pH scales and pH calibration fluid

marsh

New member
I know this is probable an old issue but I have some questions relating to pH measurement

Different pH scales for Seawater

National Bureau of Standards (NBS)
Salt Water Scale (SWS)
Total proton scale (T)
Free pH scale (F)


All of them have subtle differences as they each handle junctional potentials, ionic strength, HSO4 and HF . It is recommended by Millero that it is better to use SWS, T or F pH scales. If pH SWS reads 8.1, pH T reads 8.1096, pH F reads 8.217, ? equivalence for NBS
In addition calibration of of pH probes is dependant upon the salinity as well as the temperature of the calibration fluid.

My questions have to do with the available pH measurements reported in the hobby, the available measuring devices and calibration fluids given the different pH scales. What pH scale is commonly being reported in the hobby? According to Randy "These other standards need not concern aquarists: all data reported by reef aquarists employ the standard NBS system"
OK so we all are reporting NBS pH scales but this does not indicate how we calibrate our pH probes.
What calibration fluids out there use seawater buffers? What would be the difference if someone calibrated with a fresh water buffer vs sea water (high ionic strength) buffer?
 
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bertoni

Premium Member
Hmm, Boomer hasn't replied. I don't recall the difference between the scales being an issue for hobbyist work. A search might bring up a previous discussion. The PinPoint unit might have some work done for saltwater measurement, but that doesn't seem to be critical.
 

marsh

New member
From CO2sys document
http://cdiac.esd.ornl.gov/oceans/co2rprt.html#phscales
pH Scales
The various pH scales are inter-related by the following equations:

aH = 10( -pHNBS) = fH * Hsws ,

and
Hfree = Htot / (1 + TS/KSO4) = Hsws / (1 + TS/KSO4 + TF/KF) ,



where

aH is the activity and fH is the activity coefficient of the H+ ion (this includes liquid junction effects),
TS and TF are the concentrations of SO42- and fluoride, and
KSO4 and KF are the dissociation constants of HSO4- and HF in seawater.

The conversions depend on temperature, salinity, and pressure. At 20°C, salinity 35, and 1 atm, pH values on the total scale are (about)

0.09 units lower than those on the free scale
0.01 units higher than those on the seawater scale
0.13 units lower than those on the NBS scale

The concentration units for aH on the NBS scale are mol/kg-H2O. The concentration units used in the program CO2SYS for [H+] on the other scales are mol/kg-SW (note that the free scale was originally defined in units of mol/kg-H2O). The difference between mol/kg-SW and mol/kg-H2O is about 0.015 pH units at salinity 35 (the difference is nearly proportional to salinity). The seawater scale was formerly referred to as the total scale, and each scale is still sometimes referred to as the other in the literature.

The fit of fH used in this program is valid from salinities 20 to 40. fH has been found to be electrode-dependent, and does NOT equal 1 at salinity 0 due to the liquid junction potential.

Values on the NBS pH scale are only accurate to (at best) 0.005.

All work on pressure effects on pH has assumed that fH is independent of pressure.

For discussions of the various pH scales see Dickson (1984, 1993), Millero et al. (1993), Butler (1992), or Culberson (1981). Attention is required because in some of these references the distinction between the total and the seawater pH scales was not made.

I know most people will not be interested in the reference or the quote but the difference between the Total/SWS scales and the NBS scale is ~.13 with the Total/SWS scales reading the pH as more acidic. This still does not answer the question if calibrating in fresh water or sea water pH calibration standards will result in substantial differences in the pH result. I have asked OSIL this question.

Why care? 1. The answer may affect advice given. 2. It is a hobby and you can ask any sort of silly questions. 3. This knowledge will impresss your wife/girlfriend/mother/husband/boyfriend/dog (oh I already said husband)
 
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Boomer

Bomb Technician (EOD)
Premium Member
Marsh :wavehand:

Do not worry about that. Almost all pH levels give for seawater, even in chemical oceanography texts are NBS. Almost all texts say " pH & pKa's re as NBS unless otherwise give. I have brought this up here before. There are actually 4 pH scales.


One only needs to know SWS/NSW pH if on is doing or using something like Buch-Park equations to calculate CO2 from pH and CA. What you are talking about here is something I mentioned to you in another post, the "salt effect " error. When you use a NBS pH cal. solution the meter reads .15 pH units to high, @ 35 ppt and 25 C.

where

fh = .793 + .0307S +.0000794S^2 + .00006443 (273.15+C)- .000117S (273.15 +C) So, @35 ppt and 25 C = .70965514

and

pH(nsw) = pH(nbs) + log .70965514 = 8.15 pH

On a final note, even if doing CO2 calculations and going with all the old NBS std, the offset error for CO2 in NSW is about ~.01 ppm CO2 from the new nsw stds.

For a more full review of the pH scales see;


The pH of Estuarine Waters
http://aslo.org/lo/toc/vol_31/issue_4/0839.pdf.


also,

An Introduction to the Chemistry of the Sea by Pilson.

I might add there is more to all this than one may think, as not only is there a "salt effect" error but when doing CO2 calculations you also need to use NSW pka' and not NBS pKa's.

