International pH Scales and Certification of pH
Hans Bjarne Kristensen, Arne Salomon, and Gert Kokholm Radiometer A/S Emdrupvej 72 DK-2400 Copenhagen NV Denmark
Although the term pH is used to describe the degree of acidity or alkalinity of a solution, and pH measurem e n t s are routinely made in t h e laboratory, the meaning of the term is difficult to explain. The concept of pH is unique among physicochemical quantities because it involves a single ion activity and, by definition, cannot be m e a s u r e d directly. Because of this, pH measurements are made in relation to a pH standard solution, and an awareness of the uncertainty of the pH value of this solution is essential for understanding 0003-2700/91 /0363-885A/$02.50/0 © 1991 American Chemical Society
the overall uncertainty of the measured pH value. When reporting pH values it is also necessary to provide information about the type of reference standard solutions and their assigned pH values. In this REPORT we will describe the internationally recognized pH
REPORT scales recommended by organizations such as IUPAC (International Union of Pure and Applied Chemistry), NIST (National I n s t i t u t e of Standards and Technology, formerly the National Bureau of Standards), IFCC (International Federation of Clinical Chemistry), and OIML (Int e r n a t i o n a l Organization of Legal
Metrology). We will also describe the procedures used at the Chemical Reference Laboratory of Radiometer A/S for pH certification of primary buffer solutions using the IUPAC pH scale. Definition of pH The Danish chemist S.P.L. S0rensen originally defined pH as the negative logarithm of the hydrogen ion concentration (i) pH = -log 1 0 [H + ] (1) S0rensen was performing enzymatic studies and discovered that the degree of acidity was of fundamental importance to the biochemical reactions. Later, he and Linderstr0mLang proposed a new definition of pH as the negative logarithm of the hydrogen ion activity (2) pH = - l o g 1 0 a H
(2)
ANALYTICAL CHEMISTRY, VOL. 63, NO. 18, SEPTEMBER 15, 1991 · 885 A
REPORT This definition is equivalent to the current definition of pH pH = -log 1 0 (7 H w H )
(3)
where γ Η is the single ion activity co efficient of the hydrogen ion and m H is the molality of the hydrogen ion. The development of pH scales and the methods for pH determination are de scribed by Bates in a classic book (3). I U P A C pH scale
Because pH cannot be measured di rectly, it is defined operationally in terms of the method by which it is determined. For a number of years IUPAC has recommended standard ized methods for pH determination in aqueous solutions. The latest rec ommendations, prepared in 1984 (4), include the following. The notional definition of pH is given as pH = - l o g 1 0 a H (4) where a H is the relative hydrogen ion activity. Equation 4 is often rewrit ten as
pH = - l o g 1 0 N ^ ) ]
(5)
where nf~ is the standard-state con dition numerically equal to 1 mol/kg. It is universally agreed t h a t the definition of pH difference is an oper ational one. For this measurement, Cells I and II are prepared using identical reference electrodes. Reference electrodelKCl(aq., satd)II Solution XIH21 Pt (I) Reference electrodelKCl(aq., satd)ll SolutionSIH 2 IPt (II) The electromotive force (emf), £(X), of Cell I and the emf, £(S), of Cell II are m e a s u r e d while keeping both cells at the same temperature and hydrogen gas pressure. The pH of solution X, denoted by pH(X), is re lated to the pH of the standard refer ence solution S, denoted by pH(S), by the definition pH(X) = pH(S) +
F[E(S)-E(X)1 RT In 10
(6)
where E{S) is the potential of stand ard solution S, E(K) is the potential of solution X (the test sample), R is the gas constant, Τ is the thermody namic t e m p e r a t u r e , and F is the Faraday constant. The hydrogen electrode can be re placed by another hydrogen ion re sponsive electrode (e.g., glass elec t r o d e ) a n d s t i l l give a good approximation of pH. One can com pensate for the errors in the electrodes and m e a s u r i n g system by using a bracketing procedure to measure £(X) as well as £(S1) and £(S2) of two type
