Limit test for sulphates

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. 

Calcium Carbonate Suspend 0.5 g in 5 ml of DW and add dropwise sufficient dil. HC1 to effect solution. Add 2 ml dil HC1. Perform the test with the resultant solution. Complies with the limit test for sulphates. 

Calcium Gluconate Dissolve 1.0 g in 15 ml of DW and carry out the test as described in 2 above. Complies with limit test for sulphates (150 ppm). 

Magnesium Sulphate Dissolve 2 g in 20 ml DW. Complies with the limit test for iron. 

Starch Dissolve the residue obtained, in the test for sulphated ash in 4 ml HC1 by heating gently, dilute with DW to 50 ml and mix. 25 ml complies with the limit test for iron. 

Zinc Sulphate Dissolve 2.5 g in sufficient C02-free DW to produce 50 ml. Dilute 2 ml of this soln. to 10 ml with DW, add 2 ml of a 20% w/v soln. of iron-free-Citric acid and 0.5 ml of thioglycollie acid, mix, make alkaline with iron-free-ammonia solution, dilute to 50 ml with DW and allow to stand for 5 minutes. Complies with the limit test for iron. 

Acid radical impurities constitute a serious but unavoidable source of impurities in a large number of pharmaceutical chemicals. However, the two most commonly found acid radical impurities are chloride (Cl ) and sulphate (S042 ) that evidently arise from the inevitable use of raw tap-water in various manufacturing operations. As these two acid radical impurities are found in abundance due to contamination, the Pharmacopoeia categorically stipulates limit tests for them which after due minor modifications are applicable to a number of pharmaceutical substances. 

The results were slightly low as calcium sulphate appears to possess a small solubility in the solution, but a correction allowance gives a more accurate determination. In addition to the use of the method as a limit test for calcium in magnesium salts, since the results are unaffected by the presence of iron or phosphate, it has been applied to the determination of calcium in such preparations as Compound Syrup of Iron Phosphate. 

Metallic picrates and oxalates should be absent, and other nitrocompounds should be present only in traces, The melting point or setting point carried out on a well-dried sample should be at least 20. The moisture content is generally limited to o i-o 3S per cent., and the ash to o-oS-o t per cent. this should not be of a gritty nature. Impurities insoluble in water should not exceed the uh content by more than 0 05 per cent. The picric acid should be almost completely soluble in benzene. Sulphates are restricted to 0 05 0 i per cent, as SO,). Chlorides and nitrates are sometimes tested for. Absence of lead in any form which might give rise to the formation of lead picrate is of special importance. Lead sulphate is, however, comparatively harmless. 

Chemical Limits a new limit test introduced into API CI-4 for sulphated ash, phosphorus and sulphur, for Low SAPS . 

Packaging materials Materials to be used in contact with metals should be as free as possible from corrosive salts or acid. BS 1133, Section 7 1967 gives limits for non-corrosive papers as follows chloride, 0-05% (as sodium chloride) sulphate, 0-25% (as sodium sulphate) and pH of water extract 5 -5-8 0. Where there is doubt, contact corrosion tests may be necessary in conditions simulating those in the package. 

The H-type cell devised by Lingane and Laitinen and shown in Fig. 16.9 will be found satisfactory for many purposes a particular feature is the built-in reference electrode. Usually a saturated calomel electrode is employed, but if the presence of chloride ion is harmful a mercury(I) sulphate electrode (Hg/Hg2 S04 in potassium sulphate solution potential ca + 0.40 volts vs S.C.E.) may be used. It is usually designed to contain 10-50 mL of the sample solution in the left-hand compartment, but it can be constructed to accommodate a smaller volume down to 1 -2 mL. To avoid polarisation of the reference electrode the latter should be made of tubing at least 20 mm in diameter, but the dimensions of the solution compartment can be varied over wide limits. The compartments are separated by a cross-member filled with a 4 per cent agar-saturated potassium chloride gel, which is held in position by a medium-porosity sintered Pyrex glass disc (diameter at least 10 mm) placed as near the solution compartment as possible in order to facilitate de-aeration of the test solution. By clamping the cell so that the cross-member is vertical, the molten... 

Other purity requirements include a clarity and acidity of solution test, and limits for ionization chlorine, loss on drying, and sulphated ash. 

A small area of the outer surface of the upper arm is exposed to test and control materials under occlusive cover. The test and control materials are applied to round chambers and taped to the arm with surgical tape. A standard irritant (20% sodium dodecyl sulphate [SDS]) is used as the positive control. Each panellist has up to four patches applied, each patch being applied for an increased duration. Due to the potential irritancy of the test materials a cautious approach is used. The first patch is applied for one hour, the second, third, and fourth patches for two, three, and four hours, respectively. Each patch is applied to a different site on consecutive weeks. This approach allows any unexpected or unacceptable reactions to be limited to a minimum. 

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