Paper Chromatography Notes PDF

# 5. Paper Chromatography Paper chromatography is used to separate mixtures of soluble substances using a suitable solvent. For example, a mixture of dyes in ink can be separated by paper chromatography. ## Consider the separation of purple ink: 1. A pencil line is drawn about 2 cm at the end of...

# 5. Paper Chromatography Paper chromatography is used to separate mixtures of soluble substances using a suitable solvent. For example, a mixture of dyes in ink can be separated by paper chromatography. ## Consider the separation of purple ink: 1. A pencil line is drawn about 2 cm at the end of the chromatography paper. 2. Using a capillary tube, the purple ink is placed on the pencil line as a spot. The spot is allowed to dry and the purple ink is applied on the first spot again. 3. Dyes which are suspected to be present in purple ink are also placed on the pencil line. 4. The chromatography paper is placed in a glass tank containing a solvent. The level of the solvent should be below the pencil line. 5. The glass tank is covered with a lid and the solvent rises. 6. When the level of the solvent nearly reaches the top, the chromatography paper is removed and allowed to dry. The result obtained is called a chromatogram. A diagram of this process is shown: ``` -> lid -> glass tank -> chromatography paper -> pencil line -> solvent level (ethanol) ``` 1. Purple ink is a mixture of two dyes (2 spots formed). 2. The two dyes are red dye and blue dye. For separation to be possible in paper chromatography, the mixture should be in the solvent. If a mixture is insoluble in a solvent, the spot remains on the baseline. The more soluble a component is, the further it travels up the paper. From the above chromatogram, blue dye is the most soluble since it travels the furthest. ## Use of locating agents A locating agent is used in paper chromatography to identify invisible spots (colorless spots). For example, amino acids can be separated by paper chromatography and they produce invisible spots. By spraying the chromatogram with ninhydrin, the colorless spots become visible (brown or purple). **Note:** In some cases, a component may be insoluble in a solvent and, therefore, it remains on the baseline. It may also happen two components overlap as a result of same solubility in the solvents. In such a case, another solvent is used. ## Identifying the Purity of a Substance Paper chromatography can be used to identify pure and impure substances (mixtures). A pure substance produces only one spot while impure substances produce more than one spot. ### Retention Factor Value (Rf Value) Rf values are calculated to identify components in a mixture. The Rf value is the ratio of the distance traveled by a spot to the distance travelled by the solvent. $$Rf = \frac{distance \ travelled \ by \ a \ spot}{distance \ travelled \ by \ the \ solvent}$$ A diagram for finding Rf value is shown: ``` ->Solvent front 8.3 cm 4.8 cm ->baseline ``` $$Rf = 4.8/8.3$$ $$Rf = 0.58$$ Rf value has no units and cannot be greater than one. ## Questions 1. Explain why the baseline is drawn with a pencil and not a pen? Ink will dissolve in the solvent and spread around the chromatography paper affecting the results. 2. Explain why the level of the solvent should be below the pencil line? This prevents immediate dissolving of the spot and hence no proper separation is obtained. 3. Explain why the glass tank is covered with a lid? The lid prevents evaporation of the solvent and the inside of the tank is saturated with vapour. ## Experiment A food coloring present in a soft drink is suspected to contain one of the following dyes; E150 and E151. Plan an experiment to investigate which one of the two dyes is present in the soft drink. Assume the dye present is E150. A diagram for the experiment is shown: ``` -> lid -> glass tank -> chromatography paper -> pencil line -> solvent level M -> E150 E151 ``` A spot of the food coloring is placed on the pencil line along with spots of E150 and E151. The chromatography paper is then placed in a glass tank containing a solvent and then the glass tank is covered with a lid. The solvent rises and separation of components occur when the solvent nearly reaches the top. The chromatography paper is removed and dried, Rf values calculated and compared. The Rf value formed by the food coloring is the same as E150.