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IntraWinding

Sat May 29 2010, 05:04PM

Registered Member #2261 Joined: Mon Aug 03 2009, 01:19AM
Location: London, UK
Posts: 581

Would KCl be a better potassium source because it's very difficult to get pure anhydrous KOH due to it absorbing both moisture and CO2 from the atmosphere?

Proud Mary

Sat May 29 2010, 06:28PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

IntraWinding wrote ...

Would KCl be a better potassium source because it's very difficult to get pure anhydrous KOH due to it absorbing both moisture and CO2 from the atmosphere?

A good point, but as I shall be decanting and weighing immediately from a fresh bottle of reagent grade anhydrous KOH, it doesn't matter if the sample puts on weight from atmospheric H2O and CO2 absorption after the weighing, because the quantity of K-40 in the sample will remain the same.

In any case, I can only weigh to the nearest 10mg with my Ohaus balance, and so don't expect - or really need - a tour de force of significance arithmetic. We already know that the gamma detection efficiency is unlikely to be more than the low percents, so what we're looking for is one or even no sig figs before the point, and one afterwards.

Geigermania goes on and on...

IntraWinding

Sun May 30 2010, 02:02PM

Registered Member #2261 Joined: Mon Aug 03 2009, 01:19AM
Location: London, UK
Posts: 581

OK, it's probably too small a difference to matter here, but I've noticed that even the original spec. for Na/K hydroxide can be lower than other chemicals from a given range, and I guess that's because to keep costs down the product gets some air exposure during manufacture or packaging.

Proud Mary

Sun May 30 2010, 03:22PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

My bottle of Potassium Hydroxide Pellets AR 500g came from ReAgent Chemical Services Ltd of Runcorn, a firm accredited to ISO 9001:2000 for the manufacture and
supply of reagents and standards. Their KOH AR has a minimum purity of 99.8%, which is completely acceptable within the circle of uncertainty created by all the specified measuring instrument errors used in this calibration experiment. So there!

IntraWinding

Sun May 30 2010, 05:45PM

Registered Member #2261 Joined: Mon Aug 03 2009, 01:19AM
Location: London, UK
Posts: 581

That's some seriously pure KOH!

Proud Mary

Sun May 30 2010, 06:53PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

IntraWinding wrote ...

That's some seriously pure KOH!

The 500g cost about twenty squid with VAT and carriage, but it should last simply yonks if I find that 10 gram samples produce a count sufficiently distinct from the background to make a show of things - a sample cost of 40p per calibration.

You've got me all worried now about CO2 sneaking into the bottle as the KOH level goes down, because calibrations aren't something I'll be doing very often. I could decant just 10g to check that 10g is, in practice, enough. If it is, I can decant the remainder into sealed plastic phials (more to buy! groan) of 10g each, where the KOH should remain in good condition for a long time. I might then be able to use the samples without taking them out of the phials, as the 1.461 MeV gamma rays won't have their noses put out of joint by one millimetre of plastic, in any way I could detect. So I may very well have some samples to spare if I go down this path, and it all goes swimmingly. But as I have to teach myself as I go along, things don't always go as hoped - the best laid scams of lice and men, and all that.

With the summer weather coming on, I don't want to be cooped up here with only a rat sitting on top of a skull for company, when I could be soaking up the sun on the beach, so the experimental season will come to a close for me this summer once I've finished this project. Hurrah!

IntraWinding

Mon May 31 2010, 02:26AM

Registered Member #2261 Joined: Mon Aug 03 2009, 01:19AM
Location: London, UK
Posts: 581

I guess it's worth taking ten grams and just seeing what happens to its weight if you leave it out. Maybe it takes more than just brief exposure to the air to cause any significant abortion.

Proud Mary

Mon May 31 2010, 11:54AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

IntraWinding wrote ...

I guess it's worth taking ten grams and just seeing what happens to its weight if you leave it out. Maybe it takes more than just brief exposure to the air to cause any significant abortion.

Well, I vaguely recall that KOH was one of the ingredients in the scrubber cartridge for the Siebe Gorman Mk IV Amphibian rebreather, so it must be pretty aggressive in moist air, but I can't get too worried over this detail now, as it will only become an issue as my KOH stock ages. Nothing I could measure is going to happen in the 20 seconds it takes me to open the bottle, and decant and weigh the first 10g sample.

Proud Mary

Wed Jun 02 2010, 08:23PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

under construction

Making the K-40 Calibration Standard

Preliminary Calculations

To find the quantity of pure potassium in a 10gm sample of KOH

1 Mole KOH = 56.105 gm

Potassium: Standard Atomic Mass = 39.0983

Therefore percentage of potassium in KOH is

39.0983/56.1056 = 69.69%

So: 6.969 gm of my 10 gm sample would be potassium if my KOH were pure. But it is only 99.98%
pure, so I have

6.969 x 99.8% = 6.968 gm of pure potassium.

To work out the number of atoms of K-40 in the sample:

0.012% of natural potassium is radioactive K-40, so my 10 gm sample contains

6.968 x 0.012% = 8.362 x10-4 gm of pure radiopotassium bursting with gamma ray goodness, which is

2.089 x 10-5 Moles.

As the number of atoms in a Mole is always and forever 6.0222 x 10-23,
(Avogadroâ€™s Number) I have

1.26 x 10-19 atoms of K-40 in my 10g sample - hereinafter called No

How many atomic disintegrations per second are happening in the sample?

We need to work out the decay constant from the half life like this:

If No is the number of atoms present to start with, and Nt is the number left after time t, we can say

Nt = No e-at - where a is the decay constant.

Since everyone from Jesus down knows that the half-life of K-40 is 1.26 x 10-9 years or 6.63 x 10-14 minutes, we can say

Nt/No = 0.5 = exp(-a * 6.63 x 10-14)

Finally, we can work out the number of atomic disintegrations per minute (N).

Using N1 for the number of atoms remaining after 1 minute (t = 1), we can say:

N1 = No exp(-a)

N = (No - N1) = No - No exp(-a)

N = No {1 - exp(-a)} = 12,590 disintegrations per minute or 210 disintegrations/sec

Now, by way of illustration only, if my detector under calibration detected just 21 of these 210 disintegrations every second,
I would know that my detection system was 10% efficient.

In real life, I am expecting my G26 GM tube - big 'n butch though it may well be - to be less than 1% efficient as a gamma detector
- but let's wait and see

IntraWinding

Wed Jun 02 2010, 10:03PM

Registered Member #2261 Joined: Mon Aug 03 2009, 01:19AM
Location: London, UK
Posts: 581

Wow! My maths is so rusty, I'll take your word for it

I spent ages yesterday with a piece of acrylic plate double sided taped to the end of a motor so I could machine a 25.2 mm diameter disc to use on the end of my acrylic tube. One day I must get a lathe.

I've been worrying that the non corrosive silicone I use to support the anode end of my GM tube will get so little ventilation it could take forever to set. I now plan to have most of the silicone attached and set on the acrylic disk before I solvent weld the disc to the end of the acrylic tube. Then when I fit the GM tube I'll just need a little extra to glue it to the pre-set 'blob'. That will have very modest ventilation requirements.

Progress is very slow as I have lots of other things distracting me. Could I possibly take you up on your offer of the tiny HT supply to keep delays to a minimum? I'm sure my tube will turn out similar to yours.

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