Hydroquinone concentrations

A second investigation included an observation of an increase in absorption at 470 nm on mixing the reactants suggesting fast complex formation this absorption then decayed with first-order kinetics and with a rate linearly dependent upon hydroquinone concentration. k2 decreases with increasing acidity in a manner indicating the complex between MnOH " and hydroquinone to be the exclusive reactant. E2 = 14.0+0.7 kcal.mole and AS = 7.5+ 2.1 eu. 

Figure 10.22 Effect of increasing hydroquinone concentration on the relative stain removal after 60 min at pH 10.5 [233]...

The 02-oxidation of hydroquinone into quinone, which is very slow in the absence of a catalyst, was found to be accelerated by the addition of the ce-pyrrolinonate-bridged Pt(2.5 + )4 (19) (117). The detailed kinetic investigation revealed that the Pt(2.0+)2 species formed according to Eq. (1) plays a major role as the catalyst. The reaction rate of the quinone formation is higher than that of 02 oxidation of Pt(2.0+)2 into Pt(3.0 + )2 and was found to be rather linear to the hydroquinone concentration. Therefore, it was suggested that the quinone formation proceeds via a certain intermediate formed between the Pt(2.0+)2 species and molecular oxygen (e.g., peroxo species). The possible schematic mechanism is illustrated in Eq. (12). 

For any particular silver ion and hydroquinone concentration, the rate is proportional to the amount of nuclear sol added. When reaction... 

The reaction rate in slightly acid solution (pH range of 5.15 to 6.27) is proportional to the hydroquinone concentration and to the %rds power of the silver ion concentration. The variation of rate with pH shows that both the non-ionized and the singly-ionized hydroquinone are active in this pH range. The reaction is well represented by the equation. 

The reaction rate at pH = 9 could be followed only when the silver ion concentration was kept to a low value. This was accomplished by using as a source of silver ions the soluble silver sulfite complex ion which has a dissociation constant of about 3 X 10-9 at 25°. The undissociated complex itself is not involved in the reaction to any significant extent. The reaction rate varies as about the half power of the silver ion concentration under these conditions. The dependence upon the hydroquinone concentration, as indicated by the data in Table I, is somewhat greater than a direct proportionality (James, 7). 

There are indications that another type of catalysis is present in the reaction between hydroquinone and silver ions in alkaline solution. The increase of rate with increasing hydroquinone concentration is greater than direct proportionality. This situation is similar to that observed in the oxygen oxidation of durohydroquinone (tetramethylhydroquinone) (James and Weissberger, 16) where the quinone formed in the reaction catalyzes subsequent oxidation. A direct check on quinone catalysis of the hydroquinone-silver ion reaction was not made, since quinone is unstable in alkaline solution, particularly in the presence of sulfite which reacts with it. Experiments were made, however, on the reaction between durohydroquinone and silver ion. This reaction shows the same dependence of rate upon the square root of the silver ion concentration as the hydroquinone reaction does. Addition of duroquinone to the reaction mixture produces a definite acceleration, as shown in Table II. 

Approximately 0.7-pm diameter MIP beads with hydroquinone recognition sites have been prepared using similar one-step precipitation polymerization (Table 6) [183], These beads were immobilized in an agarose gel film deposited on GCE. The resulting MIP-modified electrode has been used as a chronoamperometric chemosensor for determination of hydroquinone. The current response of the chemosensor linearly increased with the hydroquinone concentration in the range of 2-100 pM and LOD for hydroquinone was 1 pM. 

Table 6. Effect of H202 and hydroquinone concentration on molecular mass of grafted PAN chains ...

Both evidence of the browning caused by the high-intensity absorption of conjugated chromophoric groups and upward shifts in the ultraviolet-visible spectral baselines indicate that higher hydroquinone concentrations produce more polymers. Free radical coupling is probably involved in the polymerization since the existence of free radicals is correlated to hydroquinone concentration. The ultraviolet-visible absorption profiles of these polymers formed in the systems that are not sterilized (Kung and McBride, 1988) appear to be similar to those of phenol-derived polymers in the systems free... 

The concentration dependence of the oxidation current at -i-lOO mV shows threshold characteristics. In the absence of lactate, addition of hydroquinone up to 1 mM does not lead to a typical current increase. Obviously the laccase converts its substrate completely to benzo-quinone, which is not detectable at this potential. Above 1.2 mM hydroquinone the current increase reflects the breakthrough of unreacted substrate. In the presence of lactate part of the benzoquinone formed in the laccase-catalyzed reaction is recycled to hydroquinone. Therefore, the threshold is found at lower hydroquinone concentrations. 

