The Volumetric Method

In the volumetric method, the titrant can be a solution of iodine, methanol, sulfur dioxide, and an organic base, as described previously. Such a mixture is commonly known as the Karl Fischer reagent and can be purchased from any chemical vendor. It can also be a solution of iodine in methanol solvent. In that case, a Karl Fischer solvent containing the other required components is needed for the titration vessel. 

FIGURE 14.14 Photographs of a typical volumetric unit. The complete unit is shown on the left and a close-up of the titration vessel on the right. The dual platinum wire probe is visible in the right photograph as is the tube (dark in color) that introduces the titrant to the solution. 

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Two parameters must be measured to apply the BET equation, the pressure at the sample and the amount adsorbed at this pressure. There are tlnee conmron methods for measuring the amount of gas adsorbed, called the volumetric method, the gravimetric method and the dynamic method, of which the volumetric method is the connnonest [21],... 

This method is smiple but experimentally more cumbersome than the volumetric method and involves the use of a vacuum microbalance or beam balance [22], The solid is suspended from one ann of a balance and its increase in weight when adsorption occurs is measured directly. The dead space calculation is thereby avoided entirely but a buoyancy correction is required to obtain accurate data. Nowadays this method is rarely used. 

A number of potential sources of error must be taken into account. In the volumetric method the following items need attention (a) constancy of the level of liquid nitrogen (b) depth of immersion of the sample bulb ( S cm) (c) temperature of sample (monitoring with vapour pressure thermometer close to sample bulb) (d) purity of adsorptive (preferably 99-9 per cent) (e) temperature of gas volumes (doser, dead space), controlled to 01 C. 

Isotherm measurements of methane at 298 K can be made either by a gravimetric method using a high pressure microbalance [31], or by using a volumetric method [32]. Both of these methods require correction for the nonideality of methane, but both methods result in the same isotherm for any specific adsorbent [20]. The volumetric method can also be used for measurement of total storage. Here it is not necessary to differentiate between the adsorbed phase and that remaining in the gas phase in void space and macropore volume, but simply to evaluate the total amount of methane in the adsorbent filled vessel. To obtain the maximum storage capacity for the adsorbent, it would be necessary to optimally pack the vessel. 

Air content of freshly mixed concrete by the pressure method Air content of freshly mixed concrete by the volumetric method Unit weight, yield, and air content of concrete Specific gravity, absorption, and voids in hardened concrete Resistance of concrete to rapid freezing and thawing Scaling resistance of concrete surfaces exposed to deicing chemicals... 

Few studies have been made of benzene chemisorption by the volumetric method. Zettlemoyer et al. (8) have examined the adsorption of benzene vapor at 0°C on powders of nickel and of copper. First, the monolayer coverage of argon (vm) A, was measured. The argon was then removed by pumping and the amount of benzene required to form a monolayer, (vmi) Bz, was measured. Weakly adsorbed benzene was then removed by pumping, after which further benzene adsorption provided the value (vm2) Bz. Some results are reproduced in Table I. On the assumption that the same extent of surface is accessible both for argon and for benzene adsorption, it is clear that complete monolayers of benzene were not achieved, that some (Ni) or all (Cu) of the benzene was adsorbed reversibly. It was considered that only the irreversibly adsorbed benzene was chemisorbed, the remainder being physically adsorbed. Thus chemisorption of benzene on copper appeared not to occur. The heat of adsorption of benzene on nickel at zero... 

There are two general ways by which the titration can take place. One is the volumetric method, in which the titrant is added to the sample via an automatic titrator. In this case, the titrant is either a mixture of all of the reactants above (a composite titrant) or an iodine solution (other components already in the... 

The volumetric method is used (rather than the coulometric method) when the water content is higher (greater than about 1%). 

The critical datum is not a buret reading, as it was in the case of the volumetric method. Rather, the amount of iodine used is determined coulometrically by computing the coulombs (total current over time) needed to reach the end point. The coulombs are calculated by multiplying the current applied to the anode-cathode assembly (a constant value) by the total time (seconds) required to reach the end point. The modern coulometric titrator automatically computes the amount of moisture from these data and displays it. 

Explain how a Karl Fischer titrant is standardized in the volumetric method. 

In the volumetric method, the titrant is added from an external reservoir. In the coulometric method, the titrant (iodine) is generated internally via an electrochemical reaction. 

In some cases (see below) a KF drying oven is required to get the water from a sample into the titration vessel. For these special cases, a solid sample (usually) is placed into a specially designed KF oven where the sample is heated, and the water goes into the vapor phase. A stream of dry carrier gas (usually, N2 or air) sweeps the liberated moisture into the reaction vessel, where it is titrated by either the coulometric or the volumetric method. It is critical that the carrier gas is dry and that there are no leaks along the pathway to the reaction vessel. Passing the carrier gas over activated molecular sieve prior to the sample will ensure that the gas is dry. 

