Metal indicators

Introduction. Metal indicators are organic molecules which form specifically colored soluble complexes with metal ions in aqueous media. Here, the color of the complexes and of the free indicator must be different. These reactions can be used in two analytical procedures volumetry (complexometry) and colorimetry/ photometry. In both methods, the concentration of metal ions is determined, but with different techniques. 

Colorimetry/photometry exploits the fact that the color intensity of the complex correlates to the metal ion concentration, at least within a certain concentration range. Therefore, sufficient indicator is added to the sample that the indicator forms a colored complex with all ions of the metal of interest in solution. The color intensity of this complex is then either compared visually against the color of reference samples (colorimetry) or the color intensity is measured at a defined wavelength with an instrument and then compared with a calibration curve (photometry). 

Volumetry uses the formation of complexes between the metal ions and a complexing reagent (the titrant). This reagent is added in the same manner as in 

Titrants which are most frequently used are substances like /V,/V,/V, /V -cthylc-nediaminetetraacetic acid (EDTA) and its salts. Due to their ability to form com-plexes the method is also called complexometry. 

Due to the different requirements of the two methods, the indicators for metal determination are often to be different in colorimetry/photometry and volume-try/complexometry (see Table 5.22). 

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BackTitrations. In the performance of aback titration, a known, but excess quantity of EDTA or other chelon is added, the pH is now properly adjusted, and the excess of the chelon is titrated with a suitable standard metal salt solution. Back titration procedures are especially useful when the metal ion to be determined cannot be kept in solution under the titration conditions or where the reaction of the metal ion with the chelon occurs too slowly to permit a direct titration, as in the titration of chromium(III) with EDTA. Back titration procedures sometimes permit a metal ion to be determined by the use of a metal indicator that is blocked by that ion in a direct titration. Eor example, nickel, cobalt, or aluminum form such stable complexes with Eriochrome Black T that the direct titration would fail. However, if an excess of EDTA is added before the indicator, no blocking occurs in the back titration with a magnesium or zinc salt solution. These metal ion titrants are chosen because they form EDTA complexes of relatively low stability, thereby avoiding the possible titration of EDTA bound by the sample metal ion. 

In a back titration, a slight excess of the metal salt solution must sometimes be added to yield the color of the metal-indicator complex. Where metal ions are easily hydrolyzed, the complexing agent is best added at a suitable, low pH and only when the metal is fully complexed is the pH adjusted upward to the value required for the back titration. In back titrations, solutions of the following metal ions are commonly employed Cu(II), Mg, Mn(II), Pb(II), Th(IV), and Zn. These solutions are usually prepared in the approximate strength desired from their nitrate salts (or the solution of the metal or its oxide or carbonate in nitric acid), and a minimum amount of acid is added to repress hydrolysis of the metal ion. The solutions are then standardized against an EDTA solution (or other chelon solution) of known strength. 

Indicator Chemical name Dissociation constants and colors of free indicator species Colors of metal-indicator complexes Applications... 

Finding the End Point with a Visual Indicator Most indicators for complexation titrations are organic dyes that form stable complexes with metal ions. These dyes are known as metallochromic indicators. To function as an indicator for an EDTA titration, the metal-indicator complex must possess a color different from that of the uncomplexed indicator. Furthermore, the formation constant for the metal-indicator complex must be less favorable than that for the metal-EDTA complex. 

Metallic indicator electrodes in which a metal is in contact with a solution containing its ion are called electrodes of the first kind. In general, for a metal M, in a solution of M"+, the cell potential is given as... 

Ref 2. The brazing filler metal is analyzed for those specific elements for which values are shown. If the presence of other elements is the amount of those elements is deterrnined to ensure that the maximum total of each is <0.15 wt%. Remainder of material is Cu. Value represents maximum. Remainder of material is Mn. Table 7. Chemical Composition Requirements for Nickel and Cobalt Filler Metals indicated in the analysis. ... 

Solid-phase acidic dyes ai e also used in the study of the ternai y systems with pyrazolone derivatives. In addition, the colorless complex of the investigated metals with pyrazolone derivatives has been studied by means of the metal-indicator method. 

The resulting data for liquid metals indicate a systematic relationship witlr the bonding energy of the element, which is reflected in the heat of vaporization... 

Inspect lubricant for traces of metal (indicating component wear). 

Perhaps the most obvious metallic property is reflectivity or luster. With few exceptions (gold, copper, bismuth, manganese) all metals have a silvery white color which results from reflecting all frequencies of light. We have said previously that the electron configuration of a substance determines the way in which it interacts with light. Apparently the characteristic reflectivity of metals indicates that all metals have a special type of electron configuration in common. 

In acid-base titrations the end point is generally detected by a pH-sensitive indicator. In the EDTA titration a metal ion-sensitive indicator (abbreviated, to metal indicator or metal-ion indicator) is often employed to detect changes of pM. Such indicators (which contain types of chelate groupings and generally possess resonance systems typical of dyestuffs) form complexes with specific metal ions, which differ in colour from the free indicator and produce a sudden colour change at the equivalence point. The end point of the titration can also be evaluated by other methods including potentiometric, amperometric, and spectrophotometric techniques. 

