Reef Chemistry Question of the Day #161 Using a Syringe

[Randy Holmes-Farley]

Actually on second thought, you would have to flip the syringe around to get the accurate amount its definitely not 2.8 mL because there is way to much air in the syringe but it's most likely not 1.6mL either because there is a small amount of fluid still in the tip. If you flipped the syringe around you could accurately measure it.

The answer is described in detail in post 85 (it is B, 2.9 mL).

https://www.reef2reef.com/threads/r...61-using-a-syringe.226267/page-5#post-2625685

 

[WWIII]

Great question, topic, and explanation!

 

[Jeff Hall]

And the answer is...B. 2.9 mL

Here's how to think about how a syringe works:

When the plunger is pushed all the way in, the leading edge of the plunger aligns with the zero mL marking. That is true of nearly every syringe I've seen. When you begin to use it, the tip and barrel are empty, with the tip containing air and the barrel occupied by the plunger.

As you draw back the plunger with the tip in a liquid, the plunger creates a vacuum between it and the liquid, drawing in the liquid. The movement of the plunger draws in exactly the amount of liquid equal in volume to the travel of the plunger backwards (or upwards). So you can track how much fluid is taken up exactly by the travel of the plunger against the volume markings, regardless of what you see with the fluid (assuming it is functioning properly).

Some of the drawn in liquid will be in the tip, and some (or perhaps none, if it is all in the tip) will be in the barrel of the syringe. If the volume of the tip is larger than the volume indicated by the movement of the plunger, none will show in the barrel. Many modern pipettes work this way to avoid contaminating the barrel of the device. The device stays clean and the tip is discarded after each use.

Now, when you go to dispense the liquid, the plunger is pushed in, and all of the liquid is dispensed and the air is pushed back into the tip.

If you mistakenly tipped the syringe upward during dispensing, and blew out the air before the liquid, then when you push the plunger all the way in, there will still be liquid in the tip, and you probably won't know how much so you end up with a mismeasurement.

In the medical world, it can be super important to not inject air into a patient's bloodstream. In that case, the syringe must first be overfilled. It is then tipped up and all of the air blown out and some liquid may be blown out until the end of the plunger is exactly aligned with the volume marking you want to dispense. The syringe in total now contains the amount you want to inject, plus the volume of the the needle. Then you inject and when done, the needle is still full of liquid in the exact amount as before injection, and like before, the amount dispensed is exactly determined by the movement of the plunger. So this situation is no different except that you start and end with a full tip/needle. In a non-injection setting, you start and end with an air-filled tip.

So it doesn't matter how much liquid you see in the barrel. Only the movement of the plunger is important, whatever the use.

So you are telling me there is 1.3mL of fluid in the tip of that syringe? I find that hard to believe and I think you are wrong. I understand your logic but in order to get an accurate measurement you should flip syringe the other way

 

[greg 45]

If you were to remove all the air , would it change the results
I have done this and it takes some work but it can be removed
So I am messing up my own test doing this ??

 

[Daltrey]

This is exactly the reason I bumped this. I was also trying to get the air out and wasting alot of time. If you do it the way it's explained all you have to do is draw the plunger barrel with the tip submerged to your line and you have the exact amount.

Sometimes the simple things confuse me, ha!

 

[Daltrey]

If you were to remove all the air , would it change the results
I have done this and it takes some work but it can be removed
So I am messing up my own test doing this ??

Think about it. If you managed to get out all the air the plunger tip will still be full when you push the plunger back down. There would be no way to get the extra liquid out because the plunger is all the way down. When you start with air in the plunger tip then the air will still be in the tip after you push the plunger back down.

It's a little confusing till you actually think about it, ha. Got me too.

 

[Jeff Hall]

I'm staying out of this because I am at work and have asked multiple nures and physicians about this and they agree with what I thought which is the answer is neither 1.6 or 2.9 but somewhere inbetween and closer to 1.6. You need to flip the syringe to get the answer.