II cells with standard solutions SI and S2 such that the £(S1) and £(S2) val ues are on either side of, and as near as possible to, E(X). The pH of solution X is then obtained by assuming linear ity between pH and E, that is pH(X) - pH(Sl) PH(S2)-pH(Sl)
£(X) - £(S1) (7) £(S2)-£(S1)
=
The description of pH is completed by assigning a pH value at each temperature to one or more chosen solutions d e s i g n a t e d as s t a n d a r d reference solutions. R e f e r e n c e p H s t a n d a r d . Be cause potassium hydrogen phthalate is the most studied of all pH refer ence materials, it is designated as t h e reference value pH s t a n d a r d (RVS) at the specified molality of 0.05 mol/kg. The reference value method uses a cell without a liquid junction (Cell III). Pt(Pd) I H 2 (g, p = 101 325 Pa) I RVS, C M A g C l l A g (III) The pH values of the RVS are deter mined with the following informa tion. The emf of Cell III is given by -θ-
£ = £Ag/AgCl
_
RT 12-101
F
-I
M=W]
(8)
where m represents the molality and γ the activity coefficient of the sub scripted species, £X g / A g C 1 is the stand ard potential of the Ag/AgCl elec trode, and m* = 1 mol/kg. Equation 8 can be rearranged as
-log J ( ™2i>] =
[s^ffff']^^
(9)
The standard potential of the Ag/ AgCl electrode is determined from measurements on Cell IV PtlH 2 (g,/> = 101 325 Pa) I HCK0.01 mol/kg)IAgCHAg (IV) using Equation 8 with γ Η γ € ] = y±, where γ± is the mean ionic activity coefficient of HC1 at 0.01 mol/kg. The quantity -log 10 [(m H Y H Y cl )/»i*] is calculated from measured emf val ues for each of several molalities of chloride ion, plotted against ma, and extrapolated to mcl = 0. Then pH (RVS) is calculated as
pH(RVS) = [-log 10 ( w " Y "J c ')]^ m
logioYci
886 A · ANALYTICAL CHEMISTRY, VOL 63, NO. 18, SEPTEMBER 15, 1991
„+
\mc-f>Q
(10)
w h e r e y c l is o b t a i n e d from Bates-Guggenheim convention
the
-A(I/m^)1/2 (11) 1 + 1.5(//m*) 1/2 and A is the Debye-Huckel limiting slope and / the ionic strength (/ < 0.1 mol/kg). Tabulated values of A and γ± at 0.01 mol/kg are available (3, 5). P r i m a r y pH standards. Sub stances that have high purity, solu tion stability, and low residual liquid junction potential, and t h a t meet other criteria are designated as pri mary reference standards (PS). Cur rently there are seven primary stand ards, including the RVS. The pH values of the primary standards at 25 °C and 37 °C are given in Table I. The pH(PS) values are assigned in the same way as pH(RVS) values us ing a cell without a liquid junction (Cell V). iogjoYci =
PtlH 2 (g,/> = 101325Pa)l PS, CriAgCllAg
(V)
Operational pH standards. Other substances t h a t meet certain solution stability criteria are designated as operational standards (OS). In prin ciple, their number is unlimited. The pH(OS) values are assigned by com parison with pH(RVS) values in cells with a liquid junction (the operation al cells) where the liquid junctions are formed within vertical 1-mm capillary tubes (Cell VI). PtlH 2 IOSIIKCl > 3.5 mol/dm 3 ll RVSlH 2 IPt (Pd) (VI) The pH(OS) values of several opera tional standards are given in Refer ence 4; values for some OS can be calculated from Table I. IUPAC recommends that for stand ards, three additional pieces of infor mation be provided along with the reported pH value: the name of the manufacturer and the type of glass and reference electrode, t o g e t h e r with the method by which the liquid junction of the reference electrode was formed; the manufacturer, type, and discrimination of pH meter; and the method used to calibrate the pH meter system (e.g., system calibrated with two primary standards: pH(PS) = .. and pH(PS) = .. at K, practical slope = .. mV). IFCC blood pH scale
IFCC has approved a method for pH measurement in blood (6). The IFCC blood pH scale is of special impor tance for Radiometer A/S because our main work involves clinical acidbase analysis. Therefore the buffers recommended by IFCC for calibra-
