Primary standard, defined

Calibrating the electrode presents a third complication since a standard with an accurately known activity for H+ needs to be used. Unfortunately, it is not possible to calculate rigorously the activity of a single ion. For this reason pH electrodes are calibrated using a standard buffer whose composition is chosen such that the defined pH is as close as possible to that given by equation 11.18. Table 11.6 gives pH values for several primary standard buffer solutions accepted by the National Institute of Standards and Technology. 

Because variations in accurate isotope ratio measurements typically concern only a few parts per 1000 by mass and there are no universal absolute ratios, it is necessary to define some standards. For this purpose, samples of standard substances are produced and made available at two major centers IAEA (International Atomic Energy Authority, U.K.) and NIST (National Institute for Standards and Technology, U.S.). Standards from other sources are also available. These primary standards can be used as such, or alternative standards can be employed if the primary ones are not available. However, any alternative standards need to be related accurately to the primary ones (see formulae below). For example, the material PDB (PeeDee belemnite), used particularly as a standard for the ratio of isotopes, is no longer readily available, and a new standard, VPDB,... 

The U.S. Clean Air Amendments of 1977 define two kinds of air quality standards primary standards, levels that will protect health but not necessarily prevent the other adverse effects of air pollution, and secondary standards, levels that will prevent all the other adverse effects of air pollution (Table 22-7). The amendments also define air quality levels that cannot be exceeded in specified geographic areas for "prevention of significant deterioration" (PSD) of the air of those areas. Although they are called "increments" over "baseline air quality" in the law, they are in effect tertiary standards, which are set at lower ambient levels than either the primary or secondary standards (Table 22-8). 

Notes National primary ambient air quality standards define levels of air quality which the EPA Administrator judges are necessary, with an adequate margin of safety, to protect the public health. National secondary ambient air quality standards define levels of air quality, which the Administrator judges necessary to protect the public welfare from any known or anticipated adverse effects of a pollutant. 

In titrimetry certain chemicals are used frequently in defined concentrations as reference solutions. Such substances are referred to as primary standards or secondary standards. A primary standard is a compound of sufficient purity from which a standard solution can be prepared by direct weighing of a quantity of it, followed by dilution to give a defined volume of solution. The solution produced is then a primary standard solution. A primary standard should satisfy the following requirements. 

The IUPAC definition of pH39 is based upon a 0.05M solution of potassium hydrogenphthalate as the reference value pH standard (RVS). In addition, six further primary standard solutions are also defined which between them cover a range of pH values lying between 3.5 and 10.3 at room temperature, and these are further supplemented by a number of operational standard solutions which extend the pH range covered to 1.5-12.6 at room temperature. The composition of the RVS solution, of three of the primary standard solutions and of two of the operational standard solutions is detailed below, and their pH values at various temperatures are given in Table 15.4. It should be noted that the concentrations are expressed on a molal basis, i.e. moles of solute per kilogram of solution. 

The SHE. The H" " H2 couple is the basis of the primary standard around which the whole edifice of electrode potentials rests. We call the H H2 couple, under standard conditions, the standard hydrogen electrode (SHE). More precisely, we say that hydrogen gas at standard pressure, in equilibrium with an aqueous solution of the proton at unity activity at 298 K has a defined value of of 0 at all temperatures. Note that all other standard electrode potentials are temperature-dependent. The SHE is shown schematically in Figure 3.3, while values of Eq r are tabulated in Appendix 3. 

The SHE is chosen as the ultimate reference electrode since its value is defined. By simply making a cell in which one half cell is the SHE, then straightaway we also know the potential of the second half cell. For this reason, we say that the SHE is a reference electrode. Since all potentials are ultimately cited with respect to the SHE, the latter is the reference electrode from which all other electrode potentials are derived we say that the SHE is the primary standard. It is also called the primary reference electrode. 

Lower Critical Solution Temperatures LCSTs were determined from plots of optical density at 600 nm versus temperature for 0.03% solutions of each polymer in PBS and were defined as the temperature at which Asoo = 0.1. Temperatures were raised at less than 0.3 C per minute and were measured with a thermometer that had been calibrated against an NBS primary standard thermometer. LCSTs for Figure 6 were determined from the cloud points of 0.01% solutions. 

Reference Intakes (DRIs). In the past, the recommended dietary allowances (RDAs), which are the levels of intake of essential nutrients that are considered to be adequate to meet the known nutritional needs of practically all healthy persons, were the primary reference value for vitamins and other nutrients. The DRIs also include other reference values, such as the estimated average requirement (EAR) and the adequate intake (AI). The RDA, EAR, and AI reference standards define nutritional intake adequacy. Since these recommendations are given for healthy populations in general and not for individuals, special problems, such as premature birth, inherited metabolic disorders, infections, chronic disease, and use of medications, are not covered by the requirements. Separate RDAs have been developed for pregnant and lactating women. Vitamin supplementation may be required by patients with special conditions and for those who do not consume an appropriate diet. 

First of all, the important role of platinum as the metal part of the standard hydrogen electrode (SHE), which is the primary standard in electrochemistry should be mentioned. The standard potential of an electrode reaction (standard electrode potential) is defined as the value of the standard potential of a cell reaction when that involves the oxidation of molecular hydrogen to solvated (hydrated) protons (hydrogen ions) ... 

Isotope ratios are given as deviations, in relation to a defined primary standard (zero point). The polyethylene foils CH 7 and NBS 22-oil are commercially available secondary standards, certificated and managed by the International Atomic Energy Agency. However, GC-lRMS systems cannot be calibrated without the aid of alternative peripheries like an elemental analyser (EA) or a dual inlet, owing to the lack of commonly accepted reference materials applicable in GC-IRMS techniques (Fig. 17.11). 