The effect of copolymer composition on the rate of the hydroquinone oxidation reaction has been studied using IVI-AA copolymers. Curves of the change of the oxidized hydroquinone concentration as a function of reaction time in the presence of polyampholyte-copper(II) complexes are presented in Fig. 10. An increase in the amount of basic groups in the copolymers leads to an increase in the rate of hydroquinone oxidation. Copolymers in which maleic and methacrylic acids are used as a second comonomer of IVI, exhibit lower activity than IVI-AA. 

Hydroquinone, an important xenobiotic micropollutant, could be detected by photoelectrochemical sensing due to a porphyrin/Au nanopariicles/graphene nanocomposite upon white-Ught illumination, hydroquinone was oxidized, generating photocurrent, the intensity of which increased with the increase of hydroquinone concentration [124]. 

Action of bromine water. To a concentrated aqueous solution of the phenol or to the phenol itself, add bromine water gradually. At first the bromine is decolorised and then on adding an excess a white or yellowish-white precipitate of a polybromo-derivative is produced with all except catechol, hydroquinone, i- and 2 naphthol. 

Dihydroxyacetophenone. Finely powder a mixture of 40 g. of dry hydroquinone diacetate (1) and 87 g. of anhydrous aluminium chloride in a glass mortar and introduce it into a 500 ml. round-bottomed flask, fitted with an air condenser protected by a calcium chloride tube and connected to a gas absorption trap (Fig. II, 8, 1). Immerse the flask in an oil bath and heat slowly so that the temperature reaches 110-120° at the end of about 30 minutes the evolution of hydrogen chloride then hegins. Raise the temperature slowly to 160-165° and maintain this temperature for 3 hours. Remove the flask from the oil bath and allow to cool. Add 280 g. of crushed ice followed by 20 ml. of concentrated hydrochloric acid in order to decompose the excess of aluminium chloride. Filter the resulting solid with suction and wash it with two 80 ml. portions of cold water. Recrystallise the crude product from 200 ml. of 95 per cent, ethanol. The 3 ield of pure 2 5-dihydroxyacetophenone, m.p. 202-203°, is 23 g. 

I) Hydroquinone dIacetate may be prepared as follows. Add I drop of concentrated sulphuric acid to a mixture of 55 g. of hydroquinone and 103 g. (05-5 ml.) of A.R. acetic anhydride in a 500 ml. conical flask. Stir the mixture gently by hand it warms up rapidly and the hydroquinone dissolves. After 5 minutes, pour the clear solution on to 400 ml. of crushed ice. Alter with suction and wash with 500 ml. of water. Recrystallise the solid from 50 cent, ethanol by weight (ca. 400 ml. are required). The yield of pure hydroquinone diacetate, m.p. 122°, is 89 g. 

Reduction to hydroquinone. Dissolve, or suspend, 0-5 g. of the quinone in 5 ml. of ether or benzene and shake vigorously with a solution of 1 0 g. of sodium hydrosulphite (Na2S204) in 10 ml. of N sodium hydroxide until the colour of the quinone has disappeared. Separate the alkaline solution of the hydroquinone, cool it in ice, and acidify with concentrated hydrochloric acid. Collect the product (extract with ether, if necessary) and recrystalhse it from alcohol or water. 

Krypton clathrates have been prepared with hydroquinone and phenol. 85Kr has found recent application in chemical analysis. By imbedding the isotope in various solids, kryptonates are formed. The activity of these kryptonates is sensitive to chemical reactions at the surface. Estimates of the concentration of reactants are therefore made possible. Krypton is used in certain photographic flash lamps for high-speed photography. Uses thus far have been limited because of its high cost. Krypton gas presently costs about 30/1. 

Noncnzymc-Catalyzcd Reactions The variable-time method has also been used to determine the concentration of nonenzymatic catalysts. Because a trace amount of catalyst can substantially enhance a reaction s rate, a kinetic determination of a catalyst s concentration is capable of providing an excellent detection limit. One of the most commonly used reactions is the reduction of H2O2 by reducing agents, such as thiosulfate, iodide, and hydroquinone. These reactions are catalyzed by trace levels of selected metal ions. Eor example the reduction of H2O2 by U... 

In normal practice, inhibitors such as hydroquinone (HQ) [123-31 -9] or the monomethyl ether of hydroquinone (MEHQ) [150-76-5] are added to acrylic monomers to stabilize them during shipment and storage. Uninhibited acrylic monomers should be used prompdy or stored at 10°C or below for no longer than a few weeks. Improperly iahibited monomers have the potential for violent polymerizations. HQ and MEHQ require the presence of oxygen to be effective inhibitors therefore, these monomers should be stored in contact with air and not under inert atmosphere. Because of the low concentration of inhibitors present in most commercial grades of acrylic monomers (generally less than 100 ppm), removal before use is not normally required. However, procedures for removal of inhibitors are available (67). 