A. Direct titration. The solution containing the metal ion to be determined is buffered to the desired pH (e.g. to PH = 10 with NH4-aq. NH3) and titrated directly with the standard EDTA solution. It may be necessary to prevent precipitation of the hydroxide of the metal (or a basic salt) by the addition of some auxiliary complexing agent, such as tartrate or citrate or triethanolamine. At the equivalence point the magnitude of the concentration of the metal ion being determined decreases abruptly. This is generally determined by the change in colour of a metal indicator or by amperometric, spectrophotometric, or potentiometric methods. 

B. Back-titration. Many metals cannot, for various reasons, be titrated directly thus they may precipitate from the solution in the pH range necessary for the titration, or they may form inert complexes, or a suitable metal indicator is not available. In such cases an excess of standard EDTA solution is added, the resulting solution is buffered to the desired pH, and the excess of the EDTA is back-titrated with a standard metal ion solution a solution of zinc chloride or sulphate or of magnesium chloride or sulphate is often used for this purpose. The end point is detected with the aid of the metal indicator which responds to the zinc or magnesium ions introduced in the back-tit ration. 

C. Replacement or substitution titration. Substitution titrations may be used for metal ions that do not react (or react unsatisfactorily) with a metal indicator, or for metal ions which form EDTA complexes that are more stable than those of other metals such as magnesium and calcium. The metal cation M + to be determined may be treated with the magnesium complex of EDTA, when the following reaction occurs ... 

The amount of magnesium ion set free is equivalent to the cation present and can be titrated with a standard solution of EDTA and a suitable metal indicator. 

Choice of indicators. The indicator chosen should be one for which the formation of the metal-indicator complex is sufficiently rapid to permit establishment of the end point without undue waiting, and should preferably be reversible. 

This reaction will proceed if the metal indicator complex M-In is less stable than the metal-EDTA complex M EDTA. The former dissociates to a limited extent, and during the titration the free metal ions are progressively complexed by the EDTA until ultimately the metal is displaced from the complex M-In to leave the free indicator (In). The stability of the metal-indicator complex may be expressed in terms of the formation constant (or indicator constant) Ku ... 

Xylenolorange. This indicator is 3,3 -bis[ IV,/V-di(carboxy methyl Jaminomethyl]-o-cresolsulphonphthalein it retains the acid-base properties of cresol red and displays metal indicator properties even in acid solution (pH = 3-5). Acidic solutions of the indicator are coloured lemon-yellow and those of the metal complexes intensely red. 

Goldschmidt (1926) grouped metals and covalent crystals together, and Bernal (1929) pointed out that many properties of metals indicate that metallic bonds are closely similar to covalent bonds. I developed this idea further (Pauling, 1938), and formulated a set of metallic radii in 1947, with use of the empirical equa-... 

Nuclear y-ray resonance spectra of solid solutions of Fe and Co in /3-rh boron give inconclusive results, although the large isomer shifts as compared to Fe metal indicate that the accommodation of Fe atoms in the boron structure is associated with changes in the electronic state. The magnitudes of the shifts are... 

One clue to understanding the nature of metallic bonds comes from their high electrical conductivity. Like most substances held together by ionic or covalent bonds, pure water and pure salt do not conduct electricity well. But pure copper does. Electrical conductivity is a measure of how free the electrons are to move. The high conductivity of metals indicates that their electrons are freer to move than the electrons are in salt or water. 

Conversions obtained in the hydrogenation of RCN in CH on alumina supported Group VIII metals indicated the lower activity of these catalysts compared to the Raney type catalysts (see Table 3). Almost complete conversion (except Co/A1203) was achieved on M/A1203 catalyst at 140 °C for 9 h instead of... 

Konmova, Ts.B., Popov, M.S., and Venichenko, A.S. "Study of the Interaction of Zirconium with Certain Monocarboxylic Acids by the Metal Indicator Method," Russian Journal of Inorganic Chemistry. 1975, 20(6), 861 865. 

Low resolution spectroscopy, by comparison, has the advantage of providing discrimination against cluster non-members through the use of radial velocities, and can still reach large samples of reasonably faint objects. Common metallicity indicators from low resolution work are Fe and Fe-peak blends, CN, and the Ca II infrared triplet, which are calibrated against high-resolution abundance analyses. 

Abstract. We present a new calibration of the Call triplet as metallicity indicator based in 4 globular and 11 open clusters which cover a range of metallicity -2<[Fe/H]<+0.1 and age 13<(Age/Gyr)<0.25. We use it to derive the metallicity distribution in two fields situated at 5 and 8 degrees from the center of the LMC. We show that the mean [Fe/H] of the LMC field decreases as we move away from the bar. 

If, instead, the duration of the massive stars phase is as long as 0.1 Gyr (models c3, c4, c6 and c8) then we obtain a too high metallicity for the next stellar generations, with the consequence of obtaining too high metallicity indices and too red integrated colors. Therefore these models should be ruled out. 

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