9.2. Step 2 – Model equation

Model equation
http://www.uttv.ee/naita?id=17637

https://www.youtube.com/watch?v=N45YNwToyac

The model equation (equation 9.1) enables calculating the output quantity value (result value) from the input quantity values. Input quantities are the directly measured quantities (or are calculated from directly measured quantities). In addition, the model equation has to enable accounting for all important measurement uncertainty sources. In the case of this analysis the model equation is the following:

(9.1)

The output quantity is C_{N_sample} – ammonium ion concentration in the water sample.

The input quantities are:

A_sample – absorbance of the dye solution obtained from the sample;
b₁ and b₀ – slope and intercept of the calibration graph;
f_d – dilution factor;
ΔC_dc – component taking into account uncertainty originating from possible decomposition or contamination.

The parameters A_sample, b₁, b₀ and f_d in the equation account for the directly measured input quantities. In contrast, ΔC_dc is introduced only for taking into account uncertainty due to possible decomposition of the photometric complex and due to possible contamination. Its value is zero as will be seen in section 9.4, so, it does not contribute to the value of C_{N_sample}. However, its uncertainty is different from zero and therefore will contribute to the uncertainty of C_{N_sample.}