Some of the volumetric methods described above may also be adapted to the electrometric determination of arsenic. Such methods have been described for titration of arsenites with ceric sulphate,9 iodine in the presence of sodium bicarbonate,10 chloramine (p-toluene-sulphone chloramide),11 alkaline potassium ferricyanide solution,12 potassium bromate13 or potassium iodate14 in the presence of hydrochloric acid, silver nitrate15 (by applying a secondary titration with 01N alkali to maintain the desired low H+-ion concentration), and with... 

More rapidly, and in most cases with sufficient accuracy, the invert sugar may be determined by the volumetric method as described in the general methods, but the influence of the large excess of saccharose on the reducing power is then neglected. 

If the volumetric method is used, several flasks are charged each with 10 c.c. of Fehling s solution and 40 c.c. of water, the procedure given on... 

If, however, the volumetric method is used, the procedure already indicated (see General methods, p. 112) is followed, the duration of boiling being 4 minutes. The result is calculated either as maltose (coefficient, 0 741) or as glucose (coefficient, 0 4945). 

Subsequent work on catalyst characterization via reduction studies has involved gravimetric or volumetric techniques. The gravimetric method measures weight loss (due to O loss), while the volumetric method measures amount of H2 consumed [and amount of water formed (O loss)]. In the latter case, the amount of H retained by the catalyst can also be determined (20). The gravimetric technique has also been applied to sulfided catalysts (27). From these measurements, the stoichiometric state of the catalyst can be determined. 

Specific surface areas were determined by the volumetric method of adsorption of argon at liquid N2 temperature, using the BET equation (Parfitt and Sing, 1976). 

The volumetric method is mainly used for the purpose of determining specific surface areas of solids from gas (particularly nitrogen) adsorption measurements (see page 134). The gas is contained in a gas burette, and its pressure is measured with a manometer (see Figure 5.4). All of the volumes in the apparatus are calibrated so that when the gas is admitted to the adsorbent sample the amount adsorbed can be calculated from the equilibrium pressure reading. The adsorption isotherm is obtained from a series of measurements at different pressures. 

Following the OOS result for Lot Y, the test method was updated to allow for a gravimetric sample measurement (weighing) in place of the volumetric method (pipetting). 

The application of the volumetric method for the determination of adsorption isotherms can be performed as follows. First, the adsorbate gas is introduced into the manifold volume, V and the quantity dosed is measured, usually in cubic centimeters at STP, that, is, standard temperature and pressure, specifically 273.15 K and 1.01325 x 10 Pa... 

The home-made heat-flow calorimeter used consisted of a high vacuum line for adsorption measurements applying the volumetric method. This equipment comprised of a Pyrex glass, vacuum system including a sample holder, a dead volume, a dose volume, a U-tube manometer, and a thermostat (Figure 6.3). In the sample holder, the adsorbent (thermostated with 0.1% of temperature fluctuation) is in contact with a chromel-alumel thermocouple included in an amplifier circuit (amplification factor 10), and connected with an x-y plotter [3,31,34,49], The calibration of the calorimeter, that is, the determination of the constant, k, was performed using the data reported in the literature for the adsorption of NH3 at 300 K in a Na-X zeolite [51]. 

SYSTEMS FOR THE AUTOMATIC MEASUREMENT OF SURFACE AREA AND POROSITY BY THE VOLUMETRIC METHOD... 

In Figure 6.22a, the MPSD of the DAY zeolite calculated with the help of the SF method is reported. The DAY zeolite has an FAU framework, with a 12MR three-dimensional channel system with apertures of 7.4 A (see Section 2.5.1) [83], The obtained PSD for the DAY zeolite shows a clear maximum at about 7 A. In Figure 6.22b, the S-F MPSD of the SWCNT is reported [80], The calculated PSD for the SWCNT indicates that this material has a slightly heterogeneous distribution of pores with a clear, and highly populated, maximum at 13.5 A [80], The adsorption study, in both cases, was carried out using N2 adsorption at 77 K isotherms, obtained with a Quantachrome Autosorb-1 equipment for adsorption isotherm determination by the volumetric method [21],... 

If the solid contains no macropores, the isotherm remains nearly horizontal over a range of p/p° approaching unity and the total pore volume is well-defined. In the presence of macropores the isotherm rises rapidly near p/p° = 1 and if the macropores are very wide may exhibit an essentially vertical rise. The limiting adsorption at the top of the steep rise can be identified reliably with the total pore volume only if the temperature on the sample is very carefully controlled and there are no cold spots in the apparatus (which lead to bulk condensation of the gas and a false measure of adsorption in the volumetric method). 

The static methods arc volumetric or gravimetric. The volumetric method involves the use of a vacuum system comprising two sections, a dosing section which allows the introduction of accurately measured quantities of the adsorbate, and a sample section which contains the catalyst. The precision of the volumetric method depends on accurate calibrations of volumes. 

Apparatus for measuring adsorption isotherms by the volumetric method. 

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