Gases evolution

The evolution of a gas during an electrolytic reaction produces a dispersion of bubbles in the electrolyte. Since the bubbles have virtually zero electrical conductivity the current flow path becomes restricted and ohmic voltage losses become greater than those for the electrolyte. The usual way to deal with bubbles is to assign to the electrolyte an effective conductivity which is typically correlated as a function of gas voidage There are a number of correlations of this type one of the most reliable is the so-called Bruggeman equation  

This correlation (and others) requires no knowledge of bubble size and is applicable to systems with an approximately uniform voidage distribution. Bubbles may cause a considerable effect by accumulating near the vicinity of the gas-evolving electrode and producing an electrolyte layer rich in gas bubbles, the bubble curtain effect. This layer can be as thick as a few millimeters if not controlled, it can be a major source of ohmic resistance. The usual way to meet this problem is to induce effective electrolyte circulation and/or to use perforated electrodes. The latter are used in chlorine electrolyzers in order to permit gas release at the rear, away from the potential field. 

I being the current, P the atmospheric pressure, W the width of the electrode, d the electrolyte gap, and Vg the rise velocity of the bubbles (as predicted by Stokes s law) augmented by the electrolyte flow. 

EXAMPLE 2.12. Electrolyte Voltage Losses during Gas Evolution 

THE PROBLEM Estimate the electrolyte voltage requirement for an electrolyzer with a single gas evolution reaction with a current efficiency of 100%. The rectangular electrodes are 0.1m wide and 0.25 m long the electrolyte gap is 0.01 m. 

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In a 500 ml. Pyrex round-bottomed flask, provided with a reflux condenser, place a mixture of 40 g. of freshly-distUled phenylhydrazine (Section IV.89) and 14 g. of urea (previously dried for 3 hours at 100°). Immerse the flask in an oil bath at 155°. After about 10 minutes the urea commences to dissolve accompanied by foaming due to evolution of ammonia the gas evolution slackens after about 1 hour. Remove the flask from the oil bath after 135 minutes, allow it to cool for 3 minutes, and then add 250 ml. of rectified spirit to the hot golden-yellow oil some diphenylcarbazide will crystallise out. Heat under reflux for about 15 minutes to dissolve the diphenylcarbazide, filter through a hot water funnel or a pre-heated Buchner fuimel, and cool the alcoholic solution rapidly in a bath of ice and salt. After 30 minutes, filter the white crystals at the pump, drain well, and wash twice with a little ether. Dry upon filter paper in the air. The yield of diphenylcarbazide, m.p. 171 °, is 34 g. A further 7 g. may be obtained by concentrating the filtrate under reduced pressure. The compound may be recrystallised from alcohol or from glacial acetic acid. 

Methylation with diazomethane may be carried out as follows (FUME CUPBOARD )-. Dissolve 2-3 g. of the compound (say, a phenol or a carboxylic acid) in a little anhydrous ether or absolute methanol, cool in ice, and add the ethereal solution of diazomethane in small portions until gas evolution ceases and the solution acquires a pale yellow colour. Test the coloured solution for the presence of excess of diazomethane by removing a few drops into a test-tube and introducing a glass rod moistened with glacial acetic acid immediate evolution of gas should... 

Treating a benzene suspension of sodium borohydride (4 equiv.) With glacial acetic acid (3.25 equiv.) And refluxing the mixture for 15 min under nitrogen, after the initial rapid gas evolution subsided (ca. 3 mol of Hz liberated) [No Smoking ], gave a clear solution of NaBH(OAc)3. ... 

Thionyl chloride (11.5g, 96.4 mmol) was added to 2-nitrophenylacetic acid (8.72g, 48.2mmol) and the suspension was warmed to 50°C and stirred until gas evolution was complete. The resulting solution was concentrated in vacuo and the residue dissolved in CHjClj (30 ml). This solution was added dropwise to a stirred solution of Meldrum s acid (6.94 g, 48.2 mmol) in CH2CI2 (200 ml) under nitrogen at 0 C. The solution was stirred at 0" C for 1 h after the addition was complete and then kept at room temperature for an additional hour. The reaction solution was then worked up by successively washing with dil. HC1, water and brine and dried (MgSOJ. The dried solution was concentrated in vacuo and abs. ethanol (200 ml) was added to the residue. The mixture was... 

