DIGEST ON AND ENZYMES·LAB 5 Factor decrease button Factor increase Mode selactor

Question

DIGEST ON AND ENZYMES·LAB 5 Factor decrease button Factor increase Mode selactor bution Mode indicators Digital readout Pren butiton Fiter lever and control knob absorbance control knob filter scale Sample compartment Power swiltch zero control knob In lab, your GTA will demonstrate how to use a spectrophotometer. Instructions can also be found on your lab benches. (2 points) When using a spectrophotometer, and measuring a solution in a sample tube, the spec- trophotometer indicates 00% transmittance. What does this mean? (Hint: Think about absorbance.) 6. a. b. What is a blank and what is it used for? You want to measure the concentration of starch in your samples and need to create a blank. Would you include I,KI? Why or why not? c.

Explanation / Answer

Question 6.

Question (a).

Answer. 100% Transmittance gives an optical density(absorbace) of "0".

Question (b)

Answer. The blank is a cuvette which contains everything that is in the sample (or experimental) cuvette, except the one material whose absorbance we are measuring. e.g. If a protein is dissolve in a particular buffer then that buffer will be used as a blank solution.

Question (c)

Answer.  Iodine is only sparingly soluble in water (0.3 g per litre); it is usual to dissolve it in potassium iodide solution (KI) to make a 0.01 M solution to use as a starch test reagent.

Therefore the blank for this measurement would contain everything including I2KI except starch.

Question 7.

Question (a). Different concentration of starch in two tubes will have different transmittance and hence different absorbance.

Question (b).

Answer. The tube having less transmittance and more absorbance would have more starch.

Question (c).

Answer. If there were same concentration of starch in two different tubes, both will have the same value of tansmittance and absorbance.

Question (d)

Answer. If there was no starch in the tube, that tube will have 100% transmittance and 0 absorbance.

Question 8.

Answer. The more concentrated would give high value of absorbance as compare to less concentrated one.

Question 9.

Answer.

Prepare

A. Starch solution Add a weighed amount of starch (0.5 g or 1.0 g) to a little heated water, mix to a paste, then dilute to 50 or 100 cm3.

B. Iodine solution in KI.

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