Measurement units, standard

Hundreds of chemical species are present in urban atmospheres. The gaseous air pollutants most commonly monitored are CO, O3, NO2, SO2, and nonmethane volatile organic compounds (NMVOCs), Measurement of specific hydrocarbon compounds is becoming routine in the United States for two reasons (1) their potential role as air toxics and (2) the need for detailed hydrocarbon data for control of urban ozone concentrations. Hydrochloric acid (HCl), ammonia (NH3), and hydrogen fluoride (HF) are occasionally measured. Calibration standards and procedures are available for all of these analytic techniques, ensuring the quality of the analytical results... 

Although time as a physical or philosophical concept is an extremely subtle quantity, in chemical kinetics we adopt a fairly primitive notion of time as a linear fourth dimension (the first three being spatial dimensions) whose initial value (t = 0) can be set by the experimenter (for example, by mixing two reactant solutions) and whose extent is accurately measurable in standard units. The time dimension persists as a variable until the experimenter stops observing the reaction, or until... 

In physics, the word specific implies a ratio. Weight is the measure of the earth s attraction for a body, which is called gravity. Thus, the ratio of the weight of a unit volume of some substance to the weight of an equal volume of a standard substance, measured under standard pressure and temperature, is called specific gravity. The terms specific weight and specific density are also sometimes used to express this ratio. 

The International System of Units (SI) provides a coherent system of measurement units, and all the physical quantities required for refrigeration and air-conditioning can he derived from the basic standards ... 

Ratio between the volumes, measured at standard conditions, of the gas and oil produced simultaneously (units m /m or cu.ft./barrel). The English acronym GOR is also used in France. 

Designers, manufacturers, and operators of boilers continue to use many of these terms, without undue deference to unit standardization, to define, measure, and report on plant steam-raising capacities power output) and operating parameters. (In continuance of this common practice therefore, many of these various terms are freely used in discussions throughout this book.) However, to familiarize the reader and minimize confusion, some energy terms and notes are provided here. A more complete list of units and conversion factors is provided in the appendix. 

There is often some confusion between the terms standards and reference materials. Primary standards represent the top-tier of chemical standards and, in principle, provide a means of establishing the traceability of analytical data to the SI measurement units (e.g., the kilogram, mole, meter, and second). A limited number of pure chemicals are recognized as primary standards (and thus can constitute certified reference materials). Most certified reference materials are not of themselves primary standards rather, the property values assigned to them are traceable to primary standards where practical. 

The midpoint potential of a half-reaction E, is the value when the concentrations of oxidized and reduced species are equal, [Aox] = [Aredl- In biological systems the standard redox potential of a compound is the reduction/oxidation potential measured under standard conditions, defined at pH = 7.0 versus the hydrogen electrode. On this scale, the potential of 02/water is +815 mV, and the potential of water/H2 is 414 mV. A characteristic of redox reactions involving hydrogen transfer is that the redox potential changes with pH. The oxidation of hydrogen H2 = 2H + 2e is an m = 2 reaction, for which the potential is —414 mV at pH 7, changing by 59.2 mV per pH unit at 30°C. 

Standard electrode potential, An electrode potential measured at standard temperature and pressure, where all products and reagents are present at unit activity. 

Turbidity is caused by any solid material which is dissolved or suspended in a liquid. The intensity of light scattered by a sample is measured and compared with that measured for standard formazin suspensions, and expressed as nephelometric turbidity units (NTU). Colour is determined as the absorbance (measured spectrophotometrically at 400 nm) of the sample filtered through a 0.4S micron pore size membrane filter [12, 13]. 

Note The activity of a catalyst is usually defined as a rate per unit surface area or per site, measured at standard experimental conditions. The selectivity is then a (normalized) ratio of rates or product concentrations in different reactions running simultaneously. 

The limit of quantification is numerically equivalent to six times the standard deviation of the measured unit when applying the analytical procedure to 20 representative blank samples. For recoveries less than 100% the limit of quantification must be corrected for the average recovery of the analyte. 

One may study zone broadening in gas chromatography by observing the shape of the elution peak which is Gaussian in ideal systems. The base width of the Gaussian curve is measured in standard deviation units, therefore... 

The heat capacity of coal can be measured by standard calorimetric methods that have been developed for other materials (e.g., ASTM C-351). The units for heat capacity are Btu per pound per degree Fahrenheit (Btu/lb-0F) or calories per gram per degree Celsius (cal/g °C), but the specific heat is the ratio of two heat capacities and is therefore dimensionless. 

