Nitrite ion, NO_2^-, is a preservative for bacon and other foods, but it is pote

Question

Nitrite ion, NO_2^-, is a preservative for bacon and other foods, but it is potentially carcinogenic. A spectrophotometric determination of NO_2^- makes use of the following reactions. Here is an abbreviated procedure for the determination. To 50.0 ml. of unknown solution containing nitrite is added 1.00 mL of sulfanilic acid solution. After 10 min, 2.00 mL of 1-aminonaphthalene solution and 1.00 mL of buffer are added. After 15 min, the absorbance is read at 520 nm in a 5.00-cm cell. The following solutions were analyzed. A. 50.0 mL of food extract known to contain no nitrite (that is, a negligible amount); final absorbance = 0.151. 50.0 mL of food extract suspected of containing nitrite; final absorbance = 0.630. Same as B, but with 10.0 mu L of 7.50 times 10^-3 M NaNO_2 added to the 50.0-mL sample; final absorbance = 0.953. Calculate the molar absorptivity, epsilon, of the colored product. Remember that a 5.00-cm cell was used. M^-1-cm^-1 How many micrograms of NO_2^- were present in 50.0 mL of food extract B? Mu g

Explanation / Answer

From the given data

(a) molar absortivity = 0.479/0.00125 x 5 = 76.64 M-1.cm-1

(b) concentration of standard in solution = 7.5 x 10^-3 M x 0.01 ml/50.01 ml = 1.6 x 10^-6 M

Using,

Cx/(Cx + Cs) = Ax/(Ax + As)

with,

Cx = concentration of nitrite in unknown

Cs = concentration of standard

Ax = absorbance of unknown solution of nitrite

As = absorbance of standard + unknown nitrite solution

we get,

Cx/(Cx + 1.5 x 10^-6) = (0.63 - 0.151)/(0.953 - 0.151)

0.801Cx = 0.479Cx + 7.2 x 10^-7

Cx = 2.24 x 10^-6 M

mass of nitrite in solution = 2.2.4 x 10^-6 M x 0.05 L x 46 g/mol x 10^6 = 5.152 micrograms

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