[RobbinB]

We would like to use UV absorption at 240 nm to standardize our 30% hydrogen peroxide solution.  We have found extinction coefficients listed as 35 (or 44) M-1 cm-1.  We measured absorption of the 30% peroxide in triplicate (quartz cuvette) at 240 nm and got an average reading of 0.45 AU.  Based on MW of 34 g/mol and 1 cm cuvette, using the 35 M-1 cm-1 we calculate peroxide concentration as 0.43 g/L, which is equal to 4.3% (we are assuming % is % w/v = g / 100 mL as is convention in pharmacy).

We understand the peroxide isn't stable, which is why we are measuring it, but the bottle was new so 4.3% doesn't seem to make sense.  Is there an error in the calculations or are we doing something wrong?

[Golden_4_Life]

Does the label claim say "%w/w" or "%w/v"?  Contrary to conventional wisdom, there IS a difference!

*You ought to prep a set of alternative standard solutions such that:
STD 1 = 30 mL of H2O2 upon which is added distilled water to the 100 mL mark.
STD 2 = 30 grams of H2O2 upon which is added 70 grams of distilled water.
Your neat sample.

a. Dilute each STD by 1:50
b. blank the UV-Vis with distilled water.
c. pour over STD 1, get the O.D. reading.
d. rinse throughly the cuvette, and measure/verify that the O.D. is < 0.001.
e. pour over second STD 2, get the O.D. reading.
f. rinse cuvette and get reading on di water.
g. assay your 1:50 dilution of the neat real test sample; and get the O.D.
h. compare the sample O.D. to either of the STD 1 or STD 2 and see which value agrees with your test sample.


Good luck
and measure the optical density at the wavelength of maximum absorbance.

Then