The cinnamate ester prepared as above (23.2 g. 79 mmol) was added as a solid slowly to refluxing xylene (500 ml) over a period of 3 h at a rate that prevented accumulation of unreacted azidocinnamate in the solution (monitored by gas evolution through a gas bubbler). The solution was refluxed for an additional 2 h after gas evolution ceased. The reaction mixture was cooled and the solvent removed in vacuo. The residue was recrystallized from methanol to give pure product (20.7 g, 99% yield). 

Nxylylene system, substituents affect it only to a minor extent. AH parylenes are expected to have a similar molar enthalpy of polymerization. An experimental value for the heat of polymerization of Parylene C has appeared. Using the gas evolution from the Hquid nitrogen cold trap to measure thermal input from the polymer, and taking advantage of a peculiarity of Parylene C at — 196°C to polymerize abmptiy, perhaps owing to the arrival of a free radical, a = —152 8 kJ/mol (—36.4 2.0 kcal/mol) at — 196°C was reported (25). The correction from — 196°C to room temperature is... 

Many factors other than current influence the rate of machining. These involve electrolyte type, rate of electrolyte flow, and other process conditions. For example, nickel machines at 100% current efficiency, defined as the percentage ratio of the experimental to theoretical rates of metal removal, at low current densities, eg, 25 A/cm. If the current density is increased to 250 A/cm the efficiency is reduced typically to 85—90%, by the onset of other reactions at the anode. Oxygen gas evolution becomes increasingly preferred as the current density is increased. 

Manufacture. Small cylinders of hydrogen sulfide are readily available for laboratory purposes, but the gas can also be easily synthesized by action of dilute sulfuric or hydrochloric acid on iron sulfide, calcium sulfide [20548-54-3], zinc sulfide [1314-98-3], or sodium hydrosulfide [16721 -80-5]. The reaction usually is mn in a Kipp generator, which regulates the addition of the acid to maintain a steady hydrogen sulfide pressure. Small laboratory quantities of hydrogen sulfide can be easily formed by heating at 280—320°C a mixture of sulfur and a hydrogen-rich, nonvolatile aUphatic substance, eg, paraffin. Gas evolution proceeds more smoothly if asbestos or diatomaceous earth is also present. 

Such a reaction is controlled by the rate of addition of the acid. The two-phase system is stirred throughout the reaction the heavy product layer is separated and washed thoroughly with water and alkaU before distillation (Fig. 3). The alkaU treatment is particularly important and serves not just to remove residual acidity but, more importantiy, to remove chemically any addition compounds that may have formed. The washwater must be maintained alkaline during this procedure. With the introduction of more than one bromine atom, this alkaU wash becomes more critical as there is a greater tendency for addition by-products to form in such reactions. Distillation of material containing residual addition compounds is ha2ardous, because traces of acid become self-catalytic, causing decomposition of the stiU contents and much acid gas evolution. Bromination of alkylthiophenes follows a similar pattern. 

Most mineral acids react vigorously with thorium metal. Aqueous HCl attacks thorium metal, but dissolution is not complete. From 12 to 25% of the metal typically remains undissolved. A small amount of fluoride or fluorosiUcate is often used to assist in complete dissolution. Nitric acid passivates the surface of thorium metal, but small amounts of fluoride or fluorosiUcate assists in complete dissolution. Dilute HF, HNO, or H2SO4, or concentrated HCIO4 and H PO, slowly dissolve thorium metal, accompanied by constant hydrogen gas evolution. Thorium metal does not dissolve in alkaline hydroxide solutions. 

PVC should not be melt-mixed with acetal polymers. These polymers are chemically incompatible mixing could cause rapid decomposition and gas evolution. 

