Passage Within the Renal System, Two Buffers Operate to Counter the the Production

Passage
Within the renal system, two buffers operate to counter the production of excess H⁺ from luminal H₂CO₃.  They are located within the tubular lumen, and maintain urinary pH levels between 4.5 and 8.0.The first of these is a phosphate buffer, which serves as the primary buffer for urine because its pK_a is very close to extracellular fluid pH of 7.4.  The phosphate buffer operates according to Reaction 1.  H₂PO₄⁻ is excreted in urine.
Reaction 1During acidosis when tubular pH is low, the phosphate buffer system becomes saturated.  An ammonia buffer system then becomes the primary buffer, and NH₄⁺ is excreted in urine.
Reaction 2Titratable acid (TA) is the total acid in urine, and it can be found using Folin's method.  Under normal physiological conditions, TA is approximately equal to the amount of phosphate in urine, which is a product of serum phosphate concentration and glomerular filtration rate.  This makes TA a good indicator of kidney function.Folin's method involves back titration of a urine sample to a pH of 7.4 using a strong base.  In a typical procedure, potassium oxalate is added to remove calcium from the sample prior to titration with 1.0 M NaOH.  The titration continues until the solution reaches a pH of 7.4 (measured by a pH meter) .  The amount of base required for the titration can then be used to calculate the amount of H₂PO₄⁻ in the urine sample.
-What are the predominant species in the phosphate buffer system and in the ammonia buffer system in an aqueous solution at a pH of 8?

A) H₂PO₄⁻ and NH₄⁺
B) H₂PO₄⁻ and NH₃
C) HPO₄²⁻ and NH₄⁺
D) HPO₄²⁻ and NH₃

Correct Answer:

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