The oxidising strength of Cr2O72- is greatest in a solution of pH?

And so I suppose that #pH=1.0#

In an acid milieu dichromate undergoes reduction to chromic ion...

#underbrace(Cr_2O_7^(2-))_"orange red" +14H^+ + 6e^(-)rarr#

#underbrace(2Cr^(3+))_"green" + 7H_2O(l)#

Some chromate ion, #CrO_4^(2-)#, MAY be formed in acidic solution...but this is NOT a redox solution...and chromate is still #Cr(VI+)#...

#Cr_2O_7^(2-) +H_2Orarr 2CrO_4^(2-) + 2H^+#

You can compare the relative strengths of oxidising agents by consulting a table of standard electrode potentials (#sf(E^@)#).

The more +ve the value, the greater is the ability to take in electrons and cause oxidation.

#sf(Cr_2O_7^(2-)+14H^(+)+6erightleftharpoons2Cr^(3+)+7H_2Ocolor(white)(xxxx)E^@=+1.33color(white)(x)V)#

These are under standard conditions where pH = 0 so pH 1 is the closest.

In high pH conditions chromium(VI) is present as yellow #sf(CrO_4^(2-))# ions:

#sf(CrO_4^(2-)+4H_2OrightleftharpoonsCr(OH)_3+5OH^(-)color(white)(xxxxxx)E^@=-0.13color(white)(x)V)#

As you can see the value is much less +ve so you would not expect it to be such a good oxidising agent at high pH values.