Chinese Primary Standard Seawater: Stability checks and comparisons with IAPSO Standard Seawater

Deep-Sea Research I 113 (2016) 101–106

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Deep-Sea Research I journal homepage: www.elsevier.com/locate/dsri

Chinese Primary Standard Seawater: Stability checks and comparisons with IAPSO Standard Seawater Yanan Li n, Yan Luo, Ying Kang, Tao Yu, Aijun Wang, Chuan Zhang National Center of Ocean Standards and Metrology (NCOSM), 219 Western Jieyuan Rd., Tianjin 300112, China

art ic l e i nf o

a b s t r a c t

Article history: Received 22 January 2016 Received in revised form 6 April 2016 Accepted 7 April 2016 Available online 8 April 2016

The authors give a brief introduction to the Chinese Primary Standard Seawater, with a description of its preparation procedures. IAPSO Standard Seawater (IAPSO SSW), was taken as a stable reference in the stability check of Chinese Primary Standard Seawater (CP SSW), and linear regression model as well as hypothesis testing were introduced into the analysis of check results; a demonstration check of CP SSW (batch number P8) achieved a positive conclusion. In comparisons of several batches of these two kinds of standard seawater on Practical Salinity, identical seawater samples from a homogeneous source were measured repeatedly. To evaluate the comparison results, performance criteria referred to as En numbers were adopted, the maximum of which was 0.42, indicating that no significant differences lay between these two kinds of SSWs when used to determine Practical Salinity. Measures taken to assure the reliability of measurement results are presented. & 2016 Elsevier Ltd. All rights reserved.

Keywords: Chinese Primary Standard Seawater Salinity Stability Comparison

1. Introduction IAPSO1 Standard Seawater (IAPSO SSW), a reference material derived from natural ocean water with a history of more than 100 years (Culkin and Smed, 1979), officially endorsed by the International Association for the Physical Sciences of the Oceans (IAPSO) and obtained from a single source, has long been a fundamental component of definitions of seawater salinity. It is central to the Practical Salinity Scale 1978 (PSS-78; UNESCO, 1981), and it remains an indispensable component of practical measurement procedures even using the Thermodynamic Equation of Seawater2010 (TEOS-10, IOC et al., 2010). In recent years, more attention has been focused on the physico-chemical characteristics of IAPSO SSW, especially its stability, as the precision and repeatability required for ocean measurements has increased. Mantyla (1980, 1987, 1994) inspected some early batches of IAPSO SSW and found changes of conductivity, similar work was done by Takatsuki et al. (1991) and Kawano et al. (2006). Culkin and Ridout (1998) and Bacon et al. (2007) reported on the stability in laboratory storage, while Bacon et al. (2000) evaluated some batches of IAPSO SSW used in seven cruises between 1991 and 1997. However, while necessary, it remains metrologically questionable to base all measurements on a single artifact, not traceable to fundamental parameters such as the n

Corresponding author. E-mail address: [email protected] (Y. Li). 1 IAPSO: International Association for the Physical Sciences of the Ocean.

http://dx.doi.org/10.1016/j.dsr.2016.04.005 0967-0637/& 2016 Elsevier Ltd. All rights reserved.

