Answers - Finally!

With the arrival of the Hach Colorimeter I was finally able to get good quality data and make some solid conclusions. Here's what it looks like: The meter itself is the black device that looks like a TV remote clicker. To it's right is a container of 5-in-1 test strips.

I did some practice runs on tap water from my home which is well derived and free of any baseline chlorine by making a 1:10 bleach dilution which allowed me use much smaller source volumes, everything scaled back by factors of 10: instead of 6 ml of undiluted bleach in 55 gallons I could do 0.6 ml of the 1:10 dilution into 0.55 gallons of water and get the same concentrations. Using the test strips I could get an idea if we really were in the ballpark range. Here are the results:

5 in 1 Dipstick Measurements done on well water using 1:10 bleach dilution in 0.55 gallons water
Bleach (0.825%) ml			Total Cl (ppm)		Free Cl (ppm)
Trial 1
0.5			1		0
0.6			2		0.5
0.7			4		0.5
0.8			4 to 10		1 to 2
0.9			4		0.5
1			4		1
Trial 2
0.5			2		0.5
0.6			4		1
0.7			4		1
0.8			4		1
0.9			4 to 10		1 to 2
1			<10		4

So, based on dipstick values we are pretty good in terms of being in the range. The problem with the dipstick is that they are a "blunt instrument". Rather than giving you a reliable numerical measurement the strips basically tell you one of three things: "none/not enough", "enough" or "too much". A good start for estimating but lacking in finesse. Enter the colorimeter. It works by detecting the absorption of light at a wavelength of 528nm. To perform the test water is mixed with a small pre-measured packet of DPD (dietheyl-phenylenediamene) powder; if chlorine is present it reacts to turn the solution pink.

The meter then reads the strength of absorption proportional to the amount of chlorine present. We get a number. A real quantitative value that can be used to generate data, produce equations and make predictions.

 

The challenge with the meter is that, as usual, the devil lies in the details. Every time you draw up a sample from a different source or strength, everything has to be thoroughly cleaned with tap water (sample cups, measuring tubes, syringes for drawing up, etc). So, even a few relatively simple measurements can chew up an hour or too pretty easily. Now add in going in and out to add increments of bleach to the barrel, letting it mix for 20-30 minutes with the occasional stir, siphoning out samples, labeling the jars plus doing the measurements and you can lose an entire afternoon really fast! Anyways, I was able to squeeze a few hours together yesterday and did just that. Here are the results:

Colorimeter Measurements			55 Gallon Barrel		Dipstick Measurements			55 Gallon Barrel
Bleach (8.25%) ml			Total Cl (ppm)		Total Cl (ppm)		Free Cl (ppm)
Trial 1
0			0.01		0		0
5	0.85		0.85		0		0
6	1.4		1.4		0.5		0
7	1.77		1.77		0.5		0
8	1.58	*	1.58	*	0	*	0	*
9	2.2		2.2		0.5		0
10	2.2		2.2		0.5		0
11	2.2		2.2		1		0.5
12	2.2		2.2		1		0.5

 

However, note that the value for 8 ml is a statistical outlier leading to some inaccuracy in the linear equation. If we drop that value we get a better estimate of the line and a different equation.

This improved data and equation now allow us to confidently plug in any amount or volume of bleach we may want to add to the 55 gallon drum and come up with a reliable concentration of chlorine remaining in the water, and it should stand up to measurement with the colorimeter. Here is a table of such a prediction. So, if the WHO established chlorine level is 2.5 mg/L (PPM) then a dose of somewhere between 9 and 10 ml of 8.25% bleach will reliably produce that level. But, there is a catch: Every different source water will have it's own unique chlorine requirement so that the values I have established for rainwater in Hurley, NY may be very different, from say, water taken from the Gowanus Canal in Brooklyn, NY. This is called "chlorine demand" and means that one of the first things we will need to do in Panama, in tandem with installing the new tanks, is to set up a barrel and starting testing to establish what the chlorine demand is for their source water (rain) and make up a table for them to use once the system is up and running. Still, this gives us a good idea of the variability of chlorine dosing and the fact that untested calculations alone are not enough. Very satisfying.

Table based on equation Y=0.3263x-0.6474 5 0.9841 6 1.3104 7 1.6367 8 1.963 9 2.2893 10 2.6156 11 2.9419 12 3.2682 13 3.5945 14 3.9208 15 4.2471