Although considered an active participant in the process cycle, the tetrahydroaLkylanthraquinone (10) may not be a significant part of the catalytic hydrogenation because, dependent on the concentration in the working solution, these could all be converted to the hydroquinone by the labile shift per equation 17 and not be available to participate. None of the other first- or second-generation anthraquinone derivatives produce hydrogen peroxide, but most are susceptible to further reaction by oxidative or reductive mechanisms. 

Because the reaction takes place in the Hquid, the amount of Hquid held in the contacting vessel is important, as are the Hquid physical properties such as viscosity, density, and surface tension. These properties affect gas bubble size and therefore phase boundary area and diffusion properties for rate considerations. Chemically, the oxidation rate is also dependent on the concentration of the anthrahydroquinone, the actual oxygen concentration in the Hquid, and the system temperature (64). The oxidation reaction is also exothermic, releasing the remaining 45% of the heat of formation from the elements. Temperature can be controUed by the various options described under hydrogenation. Added heat release can result from decomposition of hydrogen peroxide or direct reaction of H2O2 and hydroquinone (HQ) at a catalytic site (eq. 19). 

In humans, cases of dermatitis have been described after contact with DHBs. Combined exposure to hydroquinone and quinone airborne concentrations causes eye irritation, sensitivity to light, injury of the corneal epithelium, and visual disturbances (126). Cases with an appreciable loss of vision have occurred (127). Long-term exposure causes staining due to irritation or allergy of the conjunctiva and cornea and also opacities. Resorcinol and catechol are also irritants for eyes. 

AH operations producing dust require the usual measures to prevent dust in the atmosphere exceeding the allowable daily concentration. If this is not feasible, personal protection devices should be used. Especially when hydroquinone is present as a powder, adequate eye protection should be provided. 

Hydroquinone [123-31 -9] represents a class of commercially important black-and-white chemical reducing agents (see Hydroquinone,RESORCINOL, AND catechol). The following scheme for silver haUde development with hydroquinone shows the quantitative importance of hydrogen ion and haUde ion concentrations on the two half-ceU reactions that describe the silver—hydroquinone redox system ... 

Because of the presence of an extended polyene chain, the chemical and physical properties of the retinoids and carotenoids are dominated by this feature. Vitamin A and related substances are yellow compounds which are unstable in the presence of oxygen and light. This decay can be accelerated by heat and trace metals. Retinol is stable to base but is subject to acid-cataly2ed dehydration in the presence of dilute acids to yield anhydrovitamin A [1224-18-8] (16). Retro-vitamin A [16729-22-9] (17) is obtained by treatment of retinol in the presence of concentrated hydrobromic acid. In the case of retinoic acid and retinal, reisomerization is possible after conversion to appropriate derivatives such as the acid chloride or the hydroquinone adduct. Table 1 Hsts the physical properties of -carotene [7235-40-7] and vitamin A. 

At the end of the addition, an almost colorless ether layer swims on the surface of the strongly colored water layer. After removal of the ether layer, the water layer is concentrated to dryness under vacuum and a stream of an inert gas. An earthy precipitate is formed, which after recrystallization yields 100 grams of hydroquinone calcium sulfonate, which decomposes without melting above 250°C. 

Hydronium ion, 187 concentration calculation, 192 concentration and pH, 190 model, 186 Hydroquinone, 345 Hydrosphere, 437 composition, 439 Hydroxide ion, 106, 180 Hydroxides of lhird row, 371 Hydroxylamine, 251 Hydroxyl group, 329 Hypobromiie ion, 422 Hypochlorite ion, 361 Hypochlorous acid, structure, 359 Hypophosphorous acid, 372 Hypothesis, Avogadro s, 25, 52... 

A derivative of the triple-blend formulation to include DEHA for control of 02 (resulting from air in-leakage to the condensate line) is shown next. The concentration of DEHA must not be too high, and the blending process requires careful control because of the limited solubility of hydroquinone. 

Azelaic acid is a non-phenolic derivative (1,7-hep tanedicarboxylic acid) used at concentration of 10-20% twice a day to treat melasma with minimal side effects (allergic reactions). It acts to disturb the tyrosinase synthesis and can be used as a bleaching agent in patients sensitive to hydroquinone. Better results are obtained if a glycolic acid cream is applied sequentially to azelaic acid treatment. 

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