Table I. Values of pH(PS) for primary standard reference solutionsa, b pH(PS) at 25 °CC
pH(PS) at 37 °C°
Potassium hydrogen tartrate (saturated solution at 25 °C)
3.557
H)
3.548 (-4)
Potassium dihydrogen citrate (0.1 mol/kg)
3.776
3.756
Disodium hydrogen phosphate (0.025 mol/kg) + potassium dihydrogen phosphate (0.025 mol/kg)
6.865 (-8)
6.841 (-13)
Disodium hydrogen phosphate (0.03043 mol/kg) + potassium dihydrogen phosphate (0.008695 mol/kg)
7.413 (-7)
7.386 (-17)
Disodium tetraborate (0.01 mol/kg)
9.180
H)
9.088 (-2)
Sodium hydrogen carbonate (0.025 mol/kg) + sodium carbonate (0.025 mol/kg)
10.012 (-17)
9.910 (-21)
RVS: Potassium hydrogen phthalate (0.05 mol/kg)
4.005
4.022
Primary standard
Adapted from References 3 and 4. "Values for different lots of SRMs may differ slightly (± 0.005 pH units). "The numbers underneath the pH values are the corrections (in units of pH χ 103) to be added to the pH value to give the pH of the operational standards, pH(OS).
tion are the ones we work with most often. P r i n c i p l e s . The proposed IFCC reference method for pH measure ment in blood and other biological fluids is based on the use of a cell consisting of a pH glass electrode and a reference electrode, R, with a satu rated KC1 liquid-liquid junction ac cording to the scheme RJKCKsatd)!! Soin XIGIInner ref. solnlR 2 (Cell b) Electrodes Rx and R 2 are connected to the pH meter. The cell is calibrat ed using the primary calibrating so lutions of the NIST pH scale (7). The disodium hydrogen phosphate/ potassium dihydrogen phosphate buffers (0.025 mol/kg/0.025 mol/kg and 0.03043 mol/kg/0.008695 mol/kg) are recommended. IFCC recommends using a glass capillary electrode and a mercury/ calomel electrode. The liquid junction should be established using a bridge solution of saturated KC1. Both elec trodes and the liquid junction should be maintained at the same tempera ture. NIST pH scale The NIST method for certification of pH values, originally developed in the late 1930s, has been refined over the years, and certified standard ref erence materials (SRMs) are avail able from NIST. The NIST pH scale (described in Reference 7) is defined using multi
ple primary standards. The pH(PS) is assigned using a cell without a liquid junction (Cell III or Cell V). In this way NIST has assigned pH values to six primary standards and two sec ondary standards. The standard po tential of the Ag/AgCl electrodes is determined in 0.05 m HC1 solution using γ± = 0.8304 at 25.0 °C. OIML pH scale OIML recently published a recom mendation for a pH scale in aqueous solutions (8). This scale designates buffer solutions with reproducible pH values (as specified by NIST) as pri mary and secondary standards and is therefore a multiple primary standard pH scale. The pH values have been de termined by measuring the emf of a hydrogen-Ag/AgCl cell without a liq uid junction (Cell III or Cell V). The s t a n d a r d potential of the Ag/AgCl electrodes is assigned the values de termined by Bates and Bower (9). The buffer values of the OIML standards are compared with NIST and IUPAC values in Table II. The OIML pH values sometimes differ from the NIST values in certain buff er solutions because NIST values are valid for the most recent lots of the SRMs, whereas the OIML values are from Bates (3). Multiple versus single primary standard pH scales The single primary standard scale de fines pH using one single RVS, and the pH value is determined using a