 

[Daltrey]

I'm staying out of this because I am at work and have asked multiple nures and physicians about this and they agree with what I thought which is the answer is neither 1.6 or 2.9 but somewhere inbetween and closer to 1.6. You need to flip the syringe to get the answer.

B. It doesn't matter whether there is a tip or not or how big the tip is. The air present is a function of volume of the tip. Smaller tip, less air. Larger tip more air. In all cases, provided the plunger is pulled with the tip submerged, same amount of liquid is drawn into the syringe.

And the answer is...B. 2.9 mL

Here's how to think about how a syringe works:

When the plunger is pushed all the way in, the leading edge of the plunger aligns with the zero mL marking. That is true of nearly every syringe I've seen. When you begin to use it, the tip and barrel are empty, with the tip containing air and the barrel occupied by the plunger.

As you draw back the plunger with the tip in a liquid, the plunger creates a vacuum between it and the liquid, drawing in the liquid. The movement of the plunger draws in exactly the amount of liquid equal in volume to the travel of the plunger backwards (or upwards). So you can track how much fluid is taken up exactly by the travel of the plunger against the volume markings, regardless of what you see with the fluid (assuming it is functioning properly).

Some of the drawn in liquid will be in the tip, and some (or perhaps none, if it is all in the tip) will be in the barrel of the syringe. If the volume of the tip is larger than the volume indicated by the movement of the plunger, none will show in the barrel. Many modern pipettes work this way to avoid contaminating the barrel of the device. The device stays clean and the tip is discarded after each use.

Now, when you go to dispense the liquid, the plunger is pushed in, and all of the liquid is dispensed and the air is pushed back into the tip.

If you mistakenly tipped the syringe upward during dispensing, and blew out the air before the liquid, then when you push the plunger all the way in, there will still be liquid in the tip, and you probably won't know how much so you end up with a mismeasurement.

In the medical world, it can be super important to not inject air into a patient's bloodstream. In that case, the syringe must first be overfilled. It is then tipped up and all of the air blown out and some liquid may be blown out until the end of the plunger is exactly aligned with the volume marking you want to dispense. The syringe in total now contains the amount you want to inject, plus the volume of the the needle. Then you inject and when done, the needle is still full of liquid in the exact amount as before injection, and like before, the amount dispensed is exactly determined by the movement of the plunger. So this situation is no different except that you start and end with a full tip/needle. In a non-injection setting, you start and end with an air-filled tip.

So it doesn't matter how much liquid you see in the barrel. Only the movement of the plunger is important, whatever the use.

 

[greg 45]

Think about it. If you managed to get out all the air the plunger tip will still be full when you push the plunger back down. There would be no way to get the extra liquid out because the plunger is all the way down. When you start with air in the plunger tip then the air will still be in the tip after you push the plunger back down.

It's a little confusing till you actually think about it, ha. Got me too.

Ok now I understand why there is always extra fluid that needs to be cycled out . Meaning there is still liquid in the syringe.
But will it alter the results is my main question .

 

[Daltrey]

Ok now I understand why there is always extra fluid that needs to be cycled out . Meaning there is still liquid in the syringe.
But will it alter the results is my main question .

Your results will be the same as long as you are aligning the leading edge of the plunger with the correct ml mark. But the fluid left in the tip will be wasted unless you take apart the plunger to get the extra fluid out of the tip.

 

[Jeff Hall]

Ok now I understand why there is always extra fluid that needs to be cycled out . Meaning there is still liquid in the syringe.
But will it alter the results is my main question .

Yeah but if your syringe is not completely submerged in the fluid you could introduce more air into the syringe thus making your results innaccurate, and I know for a fact that a needle does not contain 1.3mL of air. IMO the volume of air actually in a syringe is most likely negligable because it's so small.

 

[Daltrey]

Yeah but if your syringe is not completely submerged in the fluid you could introduce more air into the syringe thus making your results innaccurate, and I know for a fact that a needle does not contain 1.3mL of air. IMO the volume of air actually in a syringe is most likely negligable because it's so small.

You are comparing two different things. These are the syringes we use in reef test kits. They can easily contain 1.3 ml of air in the tip.

