Chemical compositions physical measurements

In this primer, we will use the noun sample in a very specific way. A sample will mean the physical material in total that is taken for analysis from the lot. It may consist of severai increments (individual physical portions) that may be combined and measured or measured separately and statistically averaged. The measurement from the sample will mean either the chemical or physical measurement of interest on the sample itself (the total material composited from different physical increments) or else the statistical average of the separate chemical or physical measurements on the different physical increments. In other words, the measurement from the sample will mean the value obtained from the sampling procedure or protocol and subsequent chemical and/or physical analysis that is used to estimate the true lot value. 

The physical properties of all fats and oils, including cottonseed oil, are determined by their chemical composition. Physical characteristics are of practical importance because most applications depend on the melting behavior, solubility, flavor, density, appearance and on other physical properties to provide functionality in finished products. Appropriate analytical and physical evaluation methods are used to measure these attributes for identification, trading, and control purposes. 

The inputs or incoming signals to a process control scheme can be from a variety of measurement sources. In general these can either be measurements of the process conditions or measurements of the process materials involved in the operation. Temperature, pressure and flow are the most common process conditions used. As process control becomes more demanding then chemical and physical measurements of the materials being processed or produced can be used. These include for example colour, density and chemical composition. This is the important link to chemical measurements and the spectroscopic applications, which are the subject of this section of this handbook. 

For many years, the greater portion of quantitative x-ray spectrometry was carried out using in-type analysis in which calibration standards are used to set up calibration curves and these curves then used to estimate chemical composition from measured x-ray characteristic line intensities from unknown samples. This method was essentially borrowed from the ultraviolet emission spectroscopist the latter technique was well established at the time of the development of x-ray fluorescence methods. The in-type method is still widely employed today, even with its inherent limitations and need for many calibration standards, and its great popularity probably stems from its ease of application and minimal requirement for computational facilities. More recently, a better widespread understanding of x-ray physics, plus the advent of the cheap, high-speed minicomputer, is resulting in a much broader application of the more versatile mathematical correction procedures. 

The copolymer composition equation relates the r s to either the ratio [Eq. (7.15)] or the mole fraction [Eq. (7.18)] of the monomers in the feedstock and repeat units in the copolymer. To use this equation to evaluate rj and V2, the composition of a copolymer resulting from a feedstock of known composition must be measured. The composition of the feedstock itself must be known also, but we assume this poses no problems. The copolymer specimen must be obtained by proper sampling procedures, and purified of extraneous materials. Remember that monomers, initiators, and possibly solvents are involved in these reactions also, even though we have been focusing attention on the copolymer alone. The proportions of the two kinds of repeat unit in the copolymer is then determined by either chemical or physical methods. Elemental analysis has been the chemical method most widely used, although analysis for functional groups is also employed. 

Food processing operations can be optimi2ed according to the principles used for other chemical processes if the composition, thermophysical properties, and stmcture of the food is known. However, the complex chemical composition and physical stmctures of most foods can make process optimi2ation difficult. Moreover, the quaUty of a processed product may depend more on consumer sensory responses than on measurable chemical or physical attributes. 

Process Measurements. The most commonly measured process variables are pressures, flows, levels, and temperatures (see Flow LffiASURELffiNT Liquid-levell asurel nt PressureLffiASURELffiNT Temperaturel asurel nt). When appropriate, other physical properties, chemical properties, and chemical compositions are also measured. The selection of the proper instmmentation for a particular appHcation is dependent on factors such as the type and nature of the fluid or soHd involved relevant process conditions rangeabiHty, accuracy, and repeatabiHty requited response time installed cost and maintainabiHty and reHabiHty. Various handbooks are available that can assist in selecting sensors (qv) for particular appHcations (14—16). 

Properties of steam can be divided iato thermodynamic, transport, physical, and chemical properties. In addition, the molecular stmcture and chemical composition of steam are of iaterest. It was at the start of iadustrialization, ca 1763, that thermodynamic relationships were first measured by Watt. A century later, ia 1859, Rankiae pubUshed his Manual of the Steam Engine, which gave a practical thermodynamic basis for the design and performance of steam engines. 

In photoluminescence one measures physical and chemical properties of materials by using photons to induce excited electronic states in the material system and analyzing the optical emission as these states relax. Typically, light is directed onto the sample for excitation, and the emitted luminescence is collected by a lens and passed through an optical spectrometer onto a photodetector. The spectral distribution and time dependence of the emission are related to electronic transition probabilities within the sample, and can be used to provide qualitative and, sometimes, quantitative information about chemical composition, structure (bonding, disorder, interfaces, quantum wells), impurities, kinetic processes, and energy transfer. 

