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The meethod of standard additions was used to assess the accuracy of the Greiss method for...

The meethod of standard additions was used to assess the accuracy of the Greiss method for the measurement of nitrate. Standard nitrate was addded at 0, 1, 2, 3, and 4 mL to 5 10.00 mL aliquots of sample before being diluted to 25.00 mL in a volumetric flask. If the nitrate standard was 0.96 millimicro and the data is shown below, what is the concentration of nitrate in the original sample?

Added Nitrate (mL)    Signal    LINEST   

0                                  0.180

1.00                            0.207                     0.0289                  0.182

2.00                            0.245                     0.00189                0.004648

3.00                            0.274                      0.987                   0.00600

4.00                            0.291

Solutions

Expert Solution

The method of standard addition is a type of quantitative analysis approach often used in analytical chemistry whereby the standard is added directly to the aliquots of analyzed sample. A plot of the signal intensities of the solutions vs. the added concentrations yields a straight line. The concentration of the analyte is determined from the point at which the extrapolated line crosses the concentration axis at zero signal.

IMPORTANT NOTE: milimicro is not a concentration unit. I think you are talking about milimoles on liters, ie. milimolar. I will do the calculation using as initial concentration of the standard 0,96mM. you've probably made a mistake in transcription, review, and if another unit of concentration that provided, leave me a note and I'll help recalculating, however the procedure will show you, is the right way for develop this type of exercises.

1) Calculating standard concentration: will use following ecuation

Cf: final concentration

Vf:final volume (25mL)

Ci:initial concentration (0,96mM)

Vi:initial volumen (0,1,2,3,4mL)

The following table presents the calculated values for each concentration of standard solution

A plot of concentration vs signal allow determinate sample concentration for extrapolation

the equation of the line is as follows

Y=0,76X+0,18

Y=signal

X=concentration

extrapolation to signal de standand =0   give us the nitrate concentration in the sample.

0=0,76X+0,18

l-0,18/0,76l=X(y=0)

X=0,24mM

queen_honey_blossom answered 2 years ago
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