International System of Units of SI (Feistel et al., 2016; Pawlowicz et al., 2016). For various reasons, China has its own standard seawater, which can be traced back to the 1960s. In the 1980s, the National Center of Ocean Standards and Metrology (NCOSM), a governmental metrological organization in Tianjin, China, began to independently produce standard seawater in accordance with the procedures specified in PSS-78 (UNESCO, 1981). This Chinese Primary Standard Seawater (CP SSW) is widely used by government and academic ocean scientists in China (such as governmental monitoring stations, universities, and some observation cruises) but is not well known to the international community. Two classes of standard seawater are prepared in NCOSM, Chinese Primary Standard Seawater (Practical Salinity SP ¼ 35, expanded uncertainty U¼0.001, with coverage factor k¼2, Ma and Tian, 2004; Fig. 1) and Chinese Serial Standard Seawater (SP ¼ 5, 20, 30, 35, 40, U¼ 0.003, k¼ 2, which is actually a linearity correction pack; Tian and Gao, 2002). Both of them were approved as a certified reference material (CRM) by General Administration of Quality Supervision, Inspection and Quarantine of China (AQSIQ) in 2004, and new batch numbers (P series) have been used since then. The term “primary” need to be explained here in case of any confusion. “Primary” does not mean CP SSW is the ultimate reference material of Practical Salinity in the world; instead, it refers to the highest level of accuracy in China, in contrast with Chinese Serial Standard Seawater, which has a larger expanded uncertainty. It was adopted by NCOSM simply to follow the nomination rules set by AQSIQ. The purpose of this paper is to give an introduction to CP SSW. Not only will such a description improve comparability between

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further testing. These bottles are first rinsed with de-ionized water in an ultra-sonic bath and dried at 110 °C. The methods set out by Culkin (1986) are used for the calibration (i.e., certification) of CP SSW against the PSS-78 specified solution of Merck Suprapur KCl. Homogeneity testing is carried out for each batch to ensure that the stated expanded uncertainty of U¼0.001 with a coverage factor k ¼2 is achieved.

3. Stability checks of CP SSW 3.1. Check method After the calibration of a newly prepared batch of CP SSW is completed, stability check is immediately performed at NCOSM. The routine check procedures are summarized as: (i) set aside 18 bottles of CP SSW chosen randomly from the newly prepared batch; (ii) half a year later, pick 3 out of the 18 bottles randomly; (iii) measure the conductivity ratio of each sample three times with a Guildline Autosal 8400Bsalinometer standardized with IAPSO SSW, and calculate the average; (iv) repeat procedure (ii) and (iii) when the storage age is 1 year, 1.5 years, 2 years, respectively; (v) analyze the results. 3.2. Analysis model In step (v), as proposed by ISO (2006), a linear regression model was adopted to evaluate the result, expressed as Fig. 1. Chinese Primary Standard Seawater from batch P8, produced in 2012, with a label value of 34.997 and a shelf life of two years.

ocean measurements taken using either reference, but it will also improve our understanding of the uncertainty in current standards for salinity determination. In Section 2, we briefly describe the preparation and calibration of CP SSW. In Section 3, a method for checking the long-term stability of CP SSW is introduced and demonstrated for CP SWW batch P8, and measures adopted to improve the reliability of results are discussed. Section 4 focuses on the Practical Salinity comparison of CP SSW and IAPSO SSW. Discussion and conclusions are presented in Sections 5 and 6.

Y = β0 + β1X + ε

(1)

where X denotes time, Y the corresponding average salinity value, β0 and β1 the regression parameters, and ε denotes the random error component. For a given set of measurement results, containing n pairs of Yi and Xi , Eq. (1) can be developed as

Yi = β0 + β1Xi + εi

(2)

The regression parameters are given by

b1 =

n ∑i = 1 (Xi − X¯ )(Yi − Y¯ ) n ∑ (X − X¯ )2 i

i=1

(3)

2. Preparation and calibration of CP SSW Although the procedures used in the production of CP SSW are very similar to those documented for IAPSO SSW by Bacon et al. (2007), there are some differences which we now describe. (1) Surface seawater (the Practical Salinity of which is approximately 34.5) is collected in an open ocean area of the Western Pacific and is transported to NCOSM; (2) Pre-filtration is carried out by circulating the crude seawater through the activated carbon filter, 20 mm and 5 mm cartridge filters in sequence, for 3 days; (3) The seawater is then fine filtrated with a 0.1 mm cartridge filter, for another 3 days; all filtrations are done at room temperature and are exposed to air. (4) Practical Salinity is monitored by periodic measurement with a Guildline Autosal 8400B salinometer, and adjusted to approximately 35 by evaporation or dilution. (5) To assure its homogeneity and stability, the seawater is stirred sufficiently with an electromagnet agitator and is UV sterilized periodically during procedures above; (6) The seawater is finally sealed in pharmaceutical-grade borosilicate glass bottles (each containing ca. 220 ml), awaiting

b0 = Y¯ − b1X

(4)

where b1 is the estimator for the slope, and b0 the estimator for the intercept. The standard deviation of b1 can be used for error analysis, expressed as

s n ∑i = 1 (Xi − X¯ )2

s(b1) =

(5)

where s the deviation of linear regression is computed from n

s2 =

∑i = 1 (Yi − b0 − b1Xi )2 n −2

(6)

