Nitrogen—Inorganic Forms

Most soils contain inorganic nitrogen (N) in the form of ammonium (NHt) and nitrate (NO)"). Nitrite (NOz) also may be present, but the amount is usually too small to warrant its determination, except in cases where NHt or NHt-forming fertilizers are applied to neutral or alkaline soils. Several other forms of inorganic N have been proposed as intermediates during microbial transformations of N in soils, including hydroxylamine (NH20H), hyponitrous acid (H2N20 2), and nitramide (NH2N02), but these compounds are thermodynamically unstable and have not been detected in soil. Until the 1950s, inorganic N was believed to account for <2% of total soil N, on the assumption that NHt and NO)" are completely recovered by extracting soil with a neutral salt solution. The validity of this assumption was challenged by the finding that some soils contain NHt in a form that is not extracted by exchange with other cations (e.g., Rodrigues, 1954; Dhariwal & Stevenson, 1958; Stevenson & Dhariwal, 1959; Bremner & Harada, 1959; Bremner, 1959; Schachtschabel, 1960, 1961; Young, 1962), and by estimates that the proportion of soil N in this form can exceed 50% for some subsurface soils (Stevenson & Dhariwal, 1959; Young, 1962). In such cases, NHt is said to be fixed, and fixed NHt has subsequently been defined as the NHt in soil that cannot be replaced by a neutral potassium salt solution (SSSA, 1987), such as 1 or 2 M KCI or 0.5 M K2S04, in contrast to exchangeable NHt, which is extractable at room temperature with such a solution. Existing information indicates that fixed NHt occurs largely, if not entirely, between the layers of 2: I-type clay minerals, particularly vermiculite and illite (hydrous mica), and that fixation results from entrapment of NHt in ditrigonal voids in the exposed surfaces upon contraction of the clay lattice (Nommik & Vahtras, 1982). The term, nonexchangeable NHt, has been used by Bremner (1965) and Keeney and Nelson (1982) in previous editions of this publication as a more precise alternative to fixed NHt. The same term is used in the present treatment, with specific reference to NHt determined by the method described in "Determination of Nonexchangeable Ammonium," which involves digestion with an HF-HCI solution following treatment of the soil with alkaline KOBr to remove exchangeable NHt and labile organic-N compounds.

Nitrogen - inorganic forms.

Carrier-Based Ion-Selective Electrodes and Bulk Optodes. 1. General Characteristics.

Magnesium. An update on physiological, clinical and analytical aspects.

Antimony in the environment: a review focused on natural waters: I. Occurrence

Complexation reactions of dithiocarbamates.

The pH is one of the most important parameters for char-acterizing the chemical properties of an aqueous solution. The glass eiectrode is by far the most commonly used pH sensor. The determination of pH in special situations, e.g., in vivo applications where the fragility of the glass electrode is a draw-back, requires pH sensors that can easily be miniaturized and built into physically rugged sleeves. Also, for other applications where the volume of solution is very restricted miniaturization of pH sensors is very important. The glass electrode does not respond properly to pH in some corrosive environments (e.g., in hydrogen fluoride [HF] solutions). At present, among alternatives to the hydrogen-selective glass electrode group of sensors, potentiometric metal-metal oxide pH sensors respond to pH, ideally due to an equilibrium involving the metal and its oxide. In the case of metal oxide electrodes, the metal is not involved in the potential-determining reaction. This distinction is of course not clear in many cases because the mechanism giving the pH response is not always unequivocal.

Metal-Metal Oxide and Metal Oxide Electrodes as pH Sensors.

Platinum, palladium and rhodium content in road dust, tunnel dust and common grass in Białystok area (Poland): a pilot study.

A potentiometric ion-selective electrode based on a solid conducting poly(pyrrole) contact and neutral carrier tri-n-dodecylamine membrane was developed. The effect of the poly(pyrrole) doping anion on the characteristic of the sensor is presented and discussed. The ion-selective H+ sensitive electrode with a solid poly(pyrrole) contact is characterized by having a good stability over both short and long periods of time. Selectivity coefficients for this electrode are slightly better than those for internal solution electrodes. Stability is also improved especially when compared with coated wire type electrodes. Responses are fast and nearly Nernstian within the pH range 2–12. All solid-state sensors with plastic tri-n-dodecylamine were used for pH determinations in blood serum.

Adam Hulanicki

Papers

Complexation reactions of dithiocarbamates.

Metal-Metal Oxide and Metal Oxide Electrodes as pH Sensors.

Platinum, palladium and rhodium content in road dust, tunnel dust and common grass in Białystok area (Poland): a pilot study.

All solid-state hydrogen ion-selective electrode based on a conducting poly(pyrrole) solid contact

Model for treatment of selectivity coefficients for solid-state ion-selective electrodes