I need a few clarifications about the formaldehyde method (SE F DEC manual, B-5.1 , formaldehyde in fish using NASH's reagent )
1. NASH's reagent use in this method, I saw the leterature, that it should be freshly prepared, but in this method it is not mention. Is it ok?
2. Time period between sample prepartion and UV-VIS reading. The color develop with the time. Hence, I think thereshould be limited time before the reading (2 min or 10 mi etc). Unless the absorbance varied. Is it ok?
3. there is a factor in calculation, what will be the approximate value of the factor, this is for cross check my value
Dear
the fish samples under verification were cut into small pieces.
Then fish flesh was taken into blender for homogenization and
blended for 10 minutes. Then a 60 ml of 6% tri-chloro-acetic
acid was added for extraction of formaldehyde from the fish
flesh. The extracted solution was then filtered by a Whatman
No.1 of filter paper. Then pH of the solution was determined
by a pH meter. Though the addition of tri-chloro-acetic acid
reduced the pH value of the sample it was adjusted the pH
between 6.00-7.00 of the sample by using Potassium
hydroxide (KOH) and Hydrochloric acid (HCl). Then 5 ml of
sample solution was taken in a 50 ml of volumetric flask. Then
the sample was kept in a freeze (- 200C) for 1 h. During
analysis, the sample was taken out of the freeze and 2 ml of
previously prepared Nash’s reagent was added as indicator.
Fish sample was then heated in the water bath at 60
0 C for 30 minutes. The absorbance of the sample in cuvette was
measured at 415 nm immediately by UV/v spectrophotometer
(Thermo Fisher Scientific, Waltham, MA). Triplicate of the
absorbance was made for each sample and recorded for further
calculation. The sample reading was placed in the standard
curve for the calculation of formaldehyde content of the
sample.
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