Indicators of calcium

The jellyfish Aequorea contains a photoprotein, which emits light only when calcium ions are present.672 673 Since light emission can be measured with great sensitivity (modern photomultipliers can be used to count light quanta) the protein aequorin and related photoproteins674a are used as a sensitive indicator of calcium ion concentration.674 (In a similar way the firefly luciferin-luciferase system, which requires ATP for activation, is widely used in an assay for ATP.)... 

One major paper attacking the problem of the relationship between the preservation of calcium carbonate in shallow anoxic marine sediments and their chemistry was by Aller (1982). The study was conducted at sites in Long Island Sound. The calcium carbonate content of the sediments decreased with increasing water depth. At the shallow FOAM (Friends of Anaerobic Muds) site shell layers associated with storms resulted in irregular variations in the carbonate content of the sediment. Ca2+ loss from the pore waters, indicative of calcium carbonate precipitation, was found only at the FOAM site below -20 cm depth. During the winter, elevated Ca2+ to CL ratios were observed near the sediment-water interface... 

Two nucleation processes important to many people (including some surface scientists ) occur in the formation of gallstones in human bile and kidney stones in urine. Cholesterol crystallization in bile causes the formation of gallstones. Cryotransmission microscopy (Chapter VIII) studies of human bile reveal vesicles, micelles, and potential early crystallites indicating that the cholesterol crystallization in bile is not cooperative and the true nucleation time may be much shorter than that found by standard clinical analysis by light microscopy [75]. Kidney stones often form from crystals of calcium oxalates in urine. Inhibitors can prevent nucleation and influence the solid phase and intercrystallite interactions [76, 77]. Citrate, for example, is an important physiological inhibitor to the formation of calcium renal stones. Electrokinetic studies (see Section V-6) have shown the effect of various inhibitors on the surface potential and colloidal stability of micrometer-sized dispersions of calcium oxalate crystals formed in synthetic urine [78, 79]. 

Calcium Absorption. Phytates in cereal grains have also been reported to interfere with the absorption of calcium. However, a long-term study indicated a retention of calcium in subjects that consume large amounts of bread made with high extraction of flour (19). 

A rapid method to determine the calcium content of lead alloys is a Hquid-metal titration using lead—antimony (1%) (9). The end point is indicated by a gray oxide film pattern on the surface of a sohdifted sample of the metal when observed at a 45° angle to a light source. The basis for the titration is the reaction between calcium and antimony. The percentage of calcium in the sample can be calculated from the amount of antimony used. If additional calcium is needed in the alloy, the melt is sweetened with a lead—calcium (1 wt %) master alloy. 

Calcium Pyrophosphates. As is typical of the pyrophosphate salts of multiple-charged or heavy-metal ions, the calcium pyrophosphates are extremely insoluble ia water. Calcium pyrophosphate exists ia three polymorphic modifications, each of which is metastable at room temperature. These are formed progressively upon thermal dehydration of calcium hydrogen phosphate dihydrate as shown below. Conversion temperatures indicated are those obtained from thermal analyses (22,23). The presence of impurities and actual processing conditions can change these values considerably, as is tme of commercial manufacture. 

An estimate of world calcium consumption in 1986 indicated that lead refining uses 30% alloys, eg, with Pb, Al, and Si, 25% steel treatment, 20% calciothermic reduction, 10% calcium hydride, 10% and miscellaneous usage is 5%. More recent evidence, however, has suggested that increasing consumption of calcium in battery manufacture has made this the most significant use. 

There are 10 producers of calcium chloride solutions in the United States, three of these also make a dry product. Solution production is centered around Michigan (brines), California and Utah (brines), and Louisiana (by-product acid). The majority of dry calcium chloride is made in Michigan, lesser quantities in Louisiana, and minor quantities in California. Production involves removal of other chlorides (primarily magnesium) by precipitation and filtration followed by concentration of the calcium chloride solution, either for ultimate sale, or for feed for dry product. Commercial dry products vary by the amount of water removed and by the nature of the drying equipment used. Production and capacity figures for the United States are indicated in Table 2. 

Fig. 5. pM vs pH for M = Ca(II), L = EDTA, in the presence of excess oxalate. Sohd lines A, B, C represent 100%, 10%, and 1% excess EDTA, respectively. Broken lines indicate sohd—solution equihbria of calcium oxalate in the presence of dissolved oxalate. 

