Precipitation in limit tests

The solubility of an ionic substance in a solvent is limited by its solubility product. The solubility product is the product of the molar concentrations of the individual salt-forming ions in a saturated solution of the salt in question. The solubility formula for the general salt A Bj, is  

As different salts have very different solubility levels, usually the negative logarithm to the solubility product is used to make the vast span of solubility products encountered easier to manage. 

The first prerequisite for precipitation to take place is that the solubility of the actual substance in the solvent is exceeded. For an ionic salt this means that the ion product Q, is higher than the solubility product Ksp. The ion product is the product of the actual molar concentrations of the individual relevant ions, calculated as defined for the solubility product. 

Normally, no precipitate is formed immediately if Q is only slightly above the solubility product. What initializes the formation of a precipitate is that a number of nuclei are created spontaneously in the solution. A nuclei is a very small entity that acts as a seed upon which crystal growth can be initiated and continue. It is often a tiny crystal of the precipitating substance itself, but it can also be that an impurity acts as a nuclei. In some precipitations, the nuclei needed to start the precipitation is very small. It has been revealed that the nuclei that initiates the precipitation of barium sulfate consists of only four barium ions and four sulfate ions. 

The probability for nucleation to occur depends strongly on the degree of supersaturation of the solution. In the case of a very slight supersaturation, precipitation can take weeks or month to initiate, and at a certain degree of supersaturation nucleation and precipitation will happen immediately. This is called the critical supersaturation ratio. In the case of barium sulfate, a relatively high critical supersaturation ratio of about 32 has been determined. The supersaturation ratio is defined by the formula  

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Some salts have a tendency to form a precipitate consisting of fewer but larger particles, even though precipitation is initiated from a highly supersaturated solution. Or, they may form a colloidal precipitate, which rapidly changes in particle size distribution. Such a precipitate will appear crystalline, and particles will, unlike in a colloidal suspension, quickly settle. The factors determining the particle size distribution of a precipitate are discussed in some detail in Chapter 4 "Precipitation in Limit Tests."... 

The nature of the nephelometric value is described in Chapter 4, "Precipitation in Limit Tests." Contrary to these findings there is a more limited effect of performing the determination in test solution containing 5% of various salts... 

Barium sulfate is a hard crystal, which has a solubility that varies dramatically with particle size. The characteristic behavior of such crystals in relation to precipitations like the present test is discussed in Chapter 4, "Precipitation in Limit Tests," in many cases using barium sulfate as an example. 

Calcium oxalate is a hard crystalline precipitate and is therefore similar to barium sulfate with respect to the process of precipitation. This means that the crystal size distribuhon of the precipitated form varies a lot witir precipitation parameters such as sample solution temperature, degree of super saturation, and hme allowed from precipitation to evaluation of turbidity. For a more thorough discussion of this phenomenon, please see Chapter 4, "Precipitahon in Limit Tests."... 

Turbidimetric Agglutination Immunoassays. Agglutination—precipitation immunoassays were among the first practical appHcations of the antigen—antibody reaction in diagnostic tests. These assays are not as widely used in the 1990s as EIA and FIA because they are either not quantitative enough or lack the sensitivity limits of RIA, EIA, and EIA. 

The limit test for chlorides is based on its precipitation with silver nitrate in the presence of dilute HN03, and comparing the opalescence produced due to the formation of AgCl with a standard opalescence achieved with a known quantity of Cl ions. 

Theory The limit test for sulphates is based upon its precipitation as barium sulphate in the presence of barium chloride, hydrochloric acid and traces of barium sulphate. In this combination, hydrochloric acid exerts its common ion effect whereas traces of BaS04 aids in the rapid and complete precipitation by seeding. Thus, the opalescence caused by the sample is compared immediately with a standard turbidity produced with a known amount of the S042 ion. 

In this chapter we describe characteristic validation procedures of the Heavy Metals Limit Test in the Japanese Pharmacopoeia (JP) [1]. Although an equivalent test is commonly listed in both the United States Pharmacopoeia and the European Pharmacopoeia, there are differences in the color reagents and conditions of sample preparation of the JP procedure. Heavy metals are defined in the JP as poisonous metallic impurities such as Pb, Bi, Cu, Cd, Sn, and Hg that form colored colloidal precipitates with sodium sulfide TS in a slightly acidic solution of pH 3 to 4. The level is expressed as the equivalent quantity of lead. 