cell without a liquid junction (Cell III). In addition to this RVS, a number of operational standards are used. The pH values of these standards are determined in relation to the RVS, using a cell with a liquid junc tion (Cell VI). Therefore the pH value includes a contribution from the liq uid junction potential that increases the nonthermodynamic character of the pH values. The operational stand ards are empirical, whereas the RVS and the primary standards are nearthermodynamic. The multiple p r i m a r y s t a n d a r d scale defines pH using several pri mary s t a n d a r d s . The pH value of each standard is determined using a cell without a liquid junction (Cell III or Cell V); the pH value does not in clude a liquid junction potential. Thus the determination is as thermodynamically correct as possible. NIST and OIML pH scales are socalled multiple primary standard pH scales whereas the IUPAC pH scale allows both t h e multiple p r i m a r y standard pH scale and the single pri mary standard pH scale. The debate about whether to use the multiple or single primary stand ard pH scale has gone on for a num ber of years {10, 11). This is reflected in t h e IUPAC r e c o m m e n d a t i o n s where agreement on the use of one or the other could not be reached. We have adopted the multiple primary standard pH scale so that our meth od will give pH values comparable to those of NIST. However, we recom mend a réévaluation of international pH scales. Certification of pH buffers Our hydrogen electrode system was developed to ensure that the pH references used in production were stable and correct so that a high lot-to-lot reproducibility could be obtained. The accreditation to certify pH buffers was granted in 1983 by the Danish National Testing Board, an independent organization that specifies and audits the quality control procedures. Radiometer A/S normally certifies buffer materials similar to the primary NIST pH standards and in particular the two phosphate buffers recommended by IFCC for blood pH c a l i b r a t i o n . We use t h e I U P A C recommended method (4). Nine cells of two different types (A and B) are used simultaneously, and are configured as PtlH 2 (g,/>H 2 )IPS,NaCl(m cl )l AgCllAg (A) PtlH 2 (g, pH )IHC1(0.01 mol/kg)l AgCllAg (B)
ANALYTICAL CHEMISTRY, VOL. 63, NO. 18, SEPTEMBER 15, 1991 · 887 A
REPORT
Comment"
obtained in Step 2, which contains six values of mcl and -\og10(aHycl)m
1.679* 1.646" 3.557 3.556"
SEC.OS
at three different chloride molalities, we can calculate the limiting value -log10[aHjcl]m _^Q
Table II. pH values of buffer solutions Buffer solution Name
Chemical formula
Potassium KH 3 (C 2 0 4 ) 2 . tetraoxalate 2H 2 0 Potassium KHC 4 H 4 0 6 hydrogen tartrate Potassium KH 2 C 6 H 5 0 7 dihydrogen citrate Potassium KHC 8 H 4 0 4 hydrogen phthalate Potassium KH 2 P0 4 / dihydrogen Na 2 HP0 4 phosphate/ disodium hydrogen phosphate Sodium Na 2 B 4 0 7 · 10H2O tetraborate decahydrate Sodium NaHCCy bicarbonate/ Na 2 C0 3 sodium carbonate Calcium Ca(0H) 2 hydroxide
Cone. (mol/kg)
pH at 25 °C PHOIML
PHNIST
CI
PHlUPAC
0.05
1.679
1.679
Saturated at 25 °C
3.557
3.557
0.05
3.776
3.776
3.776
PS', PS
0.05
4.008
4.006
4.005
PS', RVS
0.025/ 0.025
6.865
6.863
6.865* 6.857**
PS', PS, OS
0.008695/ 0.03043
7.413
7.410
7.413* 7.406**
PS', PS, OS
0.01
9.180
9.180
9.180
PS', PS
0.025/ 0.025
10.012
10.010
10.012* 9.995**
Saturated at 25 °C
12.454
PS', PS, OS
1
PS', PS, OS
- l o g i o « H - log 10 [Yci] m ^>0
—
12.454
SEC
= pH(S) - log 10 [Yci] meî> 0
Six Α-cells, used for the determina tion of pH(PS), are divided into three groups (Group 1: >wci=0.005 mol/kg; Group 2: m c l = 0 . 0 1 0 mol/kg; a n d Group 3: w c l = 0 . 0 1 5 mol/kg). The three Β-cells are used for determina tion of the standard potential of the Ag/AgCl electrodes, -EXs/Agci· The emf of the cells is given by Λ
Ag/AgCl - [
ψ
JX
log 1 0(p"^'H * w*) 2
FVIH&I
F" 8 " Ag/AgCl
β
=£+
12κγ»ιο] χ
[log lo (0.01 Y± )]-[^10]x
Kfe)]
logiotYdl^O
where γ± is 0.9042 at 25 °C and 0.9020 at 37 °C (5). The emf of the three Β-cells is measured together with temperature and partial pressure of hydrogen. From Equation 13 standard potentials of the three Β-cells are cal culated. Their mean value, which is the value of £ ^ / A g C 1 for the actual lot of Ag/AgCl electrodes, is used in fur ther calculations. S t e p 2. D e t e r m i n a t i o n of t h r e e values of the acidity function. Equa tion 12 is rearranged to provide a function called the acidity function
(12)