Let us begin with the ISO definition [9] A calibration is a set of operations that establish, under specified conditions, the relationship between values of quantities indicated by a measuring instrument or measuring system, or values represented by a material measure or reference material, and the corresponding values realized by standards. Applied to amount measurements, the standards would then be the values assigned to the RMs (of defined composition) at the stated uncertainty relative to the true value of the property, expressed in SI units, or relative to an internationally recognized, certified standard RM for the relevant property, range, and matrix composition. 

The importance of using common reference scales has been recognised for centuries. For example, in England, King John introduced consistent measures throughout the land in 1215. Other countries also had their own measurements scales standards. Many city museums show the standard measures used for trade within the city or local state. As trade widened so did the need for comparability of measurement results and the use of common units widened. The many different measurements scales were harmonised with the introduction of the metric system and the SI units under the Convention of the Metre signed in 1875. An excellent summary of the historical development of units of measurement is given in the NBS Special Publication 420 [1], Under the Convention of the Metre a hierarchical chain of national and international measurement standards has been developed for the measurement of most of physical quantities. 

Mass measurements of objects are carried out with the laboratory balance. Many types of balances are available for laboratory use. The proper choice of a balance depends upon what degree of accuracy is needed for a measurement. The standard units of mass are the kilogram (kg) in the SI system and the gram (g) in the metric system. Some conversion factors are listed below. 

The standard deviation of a set of data, usually given the symbol 5, is the square root of the variance. The difference between standard deviation and variance is that the standard deviation has the same units as the data, whereas the variance is in units squared. For example, if the measured unit for a collection of data is in meters (m) then the units for the standard deviation is m and the unit for the variance is m2. For large values of n, the population standard deviation is calculated using the formula ... 

If you look at the history of measurement, it shows constant attempts to better define and refine our measurement units. Early measurement standards were arbitrary such as the Egyptian cubit (the tip of the finger to the elbow). Since then we have tried to base measurement standards on non-changing, consistent, and repeatable standards, some of which turned out to be inconsistent and/or impractical. The work of the metrologist will never be complete. The more accurately we can define our measurement standards, the better we can measure the properties of our universe. 

There are several general rules about making measurements. These rules are standard regardless of the type of equipment being used, material being studied, or the measurement units being used. 

National Institute of Standards and Technology (U.S.) http //physics.nist.gov/cuu/Units/unitsJitml National Physical Laboratory (UK) http //www.npl.co.uk/reference/measurement-units/... 

The microtiter plate array format involves immobilizing carbohydrates in wells of 96-, 384-, or 1536-well microtiter plates (see Fig. la). Each carbohydrate component is spatially separated from other components within the plate. Two of the primary advantages of a microtiter plate format are cost and simplicity. Carbohydrates can be distributed into wells using multichannel pipettors, and assay results can be measured using standard plate readers. Thus, the equipment and supplies needed for the array are relatively inexpensive and common. However, microtiter plates generally require larger amounts of each carbohydrate and can accommodate a smaller total number of components per support unit. 

The uncertainty in the pHj values for the seven primary standards in Table 3-1 (not including tetroxalate or calcium hydroxide) is judged to be about 0.006 pH unit at 25°C. At pH values much less than 3 or more than 11, or at temperatures other than 25°C, the uncertainties become greater. As to the significance of measured pH values, it may be concluded that, at best, pH may be regarded as an estimate of —log flH+ or lo.g [H ]yH+- The validity of the estimate depends on how constant the liquid-junction potential remains during measurement of standard and unknown... 

For cations and anions generally, the assumption that liquid-junction potentials are the same in the measurement of standards and unknowns is less likely to be valid than for pH measurements. It has been suggested that a quantity A ) expressed in pM or pA units be included in (13-26) and (13-27) to correct for changes injunction potential arising from differences in ionic strengths of standard and test solutions. Alternatively, these effects could be eliminated through the use of two reference half-cells composed of electrodes without liquid-junction potentials. For example, if the test solution contained chloride ion, both reference half-cells could be Ag/AgCl, and the liquid-junction potential would be eliminated. In practice, external reference half-cells without liquid junction are not always convenient. 

When all species in a cell reaction are in their standard states (pure solids, unit standard concentrations, etc.) the measured electromotive force (emf) of the... 

---