1、BRITISH STANDARD CONFIRMED AUGUST1993 BS3145:1978 Incorporating Amendment No.1 Specification for Laboratory pH meters UDC543.257.1:621.317.729BS3145:1978 This British Standard, having been prepared under the directionof the Laboratory Apparatus Standards Committee,was published underthe authority of
2、 the Executive Board on 30June1978 BSI 04-1999 First published September1959 First revision June1978 The following BSI references relate to the work on this standard: Committee reference LBC/16 Draft for comment 76/52124 DC ISBN 0 580 10009 X Cooperating organizations The Laboratory Apparatus Standa
3、rds Committee, under whose direction this British Standard was prepared, consists of representatives from the following Government departments and scientific and industrial organizations: Agricultural Research Council Association for Science Education British Laboratory Ware Association British Lamp
4、blown Scientific Glassware Manufacturers Association Ltd. British Pharmacopoeia Commission* Chemical Industries Association* Chemical Society* Chemical Society, Analytical Division Department of Health and Social Security Department of Industry, Laboratory of the Government Chemist Department of Ind
5、ustry, National Physical Laboratory* Glass Manufacturers Federation Institute of Petroleum Ministry of Defence Pharmaceutical Society of Great Britain Scientific Instrument Manufacturers Association* Society of Chemical Industry* Society of Glass Technology* Standardization of Tar Products Tests Com
6、mittee The organizations marked with an asterisk in the above list, together with the following, were directly represented on the committee entrusted with the preparation of this British Standard: British Kinematograph, Sound and Television Society Imperial Chemical Industries Limited Institute of M
7、edical Laboratory Sciences Royal Institute of Chemistry Royal Society Sira Institute Society for Applied Bacteriology Society of Dairy Technology University of Newcastle Upon Tyne Individual expert Amendments issued since publication Amd. No. Date of issue Comments 5667 October1987 Indicated by a si
8、deline in the marginBS3145:1978 BSI 04-1999 i Contents Page Cooperating organizations Inside front cover Foreword ii 1 Scope 1 2 References 1 3 Definitions 1 4 Presentation of the measured quantity 2 5 Scale 2 6 Errors of indication 2 7 Input current 3 8 Stability 3 9 Glass electrode input 3 10 Refe
9、rence electrode input 4 11 Temperature compensating devices 4 12 Set buffer adjustment 5 13 Slope factor adjustment 5 14 Electrical safety and earthing 5 15 Summary of information to be given in the literature provided with the pH meter 5 Appendix A Method of test for errors of indication 6 Appendix
10、 B Method for determination of overall instrument error 6 Appendix C Method of test for input current 6 Appendix D Methods of test for isopotential and temperature compensation 7 Table 1 Potential differences for calibrating pH scales 7 Table 2 Voltages for calibrating isopotential control and tempe
11、rature compensator 7 Publications referred to Inside back coverBS3145:1978 ii BSI 04-1999 Foreword This British Standard, first published in1959, has been revised under the direction of the Laboratory Apparatus Standards Committee. The first edition specified requirements for meters of the now obsol
12、ete manually operated null or potentiometric type. This revision covers all types of laboratory pH meters in current production including the moving coil type previously specified in BS3422 (now withdrawn), the automatic (servo-driven) potentiometric type and the more recently developed digital disp
13、lay type. In this revision, the requirements are based on the electrical performance of pH meters. Although the compliance tests, given in the appendices, have been designed with typical usage for glass and reference electrodes in mind, simulated conditions are used to make the tests independent of
14、electrode performance. These tests are conducted by applying d.c.potential differences from a potentiometer using a high value resistor to simulate a glass electrode. The committee undertook an examination of a range of pH meters in current production which showed that the new requirements of this B
15、ritish Standard are practicable. The principal cost-restricted limitations of pH meters are deflection meter linearity and scaling, input current, and dial scaling and linearity of temperature compensator potentiometers. Possible inaccuracies arising from these features can be minimized by attention
16、 to the way the pH measurements are carried out. Suitable procedures are described in BS1647. Errors due to input current can be considerable for very high electrical resistance glass electrodes, or for electrodes with resistances in the normal range (100M7 to350M7) at room temperatures when used at
17、 low temperatures. Errors arising from input current are of similar origin to those described as “grid current” errors when valve electrometer tubes were employed in pH meters up to a decade ago. Unlike grid current errors, however, the input current error may be of either sign, as present day solid