where pH2 is the partial pressure of hydrogen (Pa), and p~ = 101 325 Pa. The p H d e t e r m i n a t i o n is p e r formed in five steps. Step 1. Determination of the stand ard potential of the Ag/AgCl electrodes. Equation 12 is rearranged to yield
(13)
-l0g 1 0 («HYH) = F ( £ - £ A g / A E C l ) | 810
Hm>
(16)
Rearranging this we get pH(S) = -log10[aHya]m_^Q +
PS': Primary NIST standard. PS: Primary IUPAC standard. SEC: Secondary NIST standard. RVS: Reference value standard (IUPAC). OS: Operational IUPAC standard. * Denotes pH value determined in a cell without a liquid junction. " Denotes pH value determined in a cell with a liquid junction.
-
logi O [Yci] W| ^0 = -Α(Ι/ηΓ)1/2 (15) 1 + \MI/m~)L,z Step 5. Calculation of pH. The fol lowing calculation is carried out. -log lo [a H Yci] Weî >0 =
8
Λ
by performing a linear regression analysis. Step 4. Calculation of activity co efficient of chloride y c l . Using the Bates-Guggenheim convention {12), we can calculate the activity coeffi cient y cl , under the limiting condition ?ȍj> 0, (i.e., the limit when the total ionic s t r e n g t h / equals t h e ionic strength of the buffer without added chloride) as
L
RT\nl0
J
logio^ci - 0-5 logio l p )
+
( 14)
The right side of the equation con tains variables that can be measured directly. The emf of the three groups of Α-cells is measured together with temperature and partial pressure of hydrogen. Three sets of acidity func t i o n v a l u e s a r e c a l c u l a t e d from Equation 14. S t e p 3. Extrapolation of acidity function to mC\ - 0. From the data set
888 A · ANALYTICAL CHEMISTRY, VOL. 63, NO. 18, SEPTEMBER 15, 1991
(17)
By substituting the limiting value of the acidity function found in Step 3 and the actual value of the chloride activity coefficient found in Step 4, we can calculate the pH(S) of the buffer solution. By using this method, we can certify a n u m b e r of p r i m a r y standards that can be used to assign the pH values of working standards used in a production process. U n c e r t a i n t y . The overall uncertainty (2σ) of the pH determination is < 0.005 pH units, which conforms to the level currently achieved by nation al standard organizations and metrological laboratories. However, this un c e r t a i n t y does n o t i n c l u d e t h e uncertainty from using fixed values of Y± when £Xg/Agci i s calculated and the uncertainty from using the B a t e s Guggenheim convention for the calcu lation of YC1. If these contributions to the uncertainty were included, a n overall u n c e r t a i n t y of < 0.005 pH units could not be claimed. The speci fied uncertainty is thus very closely connected to the IUPAC method used for pH determination. pH certification measurements The first requirement for performing a pH certification is careful prepara tion of the solutions and measure ment cells. The dried and weighed
buffer salts are dissolved in high-pu rity distilled water with a conductiv ity < 1 μδ/αη at 25 °C. Carbon diox ide is removed from the water prior to dissolution of the buffer salts. The hydrogen gas, which must be free of oxygen, is passed through a palladi um purifier. Accurate pH determinations can only be achieved when the concentra tion of the HC1 solution is known with high accuracy. The HC1 solution is used to determine the standard po tential of the Ag/AgCl electrodes, Ε Ag/AgCl, and this value is then used to calculate the pH of the primary standard. The concentration of the HC1 solution is determined by using coulometric titration methods devel oped at NIST (13, 14). The uncertain ty (2σ) of the determination of the mean value of the concentration is 0.1% relative to the concentration. Electrodes. The platinized plati num electrodes are made of platinum foils ( 8 x 4 mm) attached to 0.4-mm platinum wires that are sealed into soft glass and joined, using several types of glass, to Pyrex glass so t h a t conical glass joints can be used. Each electrode is platinized at a current
Figure 1. Emf measurement cell used for the certification of primary buffer solutions at Radiometer A/S.
