Question

2. Chromatography is an analytical technique that is widely used for the separation, identification and determination of the chemical components in complex mixtures. It is a very powerful method of separation. Describe how the separation of mixture is done using a. thin layer chromatography. b. Column chromatography

Answer 1

a. Thin layer chromatography (TLC) is a chromatographic technique for the separation of components of a mixture using a thin stationary phase supported by an inert backing (silica gel). TLC is used for monitoring the progress of a reaction. TLC is a simple analytical tool widely used owing to its relative low cost and speed of separation.The principle of TLC is the same as that of chromatography i.e. a compound will have different affinities for the mobile and stationary phases, and this affects the speed at which it migrates. TLC consists of three steps spotting, development, and visualization. TLC can be used to help determine the number of components in a mixture, the identity of compounds, and the purity of a compound. The Rf value is used to quantify the movement of the materials along the plate. Rf is equal to the distance traveled by the substance divided by the distance traveled bv the solve move different distances from the original spot location and show up as separate spots TLC adsorbents are typically very polar, more polar compound in the mixture will adhere more strongly to the adsorbent and the more slowly it moves. There will be intermolecular attractions between the eluent and the compounds which inturn determine the solubility of the compounds in the mobile phase. In general, the more polar the eluent, the more rapidly a given compound moves. Polar compounds, which are strongly attracted to the adsorbent, require polar eluents to attract them away from the adsorbent. For less polar compounds, we require less polar eluents, start with non-polar solvent and slowly increasing the polarity b. Column chromatography Column chromatography is an analytical technique for the separation of a mixture of compounds using a stationary phase like silica gel and a mobile phase. The success of the separation and the contents of the fractions can be determined by spotting the fractions along with the initial mixture on TLC. It is a quicker way of separating the compounds. It helps in separating the unstable compounds. For an elutant (or mixture of elutants), it should be chosen in such a way that the spot of the target compound travels about 1/3 of the total elutant path. Or, to be more precise, Rf value should be between 0.25 and 0.35 Applications of TLC alue is always between zero and one. Different components in the original spot, having different polarities, will Thin-layer chromatography (TLC) is a simple and inexpensive analytical technique 2. Small quantities (less than 10 mg) of the compound can be efficiently separated 3. TLC is used for the rapid analysis of reagent and product purity, or to quickly determine the number of compounds in a mixture 4. Comparing the standard compound and the unknown compound's behavior TLC is used ss TLC is often used for selecting a suitable solvent before doing column chromatographic separation TLC is frequently used to monitor chromatographic separation of compounds by column 3. In both TLC and column chromatography, there are two phases, mobile and stationary. The solid stationary phase remains fixed in place, called the adsorbent and the mobile phase is the solvent or the mixture of solvents which moves through the stationary phase. The compounds partitions itself between these two phases. Compounds with less attraction to the adsorbent move rapidly with the eluent. Compounds with more attraction to the adsorbent move slowly with the eluent. Otherwise, if the compound has a greater affinity for the mobile phase, it will move along faster. If the sample has a greater affinity for the stationary phase it will spend more of its time on the stationary phase and move slower. TLC adsorbents are very polar and more polar compounds adhere strongly to the adsorbent and the more slowly it moves. Similarly, intermolecular attractions between the eluent and the compounds determine the solubility of the compounds in the mobile phase. In general, the more polar the eluent the more rapidly a given compound moves. Polar compounds, which are strongly attracted to the adsorbent, require polar eluents to attract them away from the adsorbent. 4. This separation method i TLC. Column chromatography can be used on both a large and small scale column is done with silica gel or alumina. Depending on the column size, we can load the sample from small amount to large amount. The sample can be loaded directly to the top of the column. Once the sample is loaded, the polarity of the mobile phas is slowly increased. Collecting small fractions at the bottom of the column (1-3 mL) and all these are pooled together to get the sample in large quantities to monitor the progre of a reaction by monitoring the disappearance of a reactant or the appearance of a produ Ives the same principles as TLC, but can be applied to separate larger quantities than lumn chromatography, packing of the