Sulfuric acid solutions, relative humidity

Equilibrium moisture content of a hygroscopic material may be determined in a number of ways, the only requirement being a source of constant-temperature and constant-humidity air. Determination may be made under static or dynamic conditions, although the latter case is preferred. A simple static procedure is to place a number of samples in ordinaiy laboratoiy desiccators containing sulfuric acid solutions of known concentrations which produce atmospheres of known relative humidity. The sample in each desiccator is weighed periodically until a constant weight is obtained. Moisture content at this final weight represents the equilibrium moisture content for the particular conditions. 

It is possible to control the relative humidity with sulfuric acid solutions of particular concentrations (Tab. 26) or with saturated salt solutions in contact with excess salt (Tab. 27). These liquids are placed in a conditioning chamber... 

Saturated salt solutions and sulfuric acid solutions establish relative humidity by reducing the vapor pressure above an aqueous solution (a colligative effect). Saturated salt solutions at controlled temperature maintain a constant relative humidity as long as excess salt and bulk solution are present. As water is added or removed from the solution, moisture from the head-space will either condense or evaporate (as appropriate), with subsequent dissolution or precipitation of salt to maintain the equilibrium vapor pressure. Because the degree of vapor pressure depression is dependent on the number of species in solution and, further, since the solubility of most salts is somewhat dependent on temperature, the relative humidity generated is also temperature dependent. Hence, use of the same salt at different temperatures can result in different relative humidities. Refs. can be consulted for specific saturated salt solutions that result in defined relative humidities as a function of... 

Hvgrometry related evolution. We have studied the samples of o.33[Cu-Cr-S04] at 21 C in relation to the relative humidity (%RH) of the ambient atmosphere. The hygrometry was controlled via sulfuric acid solutions or phosphor pentoxide as a drying agent. Figure 7-24 shows the evolution of interlamellar spacings for the IH and 3R polytypes. The samples are biphased, at least between 30%RH and 50%RH, and in this hygrometry domain the ratio of the two polytypes evolves. Even in a dry... 

RELATIVE HUMIDITY. The percentage of water saturation of the ambient air or chamber atmosphere at a given temperature. A relative humidity of 40-65% is normally used for TLC, and it is important to control humidity for consistent results. Control of humidity is achieved by placing a saturated salt solution or a sulfuric acid solution of specified concentration inside the chamber. 

Copper(I) chloride is insoluble to slightly soluble in water. SolubiUty values between 0.001 and 0.1 g/L have been reported. Hot water hydrolyzes the material to copper(I) oxide. CuCl is insoluble in dilute sulfuric and nitric acids, but forms solutions of complex compounds with hydrochloric acid, ammonia, and alkaU haUde. Copper(I) chloride is fairly stable in air at relative humidities of less than 50%, but quickly decomposes in the presence of air and moisture. 

Note Instead of using the potassium nitrate solution, Wilson (Ref 1) recommends the use of aqueous solutions of sulfuric acid. The following table gives the strength of H2SO4 required to give different relative humidities (RH) at various temperatures... 

Bindings covered with vegetable-tanned leather are washed with pure toilet soap, saddle soap, or mild, high quality surgical toilet soap and a 7-10% solution of potassium lactate. 0.25% of p-nitrophenol can be added as a fungicide, but this is not necessary if the temperature and relative humidity are constantly satisfactory (see p. 5). The lactate, when applied to new vegetable-tanned leather, is thought to protect the leather from the deleterious action of sulfuric acid which usually forms in leather when sulfur dioxide is absorbed. It is. probable that it has little efficacy once chemical deterioration has started, and for some years doubt has been cast on its effectiveness, even on new leather, despite favorable results in laboratory tests. However, it is used, and its use is advocated just in case it is beneficial. 

Figure 21. Mole fraction of H2SO4 (xh so ) in a spherical drop of sulfuric acid and water as a function of relative humidity and drop diameter. The weight percent (wt %) of H2SO4 in solution is related to the mole fraction by wt % = MWH2SO4 / [(MWh2o(1/xh2S04 1) + MWH2SO4] X 100%, where MWH2SO4 and MWh20 are the molecular weights of sulfuric acid and water, respectively. From Seiirfeld and Pandis (1998). Used by permission of Wiley-Interscience.

For binary solutions such as sulfuric acid and water, droplets may be distributed with respect to size, but at equilibrium all have the same composition unless the Kelvin effect is important as discussed in a later section. For ternary mixtures, the situation is more complicated the same droplet size may result from different chemical compositions in equilibrium at a given relative humidity. 

Example Taking the Kelvin effect into account, determine the percentage increase in the sulfuric acid concentration of a 0.05-jum-diameter aqueous solution droplet compared with a solution with a planar surface. The temperature is 25°C and the relative humidity is 40%. Assume that the sulfuric acid is nonvolatile. This particle size falls within the size range of sulfuric acid droplets emitted by automobiles equipped with catalytic converters. Other data ... 

At a relative humidity of 40%, p = 0.4(23,8 = 9.52 mm, By linear interpolation in the table, this corresponds to a 47.9% solution of sulfuric acid with infinite radius of curvature (planar surface). By (9.17)... 

Inorganic oxide layers can be adjusted to a defined activity level by exposure to a defined gas phase in an enclosed chamber. This is best performed after sample application in a developing chamber that allows both conditioning and development of the layer in the same chamber (e.g. a twin trough chamber). Alternatively, separate conditioning and development chambers can be used. Atmospheres of different constant relative humidity can be obtained by exposure to the vapor phase in equilibrium with solutions of concentrated sulfuric acid or saturated solutions of various salts [100]. In the same way, acid or base deactivation is carried out by exposure to concentrated ammonia or hydrochloric acid fumes. 

On the other hand, the reference salts method can indirectly determine the value of a product using graphic interpolation. This method consists in introducing an aliquot of the sample in an airtight container where different saturated saline solutions or dilutions of sulfuric acid are placed, creating a relative humidity and therefore a known value. Once equilibrium has been reached, the samples are weighed and the weight variations are represented... 

Two tests [2] are used to determine the porosity of gold contacts used in outdoor equipment (1) a nitric acid vapor test (exposure of gold-plated specimens in a desiccator containing 300 mL nitric acid for 1 h at 24 3°C and a maximum relative humidity of 60 %) tmd (2) an alkaline polysulfide test (the specimen is immersed for 60 s in a solution made of sodium sulfide, water, and excess flower of sulfur, filtered and diluted to a specific gravity of 1.142, and made alkaline with sodium hydroxide). 

Good activity control is obtained by plate development in a chamber equilibrated with the vapors of a humidity-controlling solution such as aqueous sulfuric acid or a saturated salt solution. A solution of 50% sulfuric acid will provide a relative humidity of —40%, which has proven optimum for reproducibility. In addition, Hahn-Deinstrop (1993) has tubulated saturated salt solutions that will establish constant relative humidities ranging from 15 to 95% in closed chambers. These solutions can be used by placement in either section of a twin-trough chamber or in the inner portion of a horizontal chamber (Chapter 7). 

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