0.464273 0.464263 0.464253 0.464243 0.464233 0.464223 0.464213 0.464203 0.464193 0.464183 0.464173 0.464163 0.464153 0.464143 0.464133 0.464123 0.464113 0.464103 0.464093 12.17
15.62
19.07
22.52
25.97 Time (h)
29.42
32.87
36.32
39.77
Figure 2. Emf vs. time plot of three cells filled with 0.01 m HCI. density of 60 mA/cm 2 for 4 min. in 2% chloroplatinic acid (H 2 PtCl 6 ) solu tion. A small amount of lead acetate (50 mg/L) is added to this solution to improve the plating characteristics; the plating should be uniformly black and adhere firmly to the electrode. The electrode serves as a hydrogen electrode and its potential is repro ducible within ±5 μ ν . The thermal electrolytic method is used to prepare Ag/AgCl electrodes on s u p p o r t i n g wires of platinum (15). After p r e p a r a t i o n t h e Ag/AgCl electrodes are stored in 0.005 m HC1 solution for several days before use, and during this time the lots of Ag/ AgCl electrodes are compared. The electrodes can be manufactured with such a high degree of uniformity that they can be selected to have differ ences in potential of < 10 μ ν . We have produced a number of different lots, each of which is normally used for approximately six months. The •^Xg/Agci found for an electrode lot is within 0.222360 and 0.222460 V at 25 °C. The standard deviation is ap proximately 0.000030 V. Further de tails of the preparation of the plati nized p l a t i n u m a n d t h e Ag/AgCl electrodes are given in References 3 and 15. Galvanic cell w i t h o u t liquid j u n c t i o n . The specially designed emf measurement cell (Figure 1) consists of three humidifiers, a hydrogen elec trode compartment, and a Ag/AgCl electrode compartment. The vacuumtight cell is manufactured from Pyrex
glass and has conical glass joints. Each cell is evacuated before filling so that no oxygen is present, and the cell is specially designed so that no hydrogen bubbles can enter the Ag/ AgCl electrode compartment. The emf of the cell filled with a pri mary buffer or 0.01 m HC1 is ex tremely stable over time (see Figure 2); the change in emf per hour is typ ically 1-2 μ ν . Nine cells are used s i m u l t a n e o u s l y in t h e c o n s t a n t temperature bath. C o n s t a n t - t e m p e r a t u r e bath. A double-walled glass and metal enclo sure thermally insulated with styrofoam is used as the constant-temper ature bath. The upper part contains h e a t i n g elements for t e m p e r a t u r e control of air space above the cells and a suction unit to remove the hy drogen gas used in the cells. The bath is filled with water, and its tem p e r a t u r e is controlled to w i t h i n ±0.005 °C with a unit of our own con struction and with a cooling unit (HETO Lab. Equipment A/S, Birkeroed, Denmark). A platinum resis tance thermometer (Yellow Springs Instrument Co., Yellow Springs, OH) calibrated to 0.005 °C against a stand ard reference platinum thermometer (Sensing Devices Ltd., Southport, U.K.) calibrated at Jysk Teknologisk Institut (primary Danish tempera ture calibration institute) to 0.001 °C is used for t e m p e r a t u r e m e a s u r e ments. Measuring instruments. A digi tal barometer (Druck, Leicester,
ANALYTICAL CHEMISTRY, VOL. 63, NO. 18, SEPTEMBER 15, 1991 · 889 A
REPORT U.K.) calibrated with traceability to NPL (National Physical Laboratory, U.K.) and SP (Swedish Calibration Service) is used to measure the atmo spheric pressure. A digital (8.5 digit) multimeter (Model 7081, Solartron Instruments, Farnborough, H a m p shire, U.K.) with input resistance > 10 GO., calibrated with traceability to NPL, is used to determine emf. The digital multimeter is checked be fore each measurement by measure ments of a standard cell enclosure (Model 9155, Guildline Instruments, Ontario, Canada), which is calibrated with traceability to NPL. A diagram of the hydrogen elec trode system is shown in Figure 3. Measuring r o u t i n e s are computer controlled (PC-40, Commodore Elec tronics Ltd.), and all the measure ments, calculations, and data