Hinsberg test To 0.5 ml of the amine (0.5 g, if solid) in a test tube add 1 ml of benzenesulfonyl chloride and 8 ml of 10% NaOH. Stopper the tube and shake for 3 to 5 min. Remove the stopper and warm the tube while shaking in a hot water bath ( 7CPC) for about 1 min. No reaction is indicative of a 3° amine the amine becomes soluble upon acidification (pH = 2 to 4) with 10% HC1. If a precipitate is present in the alkaline solution, dilute with 5 to 8 ml of H20 and shake. If the precipitate does not dissolve, the original amine is probably a 2° one. If the solution is clear, acidify (pH = 4) with 10% HC1. The formation of a precipitate is indicative of a 1° amine (detection limit, 100 mg compounds tested, C, to C10). 

Thus, the relatively constant arabitol production in all tested conditions can probably be explained by a combination of factors, such as high oxygen limitation and possible osmotic effects (e.g. induced by sulfate ions not fully precipitated with calcium) that may act synergistically. Arabitol yields and productivities found in the present work are similar to reported values in the literature (8,22). 

The polyimide powder of 6F + 3,3 -ODA was likewise tested for its solubility limit in DMAc. Uie imide powder was prepared by chemically imidizing the 6F + 3,3 -ODA polyamic acid with pyridine/acetic anhydride, precipitating in distilled water, thoroughly drying at 60°C and heating for 2 hours at 200 C. 

Sulfate may be detected by precipitation of barium sulphate. Dilute barium chloride solution (0.1 M) is added to the acid test solution in a test tube. If sulphate is present, the solution becomes turbid owing to precipitation of white BaS04. The limit of detection is ca. 5 p,g SO4T... 

Although the polysulfide precipitated about 10 to 20% more mercury than the sulfide, about 30 ppm mercury still remained in solution after treatment. The effect on lead removal was not as clearly defined, even though over half the lead was removed in each test. Since the concentration of lead in the feed solution was already low at only 20 ppm and the lower detectable limit for the analytical method (i.e., ICP spectroscopy) was 10 ppm, the results demonstrated only that lead concentrations less than 10 ppm could be obtained by either treatment. 

Precipitate-impregnated membrane electrodes have several disadvantages. They are usually slow in response and, in addition may be easily poisoned by many substances in the test solution under investigation. The sensitivity is as expected limited by the solubility of the precipitate substance. For example, with a molar solubility for Agl of about 10 the limit of detection for P is about lO" M for a chloride ion selective electrode with an AgCl molar solubihty of about 10 , the detection limit is ca. lO- M, about 1000 times poorer than for the P electrode. Finally, the selectivity of precipitate-membrane ISEs for other anions decreases with increasing solubility of the precipitate. 

Hard gelatin capsules should be evaluated for appearance (including brittleness), color, odor of contents, assay, degradation products, dissolution, moisture, and microbial limits. Testing of soft gelatin capsules should include appearance, color, odor of contents, assay, degradation products, dissolution, microbial limits, pH, leakage, and pellicle formation. In addition, the till medium should be examined for precipitation and cloudiness. 

This very simple test is based on the appearance of metallic arsenic formed by the reduction of arsenic ion by hypophosphorous acid. The same methodology is used in the limit test 6.2. Arsenic, method (b). Five ml of the prescribed solution is heated in a water-bath with an equal volume of hypo-phosphorous R. A brown precipitate is formed. 

Once nucleation has taken place, crystals will grow in an organized manner, determined both by the nature of the precipitate and also on the set conditions in the procedure. An important parameter in relation to limit tests is the size of the crystals. This also correlates witii tire degree of supersaturation in the mother phase, since crystal size is inversely proportional to tiie relative supersaturation as defined by the formula ... 

In relation to limit tests, the general trend will therefore be soft precipitates give a precipitate of a relative small particle size distribution regardless of the experimental procedures used and will not change in time. The particle size distribution of a hard precipitate depends on sample handling and might display crystal growth. 

The limit test for chlorides is based on the same chemistry as the identification test for chlorides, 3.16. Chlorides. The opalescence given by precipitation of the chloride present in the substance to be examined with silver is compared to the opalescence given in a similar precipitation of a chloride standard of known concentrahon. 

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