Dehydration. Residual liquid and physisorbed moisture on particle surfaces can be eliminated on beating to - 200° C. Temperatures ia excess of 1000°C may be requited to eliminate cbemisorbed water (29). Kaolin must be beated to 700°C to Hberate tbe water of crystallisation and produce tbe desired dehydrated aluminosiUcate. As with biader burnout, rapid gas evolution from rapid dehydration can result ia catastrophic stress development within a body. 

Fire Refining. The impurities in bhster copper obtained from converters must be reduced before the bUster can be fabricated or cast into anodes to be electrolyticaHy refined. High sulfur and oxygen levels result in excessive gas evolution during casting and uneven anode surfaces. Such anodes result in low current efficiencies and uneven cathode deposits with excessive impurities. Fite refining is essential whether the copper is to be marketed directly or electrorefined. 

Fermentation Processes. The efficient production of penicillin, yeasts, and single-ceUed protein by fermentation requires defoamers to control gas evolution during the reaction. Animal fats such as lard [61789-99-9] were formerly used as a combined defoamer and nutrient, but now more effective proprietary products are usually employed. Defoamer appHcation technology has also improved. For example, in modem yeast production faciHties, the defoamers are introduced by means of automatic electrode-activated devices. One concern in the use of defoamers in fermentation processes is the potential fouHng of membranes during downstream ultrafiltration (qv). SiHcone antifoams (43,44) seem less troubled by this problem than other materials. 

Tertiay Current Distribution. The current distribution is again impacted when the overpotential influence is that of concentration. As the limiting current density takes effect, this impact occurs. The result is that the higher current density is distorted toward the entrance of the cell. Because of the nonuniform electrolyte resistance, secondary and tertiary current distribution are further compHcated when there is gas evolution along the cell track. Examples of iavestigations ia this area are available (50—52). 

What are the consequences What is the maximum pressure Vapor pressure of solvent as a function of temperature Gas evolution Differential Thermal Analysis (DTA) / Differential Scanning Calorimetry (DSC) Dewar flask experiments... 

No gas evolution was observed by the checkers in some runs in which an older lot of sodium hydride was used. In this case, the cooling bath was removed and the mixture was allowed to stir at room temperature until the bubbling ceased. 

Dehydrochlorination begins at about 120°C. The temperature is raised about lO C/hr to 150 C to avoid vigorous gas evolution. The elimination of hydrogen chloride is complete after 5-6 hr. 

To 68 g. (0.5 mole) of aminoguanidine bicarbonate (p. 7) in a 500-ml. round-bottomed flask is added carefully the cold dilute sulfuric acid (0.24 mole), made from 24.5 g. of concentrated acid (sp. gr. 1.84) and 50 ml. of water. After the gas evolution has subsided, the solution is heated for 1 hour on a steam bath and then evaporated to dryness under a pressure of about 15 mm. 

Complete conversion into sodium amide is indicated by cessation of gas evolution and disappearance of the blue color of the solution. This generally requires 20-30 minutes and results in a gray suspension of sodium amide in a dark-gray reaction medium. 

When heated in a capillary tube aconitic acid decomposes rather suddenly with vigorous gas evolution at a temperature which is highly dependent upon the rate of heating and the temperature at which the sample is introduced. In the literature melting points ranging from 182.5° iQ4-5° > 16 recorded. The uncrystallized aconitic acid, when introduced at 180° into a small bath provided with mechanical agitation and heated at the rate of 2-3° per minute, usually decomposed at 189-190°. The... 

Three hundred and forty-five grams (1.7 moles) of ethyl ethoxalylpropionate, b.p., ii4-ii6°/io mm. (p. 54), is placed in a round-bottomed flask, of suitable size carrying a reflux condenser, and a thermometer is suspended from the top of the condenser into the liquid. The ethyl ethoxalylpropionate is then heated until a Aigorous evolution of carbon monoxide begins (130-150 ). The temperature of the liquid is gradually raised as the gas evolution diminishes, and finally the liquid is refluxed until no more gas comes off. The ethyl methylmalonate is then distilled. It boils at ig4-iq6 /745 mm., and the yield is 288 g. (97 per cent of the theoretical amount). 

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