But like I said the CO2 is only off about .01ppm. So who cares, if CO2 in NSW is only ~.45ppm to begin with. If you breathed on the sample the pH will change. I think Frank and some of them are nuts :) Not only that but he calibrates his probe to .001 pH units, with is own special NSW cal solution, that uses Trisma Buffers :lol: Old Franky just like being extra accurate in his pH CO2 stuff. He even wrote new equations for NSW pKa1 and pKa2, so one could get away from the pKa's from Lynman's Ph.D dissertation. The reason behind all of Millero's work her is that it is very difficult to measure CO2 in seawater, unless you have a $$$$$$$$ pCO2 probe
 
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Boomer

Bomb Technician (EOD)
Premium Member
I use this equation, often called Buch-Park equations, also in Millero, for CO2 calculations.

CO2 = CA x pH / pK1 [1 +2 pK2/pH]

or

CO2 = {CA x 10 ^ -pH / [10^-pK1 (1 + 2 x10 ^-pK2/ 10^-pH ] } x 44 (molecular wt of CO2)


Example all @ NBS std.

CA = 2.2 Carbonate Alk, the borate Alk has been subtracted, here .15 meq / l for a T-Alk of 2.35meq/ l

pH = 8.3 NBS

pK1 = 6.0 NBS ( but I can use NBS as I corrected the pH above, so Millero told me ),

pK2 = 9.1 (same as above)


CO2 = { 2.2 x -log 8.30 / [ -log 6.0 (1 + 2 x -log 9.1/ -log 8.15 ] } x 44 (molecular wt of CO2)

CO2 = 0.3683 mg / l CO2 dissolved in water

I use this equation, often called Buch-Park equations, also in Millero,



I'm glad you have an interest in this Marsh, there use to be like 6 of us here, now just you and me :lol:
 

marsh

New member
I am working real hard to read and learn chemistry stuff...hopefully my corals will survive in the meantime. Yeah your mentioning of the "salt effect" ion-pairing issue got me going.

BTW I was reading through some books on amazon.com regarding saltwater/aquatic chemistry and found some recent work regarding pCO2 probes. http://www.amazon.com/gp/reader/0470841400/ref=sib_dp_pt/102-4635832-2751304#reader-link
There is some progress on this front. Would be nice to have pH probe and pCO2 probe and get automatic Alk calculations done for you. What I don't understand is how there are pCO2 probes for the beverage industry but none that have perfected for seawater. Is it possible that there is more scientific interest in well carbonated beer than climate change? Probable a Republican conspiracy to get us all drunk so we will ignore the climate cassandras and drink more beer to cool off as the tempuratures rise...
 

Boomer

Bomb Technician (EOD)
Premium Member
Marsh

First this should not be in here. I forgot to remove it form an old post of mine here as I wanted to do it only as NBS.

( but I can use NBS as I corrected the pH above, so Millero told me )


Is it possible that there is more scientific interest in well carbonated beer than climate change?

No, it is that it is seawater and the probe is special and then there is why use or have a need for one when a calculator and equation can give a 99.9 % same result. One of the issues in CO2 measurement in seawater is "carry gas". Some now use a use equilibrator instrument designed to be free from the correction.

On your book I would buy it but not for just 1 chapter that interests me. There are other books on seawater analysis that I find much more interest in but still are to pricey.

Analysis of Seawater : A Guide for the Analytical and Environmental Chemist
http://www.amazon.com/Analysis-Seaw...5371064?ie=UTF8&s=books&qid=1178291188&sr=1-1

Methods of Seawater Analysis
http://www.amazon.com/Methods-Seawa...5371064?ie=UTF8&s=books&qid=1178291188&sr=1-2

Here are some ref on the subject
http://andrew.ucsd.edu/co2qc/bibliography.html

http://neon.otago.ac.nz/chemistry/research/mfc/pubs/co2-ph/dyeadditionpaper.pdf

http://www.bbsr.edu/Labs/co2lab/research/CO2_instrumentation.html

Some books of much more interest that I have;

Carbon Dioxide Equilibria and Their Applications (Hardcover)
http://www.amazon.com/Carbon-Dioxid...5371064?ie=UTF8&s=books&qid=1178291528&sr=1-3

The best book on the subjec there is. Gald I never paid this for mine. It is loaded with stuff despite its only 260 p. I have the 1982 edt. and it is a PB so is half this price.

CO2 in Seawater: Equilibrium, Kinetics, Isotopes
http://www.amazon.com/CO2-Seawater-...5371064?ie=UTF8&s=books&qid=1178291790&sr=1-4

This is a great book but most of it is not of intest to me , i.e., Isotopes and that is half of the book.

Also

Ocean Biogeochemical Dynamics
http://www.amazon.com/Ocean-Biogeoc...9775681-5371064?ie=UTF8&qid=1178291790&sr=1-4

This book is a flippin master piece.

Ion paring

Butlers book has tables of ion parirs against pC-pH. And so does Spotte's book Seawater Aquariums the Captive Environment. You may really want to check into this book as a used copy. I also have on the way a new marine chemistry book, 320p. I can hardly wait to see what is in 320 p, since it is a book on seawater aquariums :eek2:

Ion pair
http://www.iupac.org/goldbook/I03231.pdf
 
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