storage are performed automatically. The digital voltmeter (DVM) input multi plexer, which under computer control switches the various measurement parameters into the DVM prior to transmission to the computer for ac quisition and storage, is a specially designed multiplexer that uses read relays with very low t h e r m a l emf values. The multiplexed functions in clude temperature (platinum resis tance), standard resistance (used for determining the exact resistance of the platinum thermometer), and nine emf values from measurement cells. The software for the computer con trol is programmed at Radiometer A/S in Turbo Pascal (Borland Inter national Inc.). The DVM input multi plexer is controlled by the computer using a PIO-12 Board (Metrabyte Corporation). T h e DVM a n d t h e barometric pressure indicator are controlled with the IEEE interface (GPIB-PCII-A, National Instru ments, Austin, TX). When the software indicates that the preset t e m p e r a t u r e h a s been reached, measurements are automat ically performed at selected time in tervals. Measured values are stored, necessary calculations are per formed, results are printed, and data can be plotted on screen or paper. Reference materials. Normally, NIST SRMs are used, but we also use material from Merck ( D a r m s t a d t , Germany). Materials not classified as SRMs are examined before use for protolytic active trace elements. So dium chloride is examined for traces of bromide {16) because bromide is known to shift the potential of the Ag/AgCl electrodes. High-precision glass electrode for d e t e r m i n a t i o n of pH of sec ondary standards. It is often nec
Computer
Printer
IEEE interface
Digital barometer
Digital voltmeter
100 Ω Standard resistor
Input multiplexer
Hydrogen supply
Platinum resistance thermometer
Catalytic purifier
Temperature control unit/heater
Refrigeration unit
Constant-temperature water bath
Electrochemical cells
Figure 3. Diagram of the temperature-controlled emf measurement system.
essary to determine the pH of a sec ondary standard. The secondary s t a n d a r d s we u s e a r e n o r m a l l y placed in 3-mL glass ampoules that are heat sterilized to maintain a long shelf life. The pH of a secondary standard is measured using a capil lary glass electrode and a calomel electrode. The electrode potential is determined using an electrometer (Model 619, Keithley I n s t r u m e n t s , Cleveland, OH), and the electrode is calibrated using primary standards. The pH values of the primary stand ards are determined using the hydro gen electrode system. U s e of h i g h - p r e c i s i o n buffers u n d e r normal laboratory condi t i o n s . Recently many excellent meters and electrodes capable of pro viding pH values with a reproducibil ity of a few thousandths of a pH unit (mpH) have become available. Al though digital pH meters can be read to the nearest mpH, the fundamental meaning of these measured values is considerably less certain than the re
890 A · ANALYTICAL CHEMISTRY, VOL. 63, NO. 18, SEPTEMBER 15, 1991
producibility of the measurement. In making pH measurements it is im portant to remember that the assem bly is designed to indicate the pH dif ference between a standard buffer and a test solution, both of which should be at the same temperature. To ensure t h a t the glass electrode functions properly, t h e a s s e m b l y should be calibrated with two stand ards that bracket the pH of the sam ples. The accuracy and lot-to-lot re producibility of these pH calibrating buffers are fundamental to the total accuracy of the pH determination of the samples. If high accuracy and re producibility are required, it is neces sary to choose a high-quality pH meter, pH electrode, and pH calibrat ing solutions. Errors caused by fluc tuations in the residual liquid junc tion p o t e n t i a l a r e m i n i m i z e d by standardizing the assembly at a pH near that of the test solution and by always using the same kind of pH calibrating solutions for the same kind of samples.
