The first thing I think that you need to understand in order to answer
this question is the nature of the pH scale. The pH scale is about
the concentration of hydrogen ions, however it is not a linear scale.
This page shows the scale:
search strategy: pH scale
A one unit shift in pH is equal to 10 times more (or fewer) hydrogren
ions. Water, pH 7, is neutral. Something pH 6 is acid, and has 10
times more hydrogen ions than water. Something ph 8 is alkaline, and
has 10 times fewer hydrogen ions than water. A shift of 0.3 is
approximately a factor of 2 - either halved, or doubled.
The next thing to consider is blood pH. Enzymes work happily only
within a certain pH range. This is part of the reason that our
stomachs contain acid, and our intestines alkaline (which, when they
meet, causes the creation of gas). The body needs to take action to
maintain the correct pH of blood (7.4).
According to this page (and the pages linked from it):
There are three actions that can be takes: buffer control (i.e.
chemical), respiratory control and renal control. The different
controls are used for different reasons, but the kidneys are used to
excrete excess acid or base, to solve the balance. The kidneys can
produce urine with a pH as low as 4.4.
So we know that the kidneys will change the pH of urine to maintain
the pH of the blood a 7.4, so why does the pH of the blood change?
The first, and less obvious answer, is breathing. When a person
breathes quickly (hyperventilates) this has the effect of removing too
much carbon dioxide (CO2) from the blood, changing its acidity CO2
dissolved in water is carbonic acid. Remove the CO2, you raise the pH
of the blood. (And this can cause dizziness, because the enzymes of
the brain only work within a limited pH).
The perhaps more obvious way that blood can change its pH is through
the food that we eat. Eating tends to cause blood pH (and therefore
urine pH) to rise.
According to this page:
This is why many peoples urine is acid in the morning, and alkaline
in the evening.
However, different foods have different effects some foods make the
blood more acid, others make it more alkaline. This page shows a
Fruits, counterintuitively, tend to make the blood alkaline because
the citric acid is converted to hydrogencarbonate, which forms
substances like sodium hydrogencarbonate (sodium bicarbonate, or
baking soda), when it gets into the blood.
Search strategy: blood pH food
A final point: this page
(referenced already above) says that urine pH below 6.0 or above 8.5
is considered unhealthy. So on these measures, you are OK.
Please feel free to request clarification prior to rating this answer.
Thanks for asking this interesting question,