Hypothesis testing of linearity was carried out. If the following criterion, which means a non-significant linearity regression, is met, we can conclude that no changing trend exists in data.

b1 s(b1)

< t0.95, n −2

(7)

where t0.95, n − 2 is the critical value of t-distribution at 95% probability level and with n − 2 degrees of freedom.

Y. Li et al. / Deep-Sea Research I 113 (2016) 101–106

3.3. The reliability of check results

Table 1 Changes in Practical Salinity of CP SSW after storage.

The stability check for each batch consisted of several measurements over a time span of two years. For a given batch, the reliability of check results can be achieved by ensuring that all major factors of these separate measurements, except time, remained as consistent as possible during the check period. 3.3.1. Ambient temperature and operators The temperature in the laboratory was controlled to be (1–2) °C lower than bath temperature (WHPO, 1994). All operators involved in the operation of the salinometer followed the same procedure, and their proficiencies were validated by periodically held measurement audits. In spite of this, it was preferable that all measurements in the check of a given batch is carried out by the same operator, if possible. 3.3.2. Salinometers Similarly, the same salinometer maintained with periodic calibration with CP SSW, was used in the check. The bath temperature was set to be 24 °C every time. Although the salinometer was kept in good condition by periodic calibration, a short-term drift may arise in the measurement. It can be assumed that the salinometer stabilizes during the measurement if no significant change is detected in the following two checks: i) every time when the switch is on “standby” during the measurement, check the display and ii) when the measurement of samples is completed, the SSW used for standardization is again measured and checked against the label value. Actually, we are planning to monitor the bath temperature independently using an additionally inserted high-precision platinum resistance thermometer (PRT), which has been proved by Bacon et al. (2007) to be an effective way to check the stability of bath temperature. 3.3.3. IAPSO SSW used for standardization Given the contradictory conclusions yielded by independent investigations (Mantyla, 1980, 1987, 1994; Kawano et al., 2006; Bacon et al., 2000) on the stability and reliability of IAPSO SSW, Bacon et al. (2007) tried to reconcile this controversy by suggesting that motion and temperature resulting from transportations may cause “offsets”, while IAPSO SSW stored under stable circumstances showed no significant change. It is logical to take IAPSO SSW as a stable reference in the stability check since it is stored with care in NCOSM. Concerning that batch-to-batch difference may exist, a newly produced batch of IAPSO SSW with a validity of 3 years was preferable to be used for the salinometer standardization if possible, so that its validity could cover the whole check period. Most of the measures described in this section also apply in the comparison.

The stability check result of P8 (CP SSW) is shown in Table 1 and Fig. 2. IAPSO SSW was involved in the calibration of CP SSW as well as the stability check. In the calibration process the label salinity of CP SSW was calculated by (Culkin, 1986)

R15 ZN,15

Batch

Date

Age (months)

Practical Salinity

No. of Checks

Difference

P8

11/05/2012 25/10/2012 07/05/2013 16/10/2013 08/05/2014

0 5 12 17 24

34.9961a 34.9967 34.9963 34.9963 34.9970

10 3 3 3 3

0.0000 0.0006 0.0002 0.0002 0.0009

a The label value of P8 is 34.997, calculated from K15, while 34.9961 was derived from Rt .

Fig. 2. Linear regression of stability check results.

with a mass fraction of 32.4356 g/kg at the same temperature and pressure, and K15 the ratio of them. Herein, the same salinometer was used and remained stable, so the measurement bias introduced by the salinometer, including the effect of IAPSO SSW in the standardization, can be eliminated (Bacon et al., 2007); in other words, K15 and the label salinity as a result, was independent of the IAPSO SSW. However, in the following four checks, the salinity of CP SSW was derived from Rt . Concerning that, the salinity value of the first check (Age¼ 0 in Table 1), was calculated from Rt instead of K15, which means it was not the labeled value which had been calibrated against the KCl solution. By doing that, all the check results were related to the IAPSO SSW used for standardization. Regression parameters were calculated,

b1 = 2.315 × 10−5

s = 3.339 × 10−4 s(b1) = 1.76 × 10−5

3.4. Stability check result

K15 =

103

(8)

where R15 is the derived conductivity ratio of CP SSW at the temperature of 15 °C (ITS-68) and the pressure of one standard atmosphere, ZN,15 the calculated conductivity ratio of KCl solution

b1 s(b1)

=1. 35
Since Eq. (7) was met, we concluded that no significant changing trend of Practical Salinity was observed in this batch after 2 years' storage.

4. The comparison of CP SSW and IAPSO SSW Although label values of CP SSW and IAPSO SSW are traced to a standard KCl solution with a concentration of 32.4356 g/kg

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(UNESCO, 1981), they may perform differently in operational applications, for they still vary with each other in two aspects: the source area of crude seawater (Pacific versus Atlantic), which may result in slight composition anomalies, and the preparation technologies and processes. Periodic comparisons at NCOSM have been implemented for years for that purpose; these comparisons can also be taken as a quality assurance measure of CP SSW, in addition to the previously described stability checks and homogeneity testing. 4.1. Comparison method Direct comparison of label values is meaningless. In the method for stability check described above, CP SSW was measured while IAPSO SSW was used for the standardization of the salinometer, which seemed to be a comparison. But strictly, for its conductivity changes over time, IAPSO SSW is not a perfect reference when the same batch is involved in multiple comparisons over a long time span. Since it can not reveal differences between these two SSWs cleanly, the method for stability check is not a comparison clear enough. One feasible method is to refer these two kinds of standard seawaters to the defined KCl solution and make further comparison, while the preparation of the KCl solution is considerably time-consuming. Since one of the main applications of standard seawater is to standardize salinometers, comparisons can most usefully be achieved by making standard measurements of the same seawater sample using both types of standard seawater. Thus, in addition to the labeled bottles for each batch, a special seawater sample was also prepared, following nearly the same procedures listed in Section 2, but stored in a large narrow-necked bottle (ca. 7 L), which herein can be deemed as a sub-standard. The sub-standard was taken as the measurement object in the comparison, for which most important property was homogeneity, so there was no need to adjust its salinity to approach a specific value. Identical seawater samples from the sub-standard were first measured several times with an Autosal 8400B salinometer standardized with CP SSW, and average salinity was calculated. After that, the salinometer was standardized with IAPSO SSW, and the measurements were repeated. By controlling the measuring process strictly (similar to the description in Section 3.3), any effect introduced by the salinometer itself can be minimized. The performance of standard seawater was evaluated by analyzing the measurement results with the statistical methods modified from ISO (2005). 4.2. Arithmetic The performance criteria referred to as En numbers (ISO, 2010) were adopted to evaluate the comparison results, calculated as

En =

x1 − x2 U12 + U22

(9)

where x1 and x2 are the average measurement results obtained with the salinometer standardized with CP SSW and IAPSO SSW respectively; U1 and U2 are the expanded uncertainties of CP SSW and IAPSO SSW, both of which equal 0.001 ( k=2). When the calculated En≤1, it denotes a satisfactory result, i.e., both of them can achieve corresponding claimed uncertainties; while En>1 an unsatisfactory result. 4.3. Comparison result The comparisons included 5 batches of CP SSW and 2 batches of IAPSO SSW, among which, CP SSW P7 and P8 were compared

