iron(ii)ammonium sulfate = ferrous ammonium sulfate 12 (9-22).pdf · iron(ii)ammonium sulfate =...
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![Page 1: Iron(II)ammonium sulfate = ferrous ammonium sulfate 12 (9-22).pdf · Iron(II)ammonium sulfate = ferrous ammonium sulfate . FeSO. 4 (NH. 4) 2. SO. 4 ·6H. 2. O, M.W. = 392.14 g . Stock](https://reader030.vdocument.in/reader030/viewer/2022013104/5b3afe707f8b9a4b0a8e7e59/html5/thumbnails/1.jpg)
Expt 4: Determination of Iron by Absorption Spectrophotometry
Calibration solutions: Iron(II)ammonium sulfate = ferrous ammonium sulfate FeSO4(NH4)2SO4·6H2O, M.W. = 392.14 g Stock 1
0.210 g in 12.5 mL of 0.7 M H2SO4 diluted to 500 mL with deionized water Stock 2
25.00 mL Stock 1 diluted 10-fold with 5 mL 0.7 M H2SO4 using deionized water sufficient to make 250 mL total
Why not make stock 2 directly from scratch? Precision of mass measurement 0.210 g ± 0.1 mg
relative error = %05.0100210
1.0±=×
±mgmg
relative error = %5.010021
1.0±=×
±mgmg
less error associated with volume error of dilution
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Why add the sulfuric acid? To prevent oxidation of Fe(II) to Fe(III), which in turn reacts with water eventually to form Fe(OH)3(s) A matrix modifier!!! Next step: To various volumes of stock 2 you add 1 mL NH2OH·HCl (1.44 M) and 10 mL of sodium acetate (NaOOCH3) (1.22 M), all diluted to 100 mL Why NH2OH·HCl? Hydroxylamine is a reducing agent that helps to prevent Fe(III) formation. A matrix modifier!!! Why sodium acetate? A pH buffer to maintain solution at pH = 4.8±1 Another matrix modifier!!!
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Why maintain pH at roughly 4.8? Protonation of 1,10-phenanthroline competes with complexation with Fe(II) We want:
Fe(H2O)62+ + 3phen → Fe(phen)3
2+ + 6H2O Upon addition of third phen, complex turns red. (λmax = 508 nm) 3 matrix modifiers needed to make the method work. They prevent deleterious competing reactions (2 to minimize oxidation of Fe(II) and one to minimize protonation of 1,10 phenanthroline)
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Lab 4 Purposes: Determine an unknown ferrous concentration (via a calibration curve) using spectrophotometry data Understand quantitative relationships between transmittance, absorption, and concentration Understand relationships between measurement errors (random and systematic), sensitivity, and concentration
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Propagation of errors and sensitivity:
Measured → Calculation → Concentration response C = f(R) Measurement → Propagation of → Concentration error measurement error error ΔR, sR ΔC, sC Recall propagation of errors in mathematical calculations: If y = m + n,
222nmy sss +=
and, in terms of the standard deviations
22nmy sss +=
For cbay ⋅
= , 22222222 csbsasys cbay ++=
or, 222
⎟⎠⎞
⎜⎝⎛+⎟
⎠⎞
⎜⎝⎛+⎟
⎠⎞
⎜⎝⎛=
cs
bs
as
ys cbay
For y = log a, sy = 0.434 sa/a
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Measurement → Propagation of → Concentration error measurement error error ΔR, sR ΔC, sC ΔR = systematic (determinate) response error ΔC = systematic concentration error sR = random (indeterminate) response error sC = random concentration error Φ = sensitivity, dR/dC (ΔR/ΔC for linear relationships) In general,
RC Δ⋅Φ
=Δ1
and RC ss ⋅Φ
=1
The absolute concentration error is directly related to the response error and inversely related to sensitivity…
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Linear relationships, Φ = constant: Concentration error is directly related to response error:
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Concentration error is inversely related to sensitivity:
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Non-linear relationships: sensitivity is concentration dependent, therefore absolute concentration error becomes concentration dependent
e.g., transmittance and absorbance Beer’s Law: A = εbC = log (P0/P) = log (1/T) = -log T
ε = molar absorptivity (M-1 cm-1) at specified λ
b = path length (cm) (note, not the intercept)
C = concentration of absorbing species (M)
Slope=εb
C (M)
A
We expect a linear relationship between A and C.
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A = -log T or T = 10-A
A = εbC
In practice, we actually measure P and P0 and calculate A from T. T has a non-linear dependence on C:
T = 10-εbC
For y = log a, sy = 0.434sa/a
A = -log T, so sA = -0.434sT/T
Let’s look at this graphically…
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For a constant uncertainty in transmittance response, the absolute concentration error is highly dependent upon concentration:
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The rapidly changing sensitivity associated with transmittance versus concentration at constant error due to transmittance, sT, leads to a concentration error that is concentration dependent in the A versus C plot.
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T
C
Slope=εb
C (M)
A
sA
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T vs. C – fixed error, variable sensitivity
A vs. C – fixed sensitivity, variable error
Either way leads to:
sC
C
Non-linear increase in absolute concentration uncertainty with increasing concentration.
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What about relative concentration error (RCE)?
i.e. concentration error/concentration, (sC/C)
for a constant sC, RCE ↓ as C ↑
for a transmittance measurement, however, sc ↑ as C↑
leads to a minimum in sc versus C plot
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Stray light and wavelength error:
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Why we adjust 100% T with pure solvent or a blank solution:
The blank corrects for loss processes other than absorbance by the analyte.
The filter after the sample is usually a cut-off filter to remove long wavelengths (that can arise from second and higher order diffraction from the grating, these longer wavelengths constitute stray light.)
Who cares about some stray light?
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T
C We expect a logarithmic relationship between T and C. T = 10-A = 10-εbC
Note: in lab write up, α = intercept and β = slope (instead of b = intercept to avoid confusion with path length) In practice, from a calibration curve we get A = α + βC T = 10-(α + βC)
10-x = e-2.3x, so T = e-2.3(α + βC)
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Beer’s Law:
A = εbC = log (P0/P) = log (1/T) = -log T
T = 10-A = 10-εbC
ε = molar absorptivity
b = path length
c = concentration of absorbing species
Slope=εb
C (M)
A
We expect a linear relationship between A and C.
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Beer’s law assumes a single ε: