590. Home-made Titrino tip

Metrohm make a range of autotitrators called titrino, titrando etc. While they are great, they are expensive (ca AU$15,000 +/- 5,000) -- and the parts are ridiculously overpriced. For example, a simple pH electrode cable costs AU$100-150, which isn't a good use of taxpayer money.


I'm also not a big fan of their titration tips, one of which is shown in the photo below

The piece in this photo costs AU$30:

AU$30 -- reasonable?

Besides, in my own experience they don't work as well as the home made tips we used in a lab I used to work in.

So, here's a description of an alternative type of tip for doing titrations with a titrino.

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The design in my old lab:

This is the tip -- it's a glass capillary which is connected to 1/16 PTFE tubing using a piece of elastic silicone tubing

The titrino is connected to the 1/16 PTFE tubing via an M6 (or 1/4-28) PTFE union.

The PTFE tubing is connected to the union using an 1/4-28 (or M6) ferrule

Like this

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In my current lab:
Since I wasn't sure what the size of the capillary and the connecting silicone tubing was, I had to guess, and used the following parts:

Capillary OD 0.66 mm ID 0.5 mm
Silicone tubing OD  3 mm ID 0.5 mm
PTFE tubing 1/16 (ID 0.5 x OD 1.6 mm)
1/16 ID and 1/4 OD PEEK Ferrule
1/4-28 PTFE connector

It basically came down to what I could order online from a Chinese equivalent to ebay called taobao.

NOTE: the capillary here doesn't go that great with the silicone tubing, so I had to glue it using silicone glue. A smaller ID silicone tubing or a larger OD capillary might work better. Note also that you'd want as thin a capillary ID as possible though to avoid diffusion and leakage.

Here's the assembly (the glass capillary is a bit short since it broke):

And here's the capillary with the silicone part (the light in my office is pretty bad).

I used silicone glue to fix the capillary in place:

Some more photos of the different parts:

Very affordable Chinese capillaries -- $5 for 1,000!

1/16 inner and 1/4 outer diameter PEEK ferrule

The PEEK ferrule and the M6 (1/4-28) union

584. Build your own pH meter (analogue)

A few years ago a friend of mine with a degree in engineering helped me build a simple pH meter. In terms of quality of the readings there is no difference between it and our $15,000 titrino.

Either way, I'm putting the description of it online here in case other people are interested. Note that 'we' designed it based on what we found available from various websites, so the design is hardly unique.

Also note that it requires a multimeter/voltmeter to get a reading -- there's no USB/serial output or anything fancy like that.

(in related news I'm planning on taking night classes in electronics at a local TAFE (which is like a community college) so that I can start building more instruments/toys myself. A potentiostat would be nice...)

Anyway, back when we built this we ordered the parts from Digikey (I lived in the US at the time), and here's a list of what we got as shown on the invoice (the parts numbers of the linked items differ in some cases -- I presume that they are the same, but cannot guarantee that they are.
* Note that you'll need more than one of some items.
* Note that the parts includes stuff for using a whiteboard/prototyping board i.e. we first made a non-soldiered version, and then made a permanent assembly. 
* The stuff for the final pH meter (i.e. the stuff in the pomona box) is shown in red
* The stuff for connection the pH meter to a voltmeter/multimeter is shown in blue
* The stuff that's needed for the power supply is in bold black
* Stuff that I ordered at the same time but can't remember what we used it for is in the default colour (i.e. black)

Price     Item number               Description       
(USD)    (Digikey)
---------     ---------------------                --------------------      
3.05       LMC6081IN-ND      IC OP AMP PREC CMOS SINGLE 8-DIP  
2.96       ACX1046-ND          Conn. enc. bulkhead female jack x 2
3.78       7-1437529-5-ND     Conn socket dip 8 pos gold T/h

7.71       J6212-ND               BNC cable        

8.07       501-1032-NB          BNC female-dbl banana
47.85     945-1081-ND          Converter AC/DC 15W +/-12V out DL T/H


In addition, looking at the board (see pictures below), you'll also need
3 x female banana sockets
2 x K5M104(?) capacitor (x 2)
2 x BC1020TR-ND BC102 capacitor (x 2)
3 x female banana sockets
and lots of wires.


You'll measure the potential (emf) using a voltmeter, and by calibrating  the potential against a set of pH standards you can calculate the pH. In theory 0.0 mV should be at pH 7, and the potential should increase ca 59 mV per pH unit, so that pH 6 is +59 mV and pH 8 is -59 mV.

Whether this is actually true (it won't be) depends on your pH electrode, temperature, ionic strength etc.

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pH box:

Add caption

Annotated view

The BNC cable is connected to a double banana contact, which attaches to the voltmeter

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Power supply:
You can get a proper one, or build one yourself. I did the latter. It ain't pretty -- in fact, the following pictures should horrify you. NOTE: unless you know what you are doing you MIGHT DIE! Playing with high voltage stuff may also be ILLEGAL for unlicensed people in some jurisdictions.

Power OUT -- +12 and -12 V, and ground.

Power IN

Green goes from Ground IN to the ground pin on the right on the power supply. The thin yellow wire then goes to the ground pin on the left on the power supply. The fat yellow wire then goes to the ground out banana plug. If your ground touches ANY of the other pins you may ELECTROCUTE yourself.