No one is saying this needle has 1.3 ml of air in the tip. My wife is also a nurse. They just overdraw the liquid, invert it, thump out the air and press the plunger till fluid comes out the tip. If we did that with our test kits we would waist a whole lot of reagent.

 

[cmcoker]

Its not a needle, it's a pipette tip. The ones I used in chem lab could probably hold over a milliliter of fluid...

The air correlates to the volume of the pipette tip, if you are drawing the fluid correctly. So you just read your start to finish point on the syringe to find the volume used. The question assumes you are drawing up the reagent properly.

 

[MnFish1]

Yeah but if your syringe is not completely submerged in the fluid you could introduce more air into the syringe thus making your results innaccurate, and I know for a fact that a needle does not contain 1.3mL of air. IMO the volume of air actually in a syringe is most likely negligable because it's so small.

You are wrong. (sorry this was not the post that was wrong - this one was correct) but the one where the doctors and nurses were asked was wrong.

 

[Randy Holmes-Farley]

You are comparing two different things. These are the syringes we use in reef test kits. They can easily contain 1.3 ml of air in the tip.

No one is saying this needle has 1.3 ml of air in the tip. My wife is also a nurse. They just overdraw the liquid, invert it, thump out the air and press the plunger till fluid comes out the tip. If we did that with our test kits we would waist a whole lot of reagent.

Remember the reason that medical injections blow out the air has little to do with delivery volume accuracy and everything to do with not wanting to risk injecting an air bubble into the blood.

 

[Crabs McJones]

That depends on if the measuring on the syringe compensates for the space in the tip or not. I'm going to guess that it does and say D 1.6

 

[Servillius]

So I’m going to run this through out loud for curiosity’s sake.

The answer you give is nominally true, particularly for this size syringe.

In practice, air is compressible. When you first pull the plunger, it causes a drop in pressure. The drop in pressure pulls in liquid until the system returns to equilibrium. As the mass of the liquid increases, there will remain a pressure differential between the air in the syringe and the air outside the syringe. Because we can’t know the mass of the liquid before knowing what we’re pulling, we can’t fully account for this effect. The result should be a slight difference in volume depending on the mass.

In addition, the higher the viscosity of the liquid, the higher the likely pressure differential. If the pressure in the syringe is sufficiently low, gas could actually come out of solution to reestablish equilibrium. That gas would not return to solution as readily so produce a volume difference. The viscosity could also cause defects in the quality of the seals to become more pronounced and more air to get pulled in to reestablish equilibrium. This would result in a small error.

None of this is offered to question your answer which is of course correct. I’m simply playing out the details for the fun of it. Am I way off?

 

[Randy Holmes-Farley]

So I’m going to run this through out loud for curiosity’s sake.

The answer you give is nominally true, particularly for this size syringe.

In practice, air is compressible. When you first pull the plunger, it causes a drop in pressure. The drop in pressure pulls in liquid until the system returns to equilibrium. As the mass of the liquid increases, there will remain a pressure differential between the air in the syringe and the air outside the syringe. Because we can’t know the mass of the liquid before knowing what we’re pulling, we can’t fully account for this effect. The result should be a slight difference in volume depending on the mass.

In addition, the higher the viscosity of the liquid, the higher the likely pressure differential. If the pressure in the syringe is sufficiently low, gas could actually come out of solution to reestablish equilibrium. That gas would not return to solution as readily so produce a volume difference. The viscosity could also cause defects in the quality of the seals to become more pronounced and more air to get pulled in to reestablish equilibrium. This would result in a small error.

None of this is offered to question your answer which is of course correct. I’m simply playing out the details for the fun of it. Am I way off?

We can actually calculate both of these effects, roughly.

Let's start with the easier one, gases coming out under the reduced pressure of the syringe plunger "pull".

Table 3.6 is this pdf shows the solubility of gases in 35 ppt seawater:

N2 412 umole/kg
O2 225 umole/kg
Ar 11 umole/kg
CO2 12 umol/kg (this one is complicated. There is much more, but it is tied up as bicarbonate and carbonate and won't pull off easily)
everything else is way, way lower

Let's assume it all comes out into the gas space below the plunger.

