Problem measuring PH (it oscilates) for gastric juice

[alexepascual]

I bought a PH meter with glass electrode. I calibrated it using a buffer solution following the instructions.
When I measure the PH of tap water, vinegar, or other simple liquids it initially keeps changing but eventually it stabilizes at the correct value. But I tried to measure gastric juice and it oscillates within a range that is about 2 units wide. It never stabilizes. I considered the possibility that the meter I bought is not very good, but as so far gastric juice is the only one that is showing this behavior, I though it may have something to do with the nature of the juice. Something also strange is that I have obtained alkaline values. That may happen, but it is a little strange. Any ideas? Have you seen this oscillation with certain mixture of organic compounds?

 

[Borek]

Shouldn't happen, gastric juice is just a low pH solution of (mostly) HCl. Electrode and the meter are the suspects for me. The gastric juice pH is around 2, so outside of the buffers used for a typical calibration procedure. Do you have a 1.00 pH calibration solution to check?

 

[Dr Blizzard]

Just an educated guess, gastric juice mixtures contain HCL, NaCl and KCl and secreted hormones in the stomach lining to break down food. As foods are broken down the hormones render the food to polypeptides. Stomach enzymes cleave the polypeptides which are further broken down into amino acids. Amino acids are zwitterions which can either act as an acid or a base. I assume that the enzyme interaction change the polarity of the amino acids to alkaline and back again. I think for this reason you observe pH oscillations.

You could test this by inactivating the enzymes in gastric juice. In general, enzyme activity is temperature dependent due to ability to rearrange its structure configuration. From room temperature, 25℃, without adjusting pH meter temperature compensation function, try slowly cooling down the gastric juice to just above freezing; the rate of pH oscillations should slow or stop. Then try slowing warming the temperature, from 25℃ the gastric juice, as the gastric juice warms there should be an increase in the frequency of pH oscillations and around 45℃ the rate should stop. Note where the rate pH oscillations are most severe-may be around body temperature?

Working with amino acids and sugars are an analytical chemist worst nightmare.

 

[alexepascual]

Hi Borek
This morning I read your post and while most of it made sense, at first didn't agree with your interpretation of my problem.
My reasoning was: If it was a calibration problem , then I would just be getting the wrong result, but not that oscillation I was seeing.
My thought was "there must be something else going on" (I hadn't read Blizzard's post yet).
But I had just received some PH test strips and when I compared the values that I was getting while testing a sodium bicarbonate solution, they were very different. The PH I was getting on the meter was much lower than the one I got form the strips.
So something came to mind: What if the values for the stomach acid that are being read are so low (due to miscalibration) that they are out of range and the digital electronics is "going crazy" trying to find the value? The oscillation could be due to some algorithm that is running while trying to find the value.
The only buffer solution that I had now was a 4.0 one. The 6.9 one that came with the meter I had already used.
So I prepared the solution and found that it was way off (much lower). So I adjusted it carefully.
Then I measured vinegar and got 3.0, which seemed reasonable. Then I tested gastric juice again and got 3.3, which also seems reasonable. Not only that, but now it was not oscillating anymore. So your advice to re-calibrate worked. But I'll have to run the test several times to be sure. Thanks a lot Borek.

 

[alexepascual]

Just an educated guess, gastric juice mixtures contain HCL, NaCl and KCl and secreted hormones in the stomach lining to break down food. As foods are broken down the hormones render the food to polypeptides. Stomach enzymes cleave the polypeptides which are further broken down into amino acids. Amino acids are zwitterions which can either act as an acid or a base. I assume that the enzyme interaction change the polarity of the amino acids to alkaline and back again. I think for this reason you observe pH oscillations.
You could test this by inactivating the enzymes in gastric juice. In general, enzyme activity is temperature dependent due to ability to rearrange its structure configuration. From room temperature, 25℃, without adjusting pH meter temperature compensation function, try slowly cooling down the gastric juice to just above freezing; the rate of pH oscillations should slow or stop. Then try slowing warming the temperature, from 25℃ the gastric juice, as the gastric juice warms there should be an increase in the frequency of pH oscillations and around 45℃ the rate should stop. Note where the rate pH oscillations are most severe-may be around body temperature?
Working with amino acids and sugars are an analytical chemist worst nightmare.

Blizzard, My knowledge of the chemistry going on within the gastric juice is too elementary to be able to understand the possibility of something like what you describe. But I had a vague idea that something like that could be happening. I also wondered if the fact that there are oils in the mixture could affect the capacity of the meter to measure PH. But on the other hand, I know that gastric PH is being measured routinely during a test done on people with reflux.
This morning, before I read your post I had already recalibrated the meter (as I describe in my answer to Borek) and that apparently solved the problem.
But before that your hypothesis would have made sense. So I will have to run the test several times to be sure. Thanks a lot for your input.

 

[alexepascual]

Today I decided to measure the PH of gastric juice again. I though that the other day I had the problem solved after recalibrating the meter. But when I tried to measure it today, I got that oscillation of the number again. It was oscillating between let's say 2 to 4. So I decided to follow Blizzard's advice and put the sample in the refrigerator. I didn't put it in the freezer though, so it just got cold but not any close to freezing temperatures. I measured again and this time it was totally "crazy" and the numbers were alkaline and also out of range. It was oscillating between 11 and 25. So I am back to square one. Now I put the little flask close to the fan of my laptop to heat it up and measure again.. It looks like it is closer to room temperature now, let's see what I get... It is oscillating between 2.5 and 3.5
Something I didn't try when it was cold is to use the test strips. Maybe I'll try tomorrow. Any other ideas?