Airborne particulate matter, which includes dust, dirt, soot, smoke, and liquid droplets emitted into the air, is small enough to be suspended in the atmosphere. Airborne particulate matter may be a complex mixture of organic and inorganic substances. They can be characterized by their physical attributes, which influence their transport and deposition, and their chemical composition, which influences their effect on health. The physical attributes of airborne particulates include mass concentration and size distribution. Ambient levels of mass concentration are measured in micrograms per cubic meter (mg/m ) size attributes are usually measured in aerodynamic diameter. Particulate matter (PM) exceeding 2.5 microns (/i) in aerodynamic diameter is generally defined as coarse particles, while particles smaller than 2.5 mm (PMj,) are called fine particles. 

A difficulty that should not be overlooked is that airborne particulates are rarely homogeneous. They vary greatly in size and shape, and their chemical composition is determined by factors specific to the source and location of the emissions. The combined effects and interactions of various substances mixed with particulates have not yet been established (except for sulfur dioxide), but they are believed to be significant, especially where long-term exposure occurs. Measurement techniques and their reliability may vary across regions and countries, and so may other factors, such as diet, lifestyle, and physical fitness, that influence the human health effects of exposure to particulates. 

Characterizing an FCC feedstock involves determining both its chemical and physical properties. Because sophisticated analytical techniques, such as mass spectrometry, are not practical on a daily basis, physical properties are used. They provide qualitative measurement of the feed s composition. The refinery laboratory is usually equipped to carry out these physical property tests on a routine basis. The most widely used properties are ... 

Because the quality and health aspects of foods cannot be measured by a single index, it necessarily follows that the subject of control methods in the canned food industry is very broad, and includes chemical, physical, organoleptic, and bacteriological tests, only the first of which is discussed here. The measurement of color, odor, optical clarity, texture, viscosity, and chemical composition has been used to evaluate canned foods, but in many cases the methods that are applicable to one product are either not applicable to another, or can be used only after considerable modification. 

Definition.—The heat absorbed in producing a change of physical state or chemical composition of a system, at constant temperature and pressure, is called the latent heat of the given transition, and is measured by the number of calories absorbed during the transition of unit mass of the substance from the initial to the final state. 

In order to understand the relationship between the properties of a material and its structure, which is the raison d etre of the materials scientist, three important experimental areas of investigation may be necessary. Firstly, of course, the physical or mechanical properties in question must be measured with maximum precision, then the structure of the material must be characterised (this itself may refer to the atomic arrangement or crystal structure, the microstructure, which refers to the size and arrangement of the crystals, or the molecular structure). Finally, the chemical composition of the material may need to be known. 

Primary clay is also known as residual clay, indicating that they are either the in situ residue of one type of weathered rock or the transported residue of many types of rocks most primary clay deposits occur, however, in situ, at the location where the clay particles were formed. The clay is usually quite pure and colorless or white, but very small relative amounts of minerals mixed with the clay, such as quartz and/or iron oxides, may impart to it a yellow, brown, or green color. Primary clay is also characterized by the extreme fineness of its particles, which usually measure below 2 micrometers (0.002 mm) in diameter. The more than 20 different types of primary clay minerals can be distinguished by their chemical composition, which varies widely, and by their physical properties. Primary clays that have been used for making ceramic objects are listed in Table 55. 

The thermodynamic standard state of a substance is its most stable state under standard pressure (1 atm) and at some specific temperature (usually 25°C). Thermodynamic refers to the observation, measurement and prediction of energy changes that accompany physical changes or chemical reaction. Standard refers to the set conditions of 1 atm pressure and 25°C. The state of a substance is its phase gas, liquid or solid. Substance is any kind of matter all specimens of which have the same chemical composition and physical properties. 

The standard electrode potential [1] of an electrochemical reaction is commonly measured with respect to the standard hydrogen electrode (SHE) [2], and the corresponding values have been compiled in tables. The choice of this reference is completely arbitrary, and it is natural to look for an absolute standard such as the vacuum level, which is commonly used in other branches of physics and chemistry. To see how this can be done, let us first consider two metals, I and II, of different chemical composition and different work functions 4>i and 4>ii-When the two metals are brought into contact, their Fermi levels must become equal. Hence electrons flow from the metal with the lower work function to that with the higher one, so that a small dipole layer is established at the contact, which gives rise to a difference in the outer potentials of the two phases (see Fig. 2.2). No work is required to transfer an electron from metal I to metal II, since the two systems are in equilibrium. This enables us calculate the outer potential difference between the two metals in the following way. We first take an electron from the Fermi level Ep of metal I to a point in the vacuum just outside metal I. The work required for this is the work function i of metal I. 

Retention time is the basic measure used in GC to identify compounds. It is a physical property of the analyte and is dependant on the separation conditions such as temperature, flow rate and chemical composition of the stationary phase. Solubility of the analyte in the stationary phase, which is based on the energy of intermolecular interactions between the analyte and stationary phase, is the most important factor in determining retention time. In Fig. 14.1, the retention... 

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