Chemical compound that changes in physical appearance at a certain concentration of hydrogen cations, or pH. Acid-base indicators, or pH indicators, are used in various industries, such as chemistry labs and medical offices, to determine the concentration of hydrogen cations (H⁺), also known as pH range, of a solution. Acid-base indicators can be any chemical compound or material that changes in physical appearance, typically color, with a change in the concentration of H⁺ ions in solution (Fig. 1). This change occurs due to a large amount of H⁺ present in solution (a low pH) or a small amount of H⁺ in solution (a high pH). Indicators can therefore only reveal a pH range, rather than an exact pH value. Acid-base indicators may be natural or synthetic. Most indicators are weak organic acids used in medical, industrial, and laboratory settings. See also: Hydrogen; pH

A technique in which a neutron, charged particle, or gamma photon is captured by a stable nuclide to produce a different, radioactive nuclide which is then measured. The technique is specific, highly sensitive, and applicable to almost every element in the periodic table. Because of these advantages, activation analysis has been applied to chemical identification problems in many fields of interest.

The science of chemical separations, characterizations, and measurements, which yield information about chemical systems. Analytical chemistry has a broad range of practices from basic research to industry, including applications in food, pharmaceutical, forensic, environmental, materials, and space science as well as biotechnology, nanotechnology, and medical technology. Qualitative analysis is concerned with the description of chemical composition in terms of elements, compounds, or structural units, whereas quantitative analysis is concerned with the measurement of amount. Instrumental methods, which might better be termed physicochemical methods, have been developed to the degree that they make up the vast bulk of both qualitative and quantitative analysis (see illustration). They include measurements of purely physical characteristics, as well as the use of physical measurements, to follow the course of chemical reactions. See also: Qualitative chemical analysis; Quantitative chemical analysis

The application of chemistry to the technical examination, authentication, and preservation of cultural property. Chemists working in museums engage in a broad range of investigations, most frequently studying the chemical composition and structure of artifacts, their corrosion products, and the materials used in their repair, restoration (see illustration), and conservation. The effects of the museum environment, including air pollutants, fluctuations in temperature and relative humidity, biological activity, and ultraviolet and visible illumination, represent a second major area of research. A third area of interest is the evaluation of the effectiveness, safety, and long-term stability of materials and techniques for the conservation of works of art. Though analytical techniques appear to dominate, many other areas of chemistry, biology, physics, and engineering, including polymer chemistry, kinetic studies, imaging methodologies, biodegradation studies, dating methods, computer modeling, metallography, and corrosion engineering, play active roles in conservation science.

An analytical technique for determining the composition of a sample in terms of the chemical elements present and their quantities or concentrations. Unlike other methods of elemental analysis, however, the sample is decomposed into its constituent atoms which are then probed spectroscopically.

Auger electron spectroscopy (AES) is a widely used technique that detects the elements in the first atomic layers of a solid surface. Although many elements can be detected, hydrogen usually cannot be observed. Excellent spatial resolution can be achieved. Auger electron spectroscopy is important in many areas of science and technology, such as catalysis, electronics, lubrication, and new materials, and also understanding chemical bonding in the surface region. Auger spectra can be observed with gas-phase species.

An instrument for weighing. The word "balance," derived from the Latin bilanx ("having two pans"), is often used interchangeably with "scale" or "scales" (Old English, meaning dishes or plates). However, balance is the preferred term for an instrument used for the precise measurement of small weights or masses in amounts ranging from micrograms up to a few kilograms (Fig. 1). See also: Mass; Weight

A method for quantitatively determining the concentration, biological activity, or potency of a substance by its effect on the growth of a suitable animal, plant, or microorganism under controlled conditions. A bioassay (biological assay) is a measurement procedure that quantifies the effect of a living system's exposure to a substance. A bioassay also can quantitate the concentration or potency of a substance by some observable effect on a biological system. For example, bioassays can determine precisely how much a delivered substance affects the growth of an organism (animal, plant, or microorganism) under regulated conditions (see figure), or the relative potency of a drug, enzyme, or hormone preparation on cells. See also: Biotechnology; Cell (biology); Enzyme; Experiment; Hormone; Organism

An analytical device that converts the concentration of an analyte, a target substance of interest, into a signal by means of a biologically derived sensing element intimately connected to, or integrated into, a transducer. There are a large number of analytes that can be measured in biological media, with just a few examples being pH, partial pressure of carbon dioxide (pCO₂), partial pressure of oxygen (pO₂), and the ionic concentrations of sodium, potassium, calcium, chloride, and glucose (see figure). However, these sensors do not use biological recognition elements, and are usually considered as chemical sensors accordingly. Typically, the biological recognition element is a protein, protein complex, or oligonucleotide (nucleic acid) that is able to recognize a particular analyte in the presence of many other components in a complex biological matrix. The recognition process involves a chemical or biological reaction, and the transducer must be capable of detecting not only the reaction, but also its extent. An ideal sensor should yield a selective, rapid, and reliable response to the analyte, and the signal generated by the sensor should be proportional to the analyte concentration. Furthermore, a biosensor should ideally be self-contained, so that it is not necessary to add reagents to the sample matrix to obtain the desired response. See also: Carbon dioxide; Glucose; Ion; Nucleic acid; Oligonucleotide; Oxygen; pH; Protein; Transducer

In glass blowing, a long straight tube on which molten glass is gathered and worked, partly by blowing into the tube. The blowpipe (see illustration) is spun to shape the glass object further by centrifugal force, or by a tool, in which case the blowpipe acts as a spindle for turning. See also: Glass