Preparation of w-Phenyl-tert-Butylamine 24 grams of the urea derivative obtained as indicated above, were well mixed with 96 grams of calcium hydroxide in a flask immersed in an air bath and provided with a dropping funnel the stem of which reached the bottom of the flask. The mixture was heated to 240°-260°C (inside temperature) for 7 hours during which time 86 cc of water was slowly added. The vapors were collected in a receiver cooled with ice. After extraction with ether and distillation, the product was obtained as a colorless liquid boiling from 80°-84 C at 9 mm according to U.S. Patent 2,590,079. 

The hardness test sometimes is performed on the mud as well as the mud filtrate. The mud hardness indicates the amount of calcium suspended in the mud as well as the calcium in solution. This test usually is made on gypsum-treated muds to indicate the amount of excess CaSO present in suspension. To perform the hardness test on mud, a small sample of mud is first diluted to 50 times its original volume with distilled water so that any undissolved calcium or magnesium compounds can go into solution. The mixture then is filtered through hardened filter paper to obtain a clear filtrate. The total hardness of this filtrate then is obtained using the same procedure used for the filtrate from the low-temperature low-pressure API filter press apparatus. 

LSI (Langelier Saturation Index) an indication of the corrosive (negative) or scale-forming (positive) tendencies of the water. Hardness the total dissolved calcium and magnesium salts in water. Compounds of these two elements are responsible for most scale deposits. Units are mg/l as CaCOs. 

Although the Langelier index is probably the most frequently quoted measure of a water s corrosivity, it is at best a not very reliable guide. All that the index can do, and all that its author claimed for it is to provide an indication of a water s thermodynamic tendency to precipitate calcium carbonate. It cannot indicate if sufficient material will be deposited to completely cover all exposed metal surfaces consequently a very soft water can have a strongly positive index but still be corrosive. Similarly the index cannot take into account if the precipitate will be in the appropriate physical form, i.e. a semi-amorphous egg-shell like deposit that spreads uniformly over all the exposed surfaces rather than forming isolated crystals at a limited number of nucleation sites. The egg-shell type of deposit has been shown to be associated with the presence of organic material which affects the growth mechanism of the calcium carbonate crystals . Where a substantial and stable deposit is produced on a metal surface, this is an effective anticorrosion barrier and forms the basis of a chemical treatment to protect water pipes . However, the conditions required for such a process are not likely to arise with any natural waters. 

Thus brucite (Mg(OH)2) is also commonly found on surfaces under cathodic protection in seawater. Because more hydroxyl ions (higher pH) are required to cause magnesium hydroxide to precipitate, the magnesium is virtually always found in the calcareous deposits associated with calcium and its presence is an indicator of a high interfacial pH and thus either high cathodic current densities or relatively poor seawater refreshment. 

To predict the formula of an ionic compound, you need to know the charges of the two ions involved. Then you can apply the principle of electrical neutrality, which requires that the total positive charge of the cations in the formula must equal the total negative charge of the anions. Consider, for example, the ionic compound calcium chloride. The ions present are Ca2+ and Cl-. For the compound to be electrically neutral, there must be two Cl- ions for every Ca2+ ion. The formula of calcium chloride must be CaCl indicating that the simplest ratio of Cl- to Ca2+ ions is 2 1. 

Patton and Reeder s indicator. The indicator is 2-hydroxy-l-(2-hydroxy-4-sulpho-l-naphthylazo)-3-naphthoic acid the name may be abbreviated to HHSNNA. Its main use is in the direct titration of calcium, particularly in the presence of magnesium. A sharp colour change from wine red to pure blue is obtained when calcium ions are titrated with EDTA at pH values between 12... 

Solochrome dark blue or calcon ( C.1.15705). This is sometimes referred to as eriochrome blue black RC it is in fact sodium l-(2-hydroxy-l-naphthylazo)-2-naphthol-4-sulphonate. The dyestuff has two ionisable phenolic hydrogen atoms the protons ionise stepwise with pK values of 7.4 and 13.5 respectively. An important application of the indicator is in the complexometric titration of calcium in the presence of magnesium this must be carried out at a pH of about 12.3 (obtained, for example, with a diethylamine buffer 5 mL for every 100 mL of solution) in order to avoid the interference of magnesium. Under these conditions magnesium is precipitated quantitatively as the hydroxide. The colour change is from pink to pure blue. 

Calmagite. This indicator, l-(l-hydroxyl-4-methyl-2-phenylazo)-2-naphthol-4-sulphonic acid, has the same colour change as solochrome black, but the colour change is somewhat clearer and sharper. An important advantage is that aqueous solutions of the indicator are stable almost indefinitely. It may be substituted for solochrome black without change in the experimental procedures for the titration of calcium plus magnesium (see Sections 10.54 and 10.62). Calmagite functions as an acid-base indicator ... 

---