As part of an analytical chemistry laboratory course, a student measured the Ca2

Question

As part of an analytical chemistry laboratory course, a student measured the Ca2 content in two water samples, city-supplied drinking water and well-supplied drinking water, using two different analytical methods, flame atomic absorption spectrometry (FAAS) and EDTA complexometric titration. The results of this experiment are shown below as the mean Ca2 concentration (x) ± standard deviation (s) in units of parts per million (ppm). Each sample was measured five times (n = 5) by each method.

A) Method Comparison: For each drinking water sample (city and well), compare the Ca2 content measured with FAAS and EDTA titration. Calculate the t value (tcalc) for each sample. Do the methods produce statistically different results at the 95% confidence level when measuring the Ca2 content of the two drinking water samples (Yes or No)? A list of Student's t values at several confidence levels can be found in the Student's t table.

Map Sapling Learning As part of an analytical chemistry laboratory course, a student measured the Ca content in two water samples, city-supplied drinking water and well-supplied drinking water, using two different analytical methods, flame atomic absorption spectrometry (FAAS) and EDTA complexometric titration. The results of this experiment are shown below as the mean Ca parts per million (ppm). Each sample was measured five times (n E5) by each method. City supplied Drinking Water (Xt s) Method Well Supplied Drinking Water (xt s) FAAS 67.99 0.65 ppm 58.30 0.64 ppm 59.05 t 0.99 ppm 68.84 t 1.0 ppm EDTA Titration A) Method Comparison: For each drinking water sample (city and we compare the Number Yes Ca content measured with FAAS and No EDTA titration. Calculate the tvalue (toalc) Cit for each sample. Do the methods produce statistically different results at the 95% confidence level when measuring the Number Ca content of the two drinking water Yes samples (Yes or No)? A list of Student's t Well O No values at several confidence levels can be found in the Student's t table.

Explanation / Answer

here we use t-test and

t=(mean1-mean2)/((sp*(1/n1 +1/n2)1/2) and sp2=((n1-1)s12+(n2-1)s22)/n and with df is n=n1+n2-2

for city supplied drinking water

sp2=((5-1)*0.64*0.64+(5-1)*0.99*0.99)/8=0.6949

sp=0.8336

t=(58.3-59.05)/(0.8336*(1/5 + 1/5))=-2.24

critical t(0.05,8)=2.3 is more than the absolute value of caculated t=|-2.24|=2.24, which means there is no difference in the method

for well supplied water

sp2=((5-1)*0.65*0.65+(5-1)*1*1)/8=0.7112

sp=0.8434

t=(67.99-68.84)/(0.8434*(1/5 + 1/5))=-2.52

critical t(0.05,8)=2.3 is less than the absolute value of caculated t=|-2.52|=2.52, which means there is difference in the method

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