So the total is 660 umole/kg, or 0.66 mmole/kg or 0.00066 moles/kg

Thus, 1 kg of seawater contains about 0.00066 moles of gas.

Since at 1 atm (about the pressure inside the barrel after the liquid finishes moving up), 1 moles of gas occupies about 22.4 liters, that 0.00066 moles of gas occupies 0.015 liters of gas.

So we sucked up 1 kg of seawater (974 mL) and got a max gas release of 15 mL. That would give us an error of 15 mL/974 mL = 1.5%. Say, 1.48 mL instead of a real 1.50 mL.

That's not insignificant in some applications, but probably not enough to alter the reading that a hobbyist is able to make by eye on a syringe plunger in a barrel.

 

[Randy Holmes-Farley]

In practice, air is compressible. When you first pull the plunger, it causes a drop in pressure. The drop in pressure pulls in liquid until the system returns to equilibrium. As the mass of the liquid increases, there will remain a pressure differential between the air in the syringe and the air outside the syringe. Because we can’t know the mass of the liquid before knowing what we’re pulling, we can’t fully account for this effect. The result should be a slight difference in volume depending on the mass.

Let's calculate the effect of gravity on the internal air pressure.

Obviously, with a big enough syringe this effect is dominating since one cannot, even with complete vacuum, get water to travel higher in a tube than 33 feet.

We discussed that here in the context of a barometer using seawater as the fluid instead of the usual mercury.
https://www.reef2reef.com/threads/reef-chemistry-question-of-the-day-216-seawater-barometer.324482/

We calculated there that seawater with rise 9,784 mm against full vacuum plus water vapor pressure.

That result means that 978 cm of water pulls downward with close to 1 atmosphere of pressure.

The relationship will be approximately linear, so if we draw up 2 cm instead of 978 cm, the pull of the water downward is not 1 atmosphere, but 2/978 = 0.0020 atmospheres, or a change of 0.2% in the pressure.

Thus the pull of the water downward in a syringe containing 2 cm of seawater reduces the internal pressure in the syringe by 0.2%, which expands the gas volume in the syringe by 0.2%.

We have to make some assumption about the design of the syringe and tip to get from the trapped gas volume to liquid volume, but let's say they are similar. THen the liquid drawn up is reduced by 0.2% when the gas volume increases by 0.2%, so an intended liquid draw of 1.500 mL will only pull up 1.497 mL.

This error seems insignificant to a volume that you'd measure with a syringe.

Thanks for the interesting suggestions!

 

[Servillius]

Let's calculate the effect of gravity on the internal air pressure.

Obviously, with a big enough syringe this effect is dominating since one cannot, even with complete vacuum, get water to travel higher in a tube than 33 feet.

We discussed that here in the context of a barometer using seawater as the fluid instead of the usual mercury.
https://www.reef2reef.com/threads/reef-chemistry-question-of-the-day-216-seawater-barometer.324482/

We calculated there that seawater with rise 9,784 mm against full vacuum plus water vapor pressure.

That result means that 978 cm of water pulls downward with close to 1 atmosphere of pressure.

The relationship will be approximately linear, so if we draw up 2 cm instead of 978 cm, the pull of the water downward is not 1 atmosphere, but 2/978 = 0.0020 atmospheres, or a change of 0.2% in the pressure.

Thus the pull of the water downward in a syringe containing 2 cm of seawater reduces the internal pressure in the syringe by 0.2%, which expands the gas volume in the syringe by 0.2%.

We have to make some assumption about the design of the syringe and tip to get from the trapped gas volume to liquid volume, but let's say they are similar. THen the liquid drawn up is reduced by 0.2% when the gas volume increases by 0.2%, so an intended liquid draw of 1.500 mL will only pull up 1.497 mL.

This error seems insignificant to a volume that you'd measure with a syringe.

Thanks for the interesting suggestions!

No, thank you for working through that. I swear I meant to pay more attention in inorganic all those years ago. Funny how your interest in the outcome can change your interest in the process.