Why is blank titre value greter than back titre value?
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A blank titration is done without the analyte present to check for possible sources of error in the "blank" solution. For example, deionized water is slightly acidic and may affect the results of an acid base titration (if one want a highly accurate concentration). So he would do a blank titration to find the concentration of H3O+ in the water and use it to correct the conentration of his analyte. Blank titration allows the amount of reactive substance within the plain solvent to be determined and hence allows a determination of the error in future titration experiments using this solvent.
A back titration is used when it is diffucult to find an endpoint in a normal titration (for example, if the analyte is not very soluble in water). One react the analyte with an excess of titrant and then titrtate the excess titrant. Subtracting the moles of excess titrant from the original amount he used gives the moles of titrant reacted with the analyte. From that he can figure out moles of analyte.
A back titration is used when it is diffucult to find an endpoint in a normal titration (for example, if the analyte is not very soluble in water). One react the analyte with an excess of titrant and then titrtate the excess titrant. Subtracting the moles of excess titrant from the original amount he used gives the moles of titrant reacted with the analyte. From that he can figure out moles of analyte.
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