with IAPSO SSW P153 three times, and CP SSW P9, P10, and P11 were compared with IAPSO SSW P155 two times each. Results are shown in Table 2. Overall, all of the statistic En showed satisfactory results, which meant that CP SSW and IAPSO SSW had the accuracy of the same level when used to determine Practical Salinity. Although most seawater samples used for comparison had practical salinities slightly higher than 35, the one used on 11/05/ 2012 was 33 approximately and the one used on 04/07/2014 was as low as 20. However, the En did not show any correlation to these different salinities, which indicated the linearity of the salinometer was permissible. If the data of different batches of CP SSW are extracted from Table 2, a good batch-to-batch agreement of CP SSW in the comparison can be yielded, listed in Table 3. Note that the same two batches of SSW may be involved in multiple comparisons. For CP SSW P8 and IAPSO SSW P153, En were 0.14, 0.21, and 0 respectively; this set of comparisons can be regarded as a validation of the stability check method. This conclusion also applies for other batches.

5. Discussion In much of the European-language scientific literature salinities are implicitly or explicitly related to a particular reference material called IAPSO SSW because it is endorsed by the International Association for the Physical Sciences of the Oceans (IAPSO), an international non-governmental scientific body for the coordination of ocean science. IAPSO SSW is obtained from a single source, currently Ocean Scientific International Ltd. (UK), and the manufacture and use of IAPSO SSW is documented in both standards documents and the scientific literature. However, IAPSO SSW is not the only reference material for seawater measurements, and Chinese Primary Standard Seawater (CP SSW), a product of the Chinese National Center of Ocean Standards and Metrology is widely used in China. Important questions about CP SSW and IAPSO SSW (and hence about the comparability of salinity measurements using the two different references) concern their label comparison and relative stability. More generally, because of the use of such a reference material, oceanic salinity measurements are not traceable back to fundamental constants such as those making up the International System of Units (SI) and this is a significant problem for long-term monitoring of ocean conditions (Seitz et al., 2011; Feistel et al., 2016; Pawlowicz et al., 2016). Lacking this traceability, all aspects of the measurement process should be tested in order to form reliable uncertainty budgets. Seitz et al. (2011) illustrated the traceability chain for Practical Salinity as well conductivity measurement results in their Fig. 1, based on IAPSO SSW. With respect to CP SSW, it also fits in this schematic. As mentioned in PSS-78, “any oceanic water having a precisely known conductivity ratio of near unity at 15 °C with the KCl solution is a secondary standard for routine calibration of oceanographic instruments”. CP SSW is produced following similar procedures as IAPSO SSW, and it can be directly traced back to the KCl solution rather than IAPSO SSW; although it seems that IAPSO SSW get involved in the preparation of CP SSW, the latter is independent of the former, as discussed in Section 3.4. The uncertainty of IAPSO SSW is determined following the approach provided by Bacon et al. (2007), and the uncertainty has been assessed on CP SSW in NCOSM. Expanded uncertainty of the same order as that of IAPSO SSW was assigned to CP SSW, despite of obvious different procedures in assessment, which are not discussed here though; however, the set of good En numbers in the comparison reflect that the uncertainty assessment methods of

