PRACTICAL 2

Determination of sodium ion concentrations: Class Data

The class data for the determination of sodium by flame photometry and ion-selective electrode are provided here for simple statistical analysis (mean and standard deviation) using an Excel spreadsheet.

Instructions

A template Excel spreadsheet has already been prepared and you can download it by clicking here. Save a copy on your hard disk then open it in Excel either by double clicking it, or from the File menu if Excel is already running (do not try to open it directly then modify it or you may experience problems, depending on the browser and system set up you are using).

There are two ways you can enter the data from one class (maximum 12 values for each of the two determinations). Either (a) manually copy the data from one of the Tables below into cells A7 to B12 (flame photometer) and E7 to F12 (ISE) on the spreadsheet, or (b) download the text file "Data.doc" and cut and paste the data directly into the same cells (you need only paste into cell A7 or E7 as appropriate and the data will automatically flow into the correct cells). Do not enter data into any other cell. The spreadsheet will ignore any missing numbers that appear in the data as "-". Either way, you should end up with a table of data as in Figure 1 below. The mean and standard deviation will be calculated automatically.

Return to the practical manual and compare these values with each other and with the predicted values for accuracy and precision.

If you are keen and want to calculate the mean and S.D. for the data from both classes, then download the alternative spreadsheet and cut and paste the data from Data.doc into cells A7 to B18 and E7 to F18 (maximum 24 values each).

If you want further information on how to use Excel, the Computing Services Department has prepared Self Study documentation. Go to this page and enter Excel as the search term, then download the Word file (file 379).

All concentrations are in mM.
Class 1 (First Friday, a.m.), October 14th 2011
NOTE: The extra calculated data have not been e-mailed to me (black mark to those who promised) so you will just have to go ahead with the data we have.
It is clear that some of the results are way out i.e. the photometer results of 100, 25, 22.5 and 17.5 mM, and 25.7 for the ISE. I suspect that these are due partly to poor graph drawing rather than huge errors in the estimation, so they should be ignored for the purposes of calculation of the Standard Deviations. Have a go at doing this yourself and I will post my calculations and interpretations of the data next week. Remember that to calculate the true concentration, you need to know that there were 12 mmol of Na in the sachet.

	1	2	3	4	5	6	7	8	9	10	11	12
[Na⁺](flame)	47.5	40.0	60.0	61.7	47.5	65.0	22.5	100	67.5	25.0	17.5	66.3
[Na⁺] (ISE)	57.5	47.8	70.8	59.3	70.8	46.8	69.2	25.7	49.0	60.3	-	-

Class 2 (Second Friday, a.m.), October 21st 2011 No class this year

	1	2	3	4	5	6	7	8	9	10	11	12
[Na⁺](flame)	-	-	-	-	-	-	-	-	-	-	-	-
[Na⁺] (ISE)	-	-	-	-	-	-	-	-	-	-	-	-

CONCLUSIONS:

Using all of the submitted flame photometer results gives a sodium ion concentration of 51.7±23.4 mM. Omitting the 4 values that seem to be way out improves the data to 56.9±10.4 mM. Similarly, using all the ISE data gives 55.7±14.0 mM while omitting the one poor result gives 59.1±9.8 mM. The true, calculated value was 60 mM so the mean ISE result is very close. The FP result is not bad either. In terms of what was predicted, the ISE is indeed higher than the FP, which is consistent with its overestimation of sodium due to a partial response to potassium. However, there is no significant difference in SD, which is a measure of reproducibility. The ISE was predicted to give a smaller SD. However, both SDs are very high due to differences in the way different students carried out the experiment.

Figure 1: Your spreadsheet should look something like this. [back to instructions]

Return to

.Practicals Home Page...... Return to

.Practical 2: Theory...... Return to

.Practical 2: Methods