According to the question,

First reading = 8.48

Second reading = 8.35

Now, let the third reading be ‘x’

Average pH should be between 8.2 and 8.5

Average \[pH~=\text{ }\left( 8.48\text{ }+\text{ }8.35\text{ }+\text{ }x \right)/3\]

Multiplying each term by 3, we get

\[\Rightarrow ~24.6\text{ }<\text{ }16.83\text{ }+\text{ }x\text{ }<\text{ }25.5\]

Subtracting 16.83 from each term, we get

\[\Rightarrow ~7.77\text{ }<\text{ }x\text{ }<\text{ }8.67\]

Therefore, from the above equation,

We get that,

The third pH reading should be between 7.77 and 8.67