18、-state input stages may cause current to flow in either direction through the glass electrode/reference electrode pair. Furthermore, the current depends on the voltage, applied to the input stage. Input current tests are described in Appendix C. It is not widely appreciated that the conversion of th
19、e measured potential to the indicated pH value carried out by the temperature compensator of a pH meter is an incomplete one even if isopotential control is used. Although the compensator operates according to the slope factor, corrections for smaller effects of a physicochemical origin are not full
20、y made. Moreover, the quality of the components available and inadequacies in the scaling of the slope factor adjustment dial frequently limit the accuracy of the pH measurement. If the dial cannot be set accurately and reproducibly to a particular temperature (e.g.25C) then there may even be inaccu
21、racy in the measurement of pH values when both sample and standard buffer are temperature controlled to within 0.1C. Because of the many designs of pH meters in current production, it has not been found possible to specify all design details, but suppliers are required to provide information on some
22、 of these in their instruction manuals. A British Standard does not purport to include all the necessary provisions of a contract. Users of British Standards are responsible for their correct application. Compliance with a British Standard does not of itself confer immunity from legal obligations. S
23、ummary of pages This document comprises a front cover, an inside front cover, pagesi andii, pages1 to8, an inside back cover and a back cover. This standard has been updated (see copyright date) and may have had amendments incorporated. This will be indicated in the amendment table on theinside fron
24、t cover.BS3145:1978 BSI 04-1999 1 1 Scope This British Standard specifies requirements for pH meters for laboratory use. 2 References The titles of the standards publications referred to in this standard are listed on the inside back cover. 3 Definitions For the purposes of this British Standard the
25、 following definitions apply. 3.1 pH value this was originally intended to represent the negative common logarithm of hydrogen ion activity in aqueous solution but this notional definition is now replaced by an operational one (as given in BS1647) in terms of the electrochemical cell used for its me
26、asurement 3.2 laboratory pH meter a high impedance input instrument designed for general laboratory use in conjunction with hydrogenion responsive glass and reference electrodes (q.v.) by means of which either the pH of a solution, or the potential difference between the electrodes immersed in a sol
27、ution, can be measured 3.3 hydrogen ion responsive electrode an electrode, consisting of a bulb, or other suitable form, made of special glass, attached to a stem of high resistance glass complete with internal reference electrode and internal filling solution 3.4 reference electrode an external ele
28、ctrode system which incorporates a means of forming a liquid junction such as a ceramic plug or frit. Usually this reference electrode systemis either calomel (mercury (I) chloride) or silver/ silver chloride in contact with a concentrated solution of potassium chloride 3.5 range the interval in pH
29、or millivolts over which the instrument is usable 3.6 moving index instrument an indicating instrument in which the index is a pointer moving over a fixed scale 3.7 scale division the interval between two consecutive (minor) scale marks on an instrument with a scale or chart 3.8 scale interval the v
30、alue of the measured quantity corresponding to a scale division 3.9 scale spacing the linear or curvilinear length, measured along the scale base, between the axes of two consecutive minor scale marks of a scale or chart 3.10 digital pH meter a pH meter in which the value of the measured quantity is
31、 displayed as a group of digits 3.11 representation unit the minimum increment between two successive output states of a digital pH meter 3.12 set buffer adjustment a control for adjusting the pH meter to indicate the value of the pH of the standardizing solution 3.13 slope factor the temperature de
32、pendent proportionality factor between potential and pH 3.14 slope/temperature factor adjustment a manual control by which readings on the pH scale can be corrected for variations of temperature of the pH cell 3.15 slope factor adjustment a manual control used in conjunction with two-point standardi
33、zation procedures to correct for non-Nernstian but linear response of the electrode system 3.16 isopotential control a control which determines the point on the pH scale where the correcting effect of the temperature compensation device is zero. This additional temperature compensating device permit
