has a pronounced effect on the amount and composition of gas evolved from the work being treated (Table 15.8). It will be observed that hydrogen evolution drops to a very low figure with the zinc/nitrate baths. The formation of nitrite arises from decomposition of nitrate by reaction with primary ferrous phosphate to form ferric phosphate ... 

A 0.500-g sample of zinc-copper alloy was treated with dilute hydrochloric acid. The hydrogen gas evolved was collected by water displacement at 27°C and a total pressure of 755 mm Hg. The volume of the water displaced by the gas is 105.7 mL. What is the percent composition, by mass, erf the alloy (Vapor pressure of H20 at 27°C is 26.74 mm Hg.) Assume only the zinc reacts. 

More quantitative evidence can be obtained by carrying out the reaction between an excess of sodium and a weighed amount of ethanol and measuring the amount of hydrogen gas evolved. When this is done it is found that 46 grams of ethanol (one mole) will produce only mole of hydrogen gas. We can therefore write a balanced chemical equation for the reaction of sodium with ethanol ... 

Volumetry is concerned with measuring the volume of gas evolved or absorbed in a chemical reaction. 

Marqueyrol s Stability Test. 200g samples of double-base powds were heated at 50° under vac in a round bottom Pyrex flask. Every 3—4 days, the vapors were pumped out, collected in, a gasometer over Hg and measured. The test was contd until the powd started to decomp very rapidiy, usually after 1800—2000 days. A curve was then plotted of cc of gas evolved vs days of heating, and the gas was analyzed for C02 and NO. If any liq formed, it was collected, measured and analyzed... 

Methylamine Salt. A yellow solid mp 126—28° with decompn ign temp 112° vacuum stab 1.64cc/g of gas evolved at 60° in 2 hours, 2.04cc/g at 60° in 48 hours (Ref 43). It has been patented as an expl ingredient (Ref 44) and a proplnt ingredient (Ref 41)... 

With regard to pro pints, brisance, power and detonation velocity are not tested. It is, however, necessary to determine the burning rate, press developed on burning, vol of gas evolved, calorific value, thermal stability, etc. If one is... 

Step 3. Methane gas evolves during decarburization corrosion and builds up pressure, causing hydrogen embrittlement metal fatigue and eventual tube ... 

The kinetics of many decompositions are conveniently studied from measurements of the pressure of the gas evolved in a previously evacuated and sealed constant volume system. It is usually assumed, and occasionally confirmed, that gas release is directly proportional to a, so that the method is most suitable for reactants which yield a single volatile product by the irreversible breakdown of a substance that does not sublime on heating in vacuum. A cold trap is normally maintained between the heated reactant and the gauge to condense non-volatile products (e.g. water vapour) and impurities. The method has found wide application, notably in studies of the decomposition of azides, permanganates, etc., and has been successfully developed as an undergraduate experiment [114—116]. 

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