According to definition [3], a primary standard is a standard that is designated or widely acknowledged as having the highest metrological qualities and whose value is accepted without reference to other standards of the same quantity . Thus, once defined, primary standards require no further reference. The essential quality of a primary standard is its intrinsic, long-term stability. Primary standards are mainly used to measure and determine the value of all other standards. 

In this scheme, the primary reference material is defined as a chemical substance of the highest (and known) purity, or a well-characterized substance in a matrix, This classification of materials is, however, fairly arbitrary. It is ideal when used in connection with standards characterized in terms of biological activity. Primary standards are thus the International Reference Preparations (IRP) produced by the World Health Organization (WHO). In this case the primary standard for a particular antibiotic is the WHO reference preparation which constitutes the unit of that antibiotic. When people wish to use it they have to prepare a large batch of samples calibrated to the primary. This is then called a secondary standard. However, for well-defined chemical parameters, the term certified reference material is preferred. 

The basic definition of reflectance, as used in colorimetry, is the ratio of the light flux reflected from a material to the light flux incident on the material. Reflectance involves accounting for all of the visible radiation in the system. This is in contrast to a reflectance factor, which is defined as the ratio of the light flux reflected from a material to light flux reflected from a standard material [23], Material standards of reflectance and transmittance are difficult to obtain. Reflectance, in particular, is difficult since the primary standard of reflectance is the perfect reflecting diffuser,... 

From the analysis of air pollutants and their effects, the NAAQS described above were developed by the US EPA based on requirements of the Clean Air Act. This was done by defining the criteria pollutants to measure air quality, and then by determining the acceptable concentrations under health protection criteria (known as primary standard) a secondary standard was also defined to protect public welfare and prevent environmental and property damage. The criteria pollutants for air quality thus selected were CO, Pb, N02, O3, PM]0, PM25, and... 

Primary standard is defined as a standard that is designated or widely acknowledged as having the highest metrological qualities and whose value is accepted without reference to other standards of the same quantity . ... 

A primary standard is used to establish a secondary standard. A secondary standard is defined as a standard whose value is assigned by comparison with a primary standard of the same quantity ". It is often the secondary standard that the analyst will use. 

A reference electrode [183-185] is a half-cell that defines a potential to which all other measurements are referred. The primary standard electrode is the standard hydrogen electrode (SHE), but as this electrode is inconvenient for practical work, other reference electrodes are used. Such reference electrodes must have a potential that changes very little and is known to within 1 mV or so. In some cases of controlled potential electrolysis, it is sufficient to know the potential of the working electrode during the electrolysis within 10-20 mV, because the potential variations between different points of the electrode are of this magnitude (see earlier), and less precise electrodes may be termed comparison electrodes. In principle, any electrode at the surface of which an electrochemical reaction with a large exchange current can take place may be used as a reference electrode. 

Several electronic substituent constants were defined so as to represent both global and particular electronic effects. The a values obtained unambiguously from experimentally accessible data or from the many possible reaction series are called primary values and the corresponding set the primary standard. The o values derived from the primary values, by rescaling with modified q constants or correlation equations, are called secondary values and the corresponding set the secondary standard. 

The pH scale has been defined operationally, and standard reference solutions based on a conventional scale of hydrogen ion activity have been selected (i, 2). Measurements of the pH of seawater made with different electrodes and instruments are satisfactorily reproducible when standardized in the same way (3). The results obtained, however, do not always have a clear interpretation. Formally, this diflSculty can be attributed to the residual liquid junction potential involved in the measurement. The primary standards are necessarily dilute buffer solutions (ionic strength, I 0.1) whereas seawater normally has an ionic strength exceeding 0.6. This difference in the concentrations and mobilities of the ions coming in contact with the concentrated solution of potassium chloride of which the salt bridge-liquid junction is composed gives rise to a potential difference that is indeterminate. Consequently, the meas-m ed pH is in error by an unknown amount and does not fall exactly on the scale fixed by the primary standards. 

Under certain conditions regarded as ideal, j is probably actually close to zero. This should be the case when solution X matches closely the primary standard solutions S in pH, composition, and ionic strength (which must not exceed 0.1). Then pH(X) doubtless approaches —log (mHyn), where mn is the molality (mol/kg of water) of hydrogen ion and 7h is its conventional activity coefficient on the numerical scale defined by the convention adopted for the assignment of values to pH(S). When, as in seawater, these conditions do not prevail, the meaning of the experimental pH(X) in terms of concentrations and activities becomes unclear. 

Analytical Reference Standard An analytical reference standard (ARS), also known as primary standard or gold standard, is defined as a batch of drug substance, whose purity is independently established and accepted without reference to other standards. The ARS is the benchmark against... 

Define and illustrate the following terms clearly and concisely (a) standard solution (b) titration (c) primary standard (d) secondary standard. 

Primary standard a values are obtained from experimental determination of the dissociation constants of benzoic acids given that p is defined as unity. Secondary standard a values (Equation 25) can be obtained from any reaction series provided its p value is obtained on the basis of primary standard o values. 

Standard substances in clinical chemistry include primary standards, which can be obtained sufficiently pure to be used for the preparation of solutions by weighing or by reference to other definable physical characteristics (e.g., constant-boiling hydrochloric acid). Primary standard chemicals are available for acid-base reactions, precipitation reactions, oxidation-reduction reactions, etc. (V3), and are used in these various categories of analytical determination to validate the preparation of solutions of other chemical substances which cannot be obtained in a form suitable to meet the criteria demanded for a primary standard. Following their calibration in terms of a primary standard, these other chemieals can act as secondary standards. 

Most standards define the minimum number of cuts that the secondary divider has to take from each primary increment ASTM D2234, for example, requires six secondary increments to... 

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