34、s a partial correction to be made for those cell potential difference changes which occur only as a result of: a) temperature differences between the test solution and the standard buffer solution;BS3145:1978 2 BSI 04-1999 b) temperature differences between the glass electrode and the reference elec
35、trode. 3.17 zero point the value of the pH of a solution, which, for a given glass electrode in combination with a stated outer reference electrode system, gives zero potential difference from the system 3.18 error (of indication) the difference (v i v c ) between the value indicated by the pH meter
36、 (v i ) and the true value of the measured quantity (v c ), taken as equivalent to the d.c.potential difference applied from a potentiometer 3.19 calibration the operation of determining the errors (of indication) of a pH meter 3.20 intrinsic error the error of indication of a pH meter under referen
37、ce conditions 3.21 input current the current flowing through the glass electrode and reference electrode cell under specified conditions of input voltage 3.22 (overall) instrument error the whole error of indication of a pH meter under specified conditions of use 3.23 influence quantity any quantity
38、, generally external to the pH meter, which may affect the performance of the pH meter 3.24 common mode imput voltage a voltage applied to both input terminals together 3.25 common mode rejection ratio (CMRR) ratio input voltage range to maximum change in input offset voltage over this range 4 Prese
39、ntation of the measured quantity The value of the measured quantity may be presented by: a) a moving index instrument; b) a digital instrument; c) a recording instrument; d) a computer-readable output e.g.RS232. The principal form of presentation is usually a) or b) with c) and/or d) as optional acc
40、essories. The manufacturer shall state in the literature provided with the pH meter which is the principal method of presentation adopted and which optional methods are available together with a description of these, e.g.parallel b.c.d.,isolated potentiometric recorder output, etc. (see clause15). 5
41、 Scale 5.1 The scale of the instrument shall be graduated both in pH and in millivolts. 5.2 The principal scale of the instrument shall cover the pH range0 to14. 5.3 For an instrument with scales or charts, the scale spacing (in millimetres) and the scale interval on both pH and millivolt scales and
42、 on any expanded scale shall be stated in the literature provided with the pH meter. 5.4 The instrument shall be described in terms of its most sensitive pH scale interval or representation unit. For example, a pH meter with0.01 pH scale interval shall be designated a0.01 pH meter and a digital pH m
43、eter with0.001 representation unit shall be designated a0.001 pH meter. 5.5 For moving index instruments, the scales should be divided and marked to comply with the recommendations of BS3693-2. The index (knife edge) and (minor) scale mark thickness shall be about of the scale division in order to f
44、acilitate interpolation by eye into fifths. 5.6 For a digital instrument, the representation unit on both pH and millivolt scales and on any expanded scale, and the height in millimetres of the numerical display shall be stated in promotional and accompanying literature. 5.7 In a multi-range instrum
45、ent, the range in use shall be clearly indicated on the front panel. 5.8 The manufacturer shall state in accompanying literature whether a positive millivolt scale refers to positive polarity of the glass electrode input or of the reference electrode input. 6 Errors of indication 6.1 When determined
46、 by the methods given in Appendix A, the intrinsic error of indication arising from graduation and non-linearity at any point shall not exceed, on any range,20% of the scale interval for instruments with scales, or one representation unit for digital instruments. 1 10 BS3145:1978 BSI 04-1999 3 6.2 W
47、hen determined by the method given in Appendix B, the (overall) instrument error shall not exceed50% of the scale interval at any point on any range or two representation units on that range. 6.3 On multi-range instruments, unless the manufacturer states in the literature provided with the pH meter
48、(see clause15) that the electrode system should be separately standardized on each pH scale, the error of indication arising from scale incompatibility shall not exceed20% of the principal scale interval or one representation unit when switching between the principal scale and an expanded scale, or5
49、0% of the expanded scale interval or two representation units when switching between expanded scales of the same scale interval but different range. 7 Input current 7.1 The input current at zero voltage shall not cause an error of indication of more than one scale interval or five representation units on the most sensitive scale of the instrument when a glass electrode with1G7 resistance is used. Compliance with this requirement shall be ascertained by testing according to the procedure detailed inC.1 in which a resistor is used to simulate the glass electrode. NOTE 1Th