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105

Table 2 Practical Salinity comparison results of CP SSW and IAPSO SSW. Date

SSW Batch Number CP

11/05/2012

Measurement Results

33.0802 33.0802

33.0800 33.0800

33.0800 33.0802

33.0801

0

0

P8

33.0802 33.0802

33.0802 33.0802

33.0802 33.0806

33.0803

2

0.14

33.0800 33.0800

33.0800 33.0800

33.0800 33.0806

33.0801

–

–

P7

35.2515 35.2513

35.2513 35.2515

35.2513 35.2515

35.2514

6

0.42

P8

35.2515 35.2519

35.2515 35.2519

35.2515 35.2519

35.2517

3

0.21

35.2521 35.2521

35.2521 35.2519

35.2519 35.2519

35.2520

–

–

P7

35.4307 35.4313

35.4307 35.4311

35.4306 35.4313

35.4310

5

0.35

P8

35.4315 35.4317

35.4315 35.4313

35.4315 35.4315

35.4315

0

0

35.4315 35.4317

35.4313 35.4315

35.4315 35.4317

35.4315

–

–

35.4624 35.4626

35.4626 35.4628

35.4624 35.4626

35.4626

4

0.28

35.4632 35.4630

35.4630 35.4630

35.4628 35.4628

35.4630

0

–

P10

20.0833 20.0837

20.0835 20.0837

20.0835 20.0833

20.0835

1

0.07

P11

20.0831 20.0835

20.0831 20.0833

20.0833 20.0831

20.0833

3

0.21

20.0835 20.0835

20.0837 20.0837

20.0835 20.0835

20.0836

0

–

P9

35.4906 35.4906

35.4904 35.4908

35.4908 35.4908

35.4907

2

0.14

P10

35.4908 35.4906

35.4908 35.4908

35.4910 35.4910

35.4908

3

0.21

P11

35.4906 35.4902

35.4904 35.4906

35.4906 35.4906

35.4905

0

0

35.4904 35.4904

35.4904 35.4904

35.4906 35.4906

35.4905

–

–

P153

07/03/2014

P9

P155

04/07/2014

P155

27/05/2015

P155

Table 3 Differences between some batches of CP SSW. Date

CP SSW Batch

Average

11/05/2012

P7 P8 P7 P8 P7 P8 P10 P11 P10 P11

33.0801 33.0803 35.2514 35.2517 35.4310 35.4315 20.0835 20.0833 35.4908 35.4905

07/05/2013 19/10/2013 04/07/2014 27/05/2015

En

P7

P153

19/10/2013

Difference (  10  4)

IAPSO

P153

07/05/2013

Average

Difference (  10  4)

En

2

0.14

3

0.21

5

0.35

2

0.14

3

0.21

these two kinds of SSWs are both reasonable. While Millero et al. (2008) provided the reference composition of IAPSO SSW, the composition anomaly of CP SSW which is essential to

the calculation of Absolute Salinity, is currently unknown. Pawlowicz et al. (2016) outlined the intended near-future use of density standards for SI-traceable measurements, to replace the current conductivity-based PSS-78. In light of TEOS-10, conversion between Practical Salinity and Absolute Salinity is quantitatively well-known for IAPSO SSW already, but not for CP SSW. Hence, although it can be claimed that the comparability between CP SSW and IAPSO SSW is acceptable when used to determine Practical Salinity, it does not necessarily apply for density and consequently Absolute Salinity. Research work on the composition, density, Absolute Salinity, thermodynamic properties of CP SSW will be done and presented by NCOSM in the future, in order to ascertain the potential inconsistency between these two SSWs. 6. Conclusions Here we describe the procedures behind, and quality of, CP SSW.

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The preparation procedures of Chinese Primary Standard Seawater are different from those of IAPSO Standard Seawater, mainly in the collection location of crude seawater and filtration operations. However, these two kinds of standard seawaters share the same calibration method which was documented by PSS-78 (UNESCO, 1981). A stability check of CP SSW was implemented by measuring random samples taken from the same batch at five times over two years, no significant trend was detected in batch P8. In addition, a number of comparisons were performed between different batches of CP SSW and IAPSO SSW, at different ages, by using each to repeatedly measure water samples from a single homogeneous source. For each of the 12 comparisons, a corresponding statistic En was calculated to evaluate whether the two measurements agreed to within their individual uncertainty budgets, and this was found to be the case. For the purpose of assuring the reliability of measurement results, many steps were taken to maintain the operation consistency, and to eliminate the short-term drift of the salinometer, while there is still space for further improvement.

Acknowledgements The authors thank Dr. R. Pawlowicz of University of British Columbia for his encouragement and comments. This paper contributes to the tasks of the Joint SCOR/IAPWS/ IAPSO Committee on the Properties of Seawater (JCS).

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