To select the electrode type for optimal performance, consider the pH and temperature range at which the measurements are to be made. For measurements in alkaline solutions, select a glass electrode with a low Na + sensitivity so that the alkaline error will be as low as possible. For measurements at temperatures above 60 °C, select a reference electrode other than the mercury/ mercurydVchloride (calomel) electrode, which exhibits instability at elevated temperatures. At elevated temperatures the Ag/AgCl electrode is a good choice because it has excellent temperature stability. A detailed discussion of sources of error in potentiometric measurements can be found in Reference 17. The future Because the pH concept is fundamental to many chemical and biochemical processes, it is important to obtain international agreement on the use of pH scales. Currently several different scales are in use. Reevaluation of these international pH scales and, ultimately, selection of one recommended scale would be beneficial.
(17) Durst, R. A. In Ion-Selective Electrodes in Analytical Chemistry; Freiser H., Ed.; Plenum Publishing Corp.: New York, 1978; pp. 311-38.
Hans Bjarne Kristensen is head of the Chemical Reference laboratory of Radi ometer A/S, Denmark. He received his M.Sc. degree in physics and chemistry at the University of Copenhagen in 1982. Since 1982 he has been working with ref erence systems for aqueous electrolyte solu tions of sodium, potassium, calcium, and in particular the reference system for the hydrogen ion activity, pH. He serves as Denmark's representative to the IUPAC commission V.5 (electroanalytical chem istry).
References (1) Sorensen, S.P.L. Comtes-Rendus des Travaux du Laboratoire de Carlsberg 1909, 8, 1. (2) Sdrensen, S.P.L.; Linderstrém-Lang, K. Comtes-Rendus des Travaux du Laboratoire de Carlsberg 1924, 15, 40. (3) Bates, R. G. Determination ofpH, Theory and Practise, 2nd éd.; John Wiley and Sons: New York, 1973. (4) Covington, A. K.; Bates, R. G.; Durst, R. A. Pure. Appl. Chem. 1985, 57, 531-42. (5) Bates, R. G.; Robinson, R. A. / Solution Chem. 1980, 9, 455-56. (6) Maas, A.H.J; Weisberg, H. F.; Burnett, R. W.; Muller-Plathe, O.; Wimberley, P. D.; Zijlstra, W. G.; Durst, R. Α.; Siggaard-Andersen, O. /. Clin. Chem. Clin. Biochem. 1987, 25, 281-89. (7) Durst, R. Α.; Koch, W. F.; Wu, Y. C. Ion-Sel. Electrode Rev. 1987, 9, 173-96. (8) "International Recommendation No. 54, pH Scale for Aqueous Solutions"; re port to OIML, Sixth International Con ference of Legal Metrology, June 1980. (9) Bates, R. G.; Bower, V. E. / Res. Natl. Bur. Stand. 1954, 53, 283-90. (10) Bates, R. G. Crit. Rev. Anal. Chem. 1981, 10, 247-78. (11) Covington, A. K. Anal. Chim. Acta. 1981 127 1—21 (12) Bates,' R. G.; Guggenheim, E. A. Pure. Appl. Chem. 1960, 1, 163-68. (13) Taylor, J. K.; Smith, S. W.J. Res. Natl. Bur. Stand. 1959, 63A, 153-59. (14) Marinenko, G.; Taylor, J. K. Anal. Chem. 1968, 40, 1645-51. (15) Reference Electrodes; Ives, D.J.G.; Janz, G., Eds.; Academic Press: New York, 1961. (16) Pinching, G. D.; Bates, R. G. /. Res. Natl. Bur. Stand. 1946, 37, 311-19.
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ANALYTICAL CHEMISTRY, VOL. 63, NO. 18. SEPTEMBER 15, 1991 · 891 A
