Platinum wire tables

Flammable atmospheres can be assessed using portable gas chromatographs or, for selected compounds, by colour indicator tubes. More commonly, use is made of explos-imeters fitted with Pellistors (e.g. platinum wire encased in beads of refractory material). The beads are arranged in a Wheatstone bridge circuit. The flammable gas is oxidized on the heated catalytic element, causing the electrical resistance to alter relative to the reference. Instruments are calibrated for specific compounds in terms of 0—100% of their lower flammable limit. Recalibration or application of correction factors is required for different gases. Points to consider are listed in Table 9.10. 

A cell with a capacity of 1 L was made of mild steel. An amorphous carbon rod (diameter 25 mm length 15 cm) was used as anode, the inside wall of the cell as cathode and a platinum wire was used as reference electrode. The anode compartment of the cell was separated from the cathode compartment by a skirt of steel welded to the cell cover. The anode gas was passed through a tube filled with tablets of NaF to absorb anhyd HF gas and then led to a gas sampler. Fluorine was detected with K.I soln. After the starting material was added into the molten KIIF2/HF salt, the electrolyte was pre-electrolyzed at a low current density until NF2 was detected, and then current efficiency of each product and polarization curves by galvanostatic or potential sweep method were determined (Table 1). At optimum conditions the current efficiency of NF3 was 55%. 

Screen-printed carbon electrodes were also evaluated in the Topas microchip for separation and detection of pAP and AsA. The theoretical plate number (AO, half-peak width (wy2), peak current and resolution are also shown in Table 34.2. Peak current at SPEs was higher than those obtained with gold and platinum wires. 

The electrodes usually are rotated at about 600 rpm. Contact to the platinum wire is made internally by filling the electrode with mercury. A stationary wire dips into the mercury pool at the top to make external contact to the potential-control circuitry. The platinum-glass seals are prone to crack, which causes erratic currents that are associated with the leaking of mercury to the electrode surface. Once cracked, the electrodes are not easily repaired and should be discarded. Table S.9 indicates the dependence of the current on the rotational speed of the electrode. 

Explain the relations between orthoboric acid, metaboric acid, tetraboric acid, and boric anhydride. Experiment Place a few grams of boric acid on a watch glass upon the steam table (100-110°) and leave for hour. What is formed What would be formed if the acid were heated to 140° Suspend a little of the acid in a loop of platinum wire, and heat in the Bunsen flame. What is formed ... 

Adsorption experiments were conducted on chromium, platinum, cadmium, and zinc the sources and preparation of these metal specimens have been reported previously (16). In preparing adsorbed, mono-molecular layers by adsorption directly from the molten pure acid (5), the clean adsorbing substrate was first heated to a temperature just above the melting point of the acid (see Table I), a few crystals of the acid were sprinkled on the surface, and the resulting pool of molten acid was teased over the whole surface with a previously freshly flamed platinum wire. If spontaneous retraction of the liquid acid did not occur, the specimen was allowed to cool and all of the solidified material adhering on top of the adsorbed monolayer was removed by appropriate solvent treatments as discussed below. 

In general, it may be stated that the borax beads are more viscous than the phosphate beads. They accordingly adhere better to the platinum wire loop. The colours of the phosphates, which are generally similar to those of the borax beads, are usually more pronounced. The various colours of the phosphate beads are collected in the following table. 

Flame colourations Place a small quantity (3-4 mg) of the substance on a watch glass, moisten with a little concentrated hydrochloric acid, and introduce a little of the substance on a clean platinum wire into the base of the non-luminous Bunsen flame (Table V.2). An alternative method is to dip the platinum wire into concentrated hydrochloric acid contained in a watch glass and then into the substance sufficient will adhere to the platinum wire for the test to be carried out. 

Borax bead reactions (T able V.7.) Prepare a borax bead in a loop of platinum wire by dipping the hot wire into borax and heating until colourless and transparent. Bring a minute quantity of the substance into contact with the hot bead and heat in the outer or oxidizing flame. Observe the colour when the bead is hot and also when it is cold. Heat the bead in the inner or reducing flame and observe the colours in the hot and cold states. Coloured beads are obtained with compounds of copper, iron, chromium, manganese, cobalt, and nickel (see however, Section VII.25, Table VII.8). ... 

A reference electrode is needed to provide a potential scale for E° valnes as all voltages are relative. Any electrochemical reaction with a stable, well known potential can be nsed as a reference electrode. The NHE or standard hydrogen electrode (SHE) (Pt/H2,1.0 M H+) was the first well known reference electrode and is used as a reference in most tables of redox potentials. An NHE is difficult to construct and operate and therefore, is not typically used experimentally. Since the NHE is widely accepted, potentials are still often referenced to the NHE, converted from other reference electrodes. For aqueous solvents the SCE (Hg/Hg2Cl2 (KCl)) and the silver/silver chloride (Ag/AgCl) electrode are now commonly used as reference electrodes. To convert from the SCE to the NHE, E (vs. NHE) = E (vs. SCE) + 0.24 V. For nonaqueous solvents the silver/silver nitrate (Ag/AgNOs) reference electrode is often used. A pseudo-reference electrode can also serve as a reference point for aqueous or nonaqueous solutions. A silver or platinum wire can be used as a... 

Table XIII summarizes the results of Chaudhri and Field [133]. The observations were made on freshly prepared crystals, 10 mm long and chosen to be free of macroscopic defects. Initiation was by heated platinum wires.

The metals and their compounds display characteristic flame colours. If a moistened platinum wire is dipped into a substance containing the alkali metal and introduced into a Bunsen flame, the flame becomes coloured. The colours are shown in Table 12.1. 

Platinum wire is the metal of choice because of its linearity, its chemical inertness, and its accuracy, which increases with purity. Other metals are also used and their characteristics are given in Table 5.5. Temperature ranges for Pt RTDs range from -260 to 660 °C, which is significantly greater than for thermistors. Temperatures as high as 850 °C are also possible but it is difficult to avoid contamination of the platinum by the thermometer metal sheath. 

Based on the procedure developed by CIBA-Geigy [8], the material is rated from 1 to 6 depending upon its reaction to an electrically heated platinum wire. The sum of the two ratings is then taken as the overall index of the drying hazard and subsequently a hazard classification is determined (Table 56.1). Products with a hazard class Tr 3 should not be dried in heated vessels since safe handling of the dry product cannot be assumed. 

One design of electrode is illustrated in Fig. 3.4. Pure mercury covers a platinum wire sealed through the bottom of a glass tube. The mercury is covered with powdered mercurous chloride, which is only slightly soluble in potassium chloride solution, the latter filling the cell. The activity of Hgi depends on the concentration of KCl since the solubility product (Hg2" )(Cl is a constant. Potentials on the standard hydrogen scale for various KCl concentrations are listed in Table 3.4. 

Phosphate Beads. If a small quantity of microcosmic salt (Chapter 3.2) is heated on a loop of platinum wire, it fuses and forms sodium metaphosphate glass (14.12). The molten glass will react with trace metal ions, if present, to form coloured glass beads of double orthophosphates, for example, (14.13) and (14.14) (Table 14.2). 

In the case of the catalytic decomposition of ammonia on a hot platinum wire (first entry in the table above), the reaction order is zero initially because the reaction occurs on the surface of the wire, and the surface coverage is independent of concentration. The rate of a zero-order reaction is independent of concentration until the reactant is nearly exhausted or until equilibrium is reached. Reaction rates may depend on species that do not appear in the overall chemical equation. 

---