Situation: The amount of protein in a sample of cheese is determined by a Kjeldahl analysis for nitrogen. After digesting a 0.9814-g sample of cheese, the nitrogen is oxidized to NH4+, converted to NH3 with NaOH, and distilled into a collection flask containing 50.00 mL of 0.1047 M HCl. The excess HCl is back titrated with 0.1183 M NaOH, requiring 22.84 mL to reach the bromothymol blue end point. Questions: Write the reactions involved in the above Kjeldahl analysis? Bromothymol blue is used as the indicator of titration; can you give the reason for this?

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Author:Steven S. Zumdahl, Susan A. Zumdahl, Donald J. DeCoste
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Situation: The amount of protein in a sample of cheese is determined by a Kjeldahl analysis for nitrogen. After digesting a 0.9814-g sample of cheese, the nitrogen is oxidized to NH4+, converted to NH3 with NaOH, and distilled into a collection flask containing 50.00 mL of 0.1047 M HCl. The excess HCl is back titrated with 0.1183 M NaOH, requiring 22.84 mL to reach the bromothymol blue end point.

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  1. Write the reactions involved in the above Kjeldahl analysis?
  2. Bromothymol blue is used as the indicator of titration; can you give the reason for this? 
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