Question

Most analytical machines used in science can either measure absorbance of light, like the spectrophotometers used in BioZ 151 and 152 labs, or can measure change in electrical conductance.

In order to convert things like absorbance values into concentration data, a linear regression is used.

In order to determine nitrite concentration in water samples, nitrite is reacted with several chemicals to produce a purple color. The absorbance of the solution is measured from known amounts of nitrite to produce a standard curve to analyze samples.

Use the data in the provided table, or from the linked spreadsheet to make a graph and perform a linear regression using treadline.

mg/L NO3-	Absorbance
0	0
0.1	0.183
0.3	0.542
0.5	0.914
0.7	1.298
1	1.61

In order for this to work, samples must be diluted prior to measurement so that the concentration falls within the range of the standard curve. If a sample underwent a 0.01 dilution, and produce an absorbance value of 0.236, what is the resultant concentration of NO₂^- in mg per L? Report your answer to two decimal places.

Tip: The regression equation will give you the final concentration. The dilution is equal to V₁/V₂. The question is asking for the initial concentration.

Answer 1

using excel

got to data - > data analysis -> regression

note that you should click on "constant is zero "

result

SUMMARY OUTPUT
Regression Statistics
Multiple R	0.997735157
R Square	0.995475444
Adjusted R Square	0.795475444
Standard Error	0.07015919
Observations	6
ANOVA
	df	SS	MS	F	Significance F
Regression	1	5.41494144	5.414941	1100.081	4.93E-06
Residual	5	0.02461156	0.004922
Total	6	5.439553
	Coefficients	Standard Error	t Stat	P-value	Lower 95%	Upper 95%	Lower 95.0%	Upper 95.0%
Intercept	0	#N/A	#N/A	#N/A	#N/A	#N/A	#N/A	#N/A
mg/L NO3-	1.71548913	0.051722041	33.16747	4.68E-07	1.582533	1.848445	1.582533	1.848445

y = 1.715489 x

0.236 = 1.715489 *x

x = 0.236/1.715489

= 0.13757

c1 *v1 = c2 * v2

c2 = x = 0.13757

hence

c1 = c2 * v2/v1 = 0.13757/0.01 = 13.757