Absorption methylene blue

The affinity of variously oxidized cellulosic materials for certain basic dyes, notably methylene blue, has long been known. Kenyon and coworkers have recently shown that in the case of nitrogen dioxide oxycelluloses, the methylene blue absorption is a function of the carboxyl group content. Application of the reversible methylene blue method to the determination of carboxyl groups in purified cellulose has been made by Davidson and by Weber, who was able to show good agreement with the cation-exchange methods previously considered as well as with viscosity and osmotic pressure measurements. 

Figure 12-8. Effect of pH of hypochlorite solutions on the copper number and methylene blue absorption of oxycellulose [29].

The damages that occur during various pre-treatment processes of cellulosic fibres can be detected by various chemical tests like fluidity, copper number. Methylene Blue absorption and silver nitrate staining etc. 

Methylene Blue test can also be used to differentiate between oxycellulose and hydrocellulose. Methylene Blue is a cationic dye. Standard cellulose generally has no affinity for Methylene Blue, but oxycellulose with the formation of carboxyl groups confer an affinity and can be sorbed onto cotton. For this purpose. Methylene Blue absorption tests are carried out both at pH of 7.0 and at a pH of 2.7 (acidic). Two pieces of fabrics to be tested are taken and treated one with Methylene Blue at a pH of 2.7 and the other at pH 7.0. If oxycellulose is present, the material will absorb less dye in the acid than in the neutral solution, whilst reverse is the case if hydrocellulose is present. The degree of staining will indicate the extent of degradation. 

Scoured and bleached American cotton have carboxyl contents of 0.8-0.9 Egyptian cotton 1.16-1.74. Unbuffered Methylene Blue solutions give lower values viz., 0.45 and 0.65 respectively. Methylene Blue absorption of cotton depends on the ash alkalinity of the cotton and the value decreases as the ash alkalinity decreases. 

Methylene blue does not react with aldehyde groups which are characteristic of hydrocellulose or oxidation of cellulose under acid conditions. Thus a sample with a high copper number and low methylene blue affinity indicates either hydrocellulose or acid oxidation. Hydrocellulose is usually accompanied by residual traces of acid. If there is a reduction in the methylene blue absorption brought about by boiling in dilute sodium hydroxide, it indicates the presence of acid, because this treatment has no effect upon the number of carbosyl groups. The latter will only be converted into sodium carbosylate which will revert to sodium chloride and carboxylic acid groups with the hydrogen chloride in the methylene blue hydrochloride. 

A test that measures the quantity of carboxyl groups uses methylene blue absorption. A sample of the paper is immersed in a solution of methylene blue and the absorbance of the dye solution at 620 nm is measured before immersion of the paper and at the end of the experiment. The amount of sample needed to produce 50% exhaustion of the dye solution is used to calculate the carboxyl content. 

Limestone used as a filler should be free of the organic compounds—French Standard limits their content to 0.5 %. The clay admixture should be as low as possible, it is determined from the methylene blue absorption test, which should not exceed 1.5 g per 100 g of filler with the fineness below 80 pm [16]. Simultaneously, cement with limestone addition should give the paste of standard consistency, at water content lower than 30% [171]. 

Hang, P. T., and G. W. Brindley. 1970. Methylene blue absorption by clay minerals, determination of surface areas and cation exchange capacities (clay-organic studies XVIII). Clays and Clay Minerals 18, no. 4 203-212. doi 10.1346/CCMN.1970.0180404. 

Table 2. Decay constants of the methylene blue absorption in l Oj films deposited by decom-...

Reduction to Cl via T1+ wbicb oxidizes tbe leuco form of methylene blue Absorption band at 973